The concentration of serum ionized calcium (physiologically “active” calcium) is normally maintained within narrow limits by the action of parathyroid hormone (PTH) on bone resorption, renal calcium excretion, and metabolism of vitamin D (Table 16-1). PTH (primarily an 84–amino acid, single-chain polypeptide) is synthesized, stored, and secreted by chief cells in the four parathyroid glands. Secretion is regulated, in turn, by the serum calcium concentration: high concentrations inhibit secretion of PTH, and low concentrations stimulate it (Aurbach et al, 1985a; Brown et al, 1999). The amino acid sequence of PTH is known for humans, dogs, cows, pigs, rats, chickens, and cats. Based on immunologic reactivities, most mammals appear to have similar amino-terminal portions of the molecule (Toribio et al, 2002). It is recognized that the amino terminal of PTH binds to cell membrane receptors. The carboxyl terminus is thought to serve only as a guide for PTH through the cellular secretory pathway (Orloff and Stewart, 1995). A synthetic 1-34 amino-terminal fragment is biologically active, but relatively minor modifications at the amino terminal (especially at the first two residues) can completely abolish its biologic activity (Marx, 2000). The effect of PTH on mineral metabolism is initiated by binding of PTH to type 1 receptors in target tissues, which results in regulation of large calcium fluxes across bone, kidneys, and intestines (Fig. 16-1).

TABLE 16-1 NORMAL SERUM CONCENTRATIONS ^*

	Dog	Cat
Total Calcium
(mg/dl)	9.0–11.7	8.0–10.5
(mmol/L)	2.2–3.9	2.0–2.6
Ionized Calcium
(mg/dl)	4.6–5.6	4.5–5.5
(mmol/L)	1.12–1.42	1.1–1.4
Parathyroid Hormone
(pmol/L)	2–13	0–4
Parathyroid Hormone-
Related Protein (PTH-P)
(pmol/L)	<2	<2
1,25-Dihydroxyvitamin
D (Calcitriol)
(pg/ml)
Adults	20–50^*	20–40^*
10–12 wk old	60–120^*	20–80^*

* From Rosol, et al, 2000.

FIGURE 16-1 The parathyroid axis. The synthesis of parathyroid hormone (PTH) and parathyroid hormone–related peptide (PTHrP) is shown on the left and the target sites on the right. Both act by means of the same receptor (also called the type 1 receptor). Negative feedback of 1,25-dihydroxyvitamin D is not shown.

(Modified from Marx SJ: N Engl J Med 343:1863, 2000.)

Parathyroid hormone–related peptide (PTHrP) is a distant homologue of PTH and is not considered a true hormone. It is produced widely in the body and has numerous actions in the normal fetus and adult. However, this substance is best recognized as having a central role in the pathogenesis of humoral hypercalcemia of malignancy and is used as a “tumor marker.” Since its discovery in the early 1980s, assay of PTHrP has been used in the evaluation of hypercalcemic patients in whom cancer is a possibility (Philbrick et al, 1996).

ETIOLOGY

The essential disorder in primary hyperparathyroidism (PHPTH) is the excessive synthesis and secretion of PTH by abnormal, autonomously functioning parathyroid “chief” cells. The cause of this hormonal excess is usually a solitary adenoma, but adenomatous hyperplasia of one or more parathyroid glands and parathyroid carcinomas have been identified in both dogs and cats. In contrast, other forms of hyperparathyroidism (e.g., renal secondary or nutritional secondary hyperparathyroidism) are usually the result of alterations in calcium and phosphorus homeostasis caused by nonendocrine disturbances. Such disturbances indirectly affect the parathyroid glands, causing diffuse parathyroid gland hyperplasia (see Differential Diagnosis of Hypercalcemia, page 669). Depending on the underlying cause, the serum calcium concentration in secondary disorders may range from low to normal to increased. Primary hyperparathyroidism is virtually always associated with hypercalcemia, as reflected by increases in circulating concentrations of the total or ionized calcium concentration or both.

PATHOPHYSIOLOGY

Calcium–Parathyroid Hormone Feedback System

In primary hyperparathyroidism, normal negative-feedback homeostatic control is lost, and PTH secretion is increased either autonomously or as a result of a change in the set point. The autonomous secretion of PTH is not suppressible by the increased concentration of calcium perfusing the parathyroid glands. Conversely, the parathyroid glands in secondary hyperparathyroidism are normally suppressible by increased concentrations of calcium.

Severe Hypercalcemia, Bone Resorption, and the Kidneys

BONE.

The process leading to severe hypercalcemia is initiated by accelerated bone resorption caused by the activation of osteoclasts in the skeleton (Attie, 1989). The activity of these cells, caused by PTH or PTHrP, is the foundation for virtually all cases of marked hypercalcemia. Excessive absorption of calcium from the gastrointestinal tract is not usually an important cause of hypercalcemia, although it is a contributor to the hypercalcemia resulting from vitamin D toxicosis. Hypercalcemia develops when the entry of calcium into the extracellular fluid (regardless of the source) overwhelms the mechanisms that maintain normocalcemia. One of these mechanisms is suppressed secretion of PTH, a process obviously negated when the cause of the hypercalcemia is the autonomous secretion of PTH. In cancer-associated hypercalcemia, the secretion of PTH is suppressed, but the humoral factor that activates osteoclasts is the autonomous secretion of PTHrP. PTHrP, as previously discussed, has biologic activity quite similar to that of PTH (Sleeboom and Bijvoet, 1982; Harinck et al, 1987). In the setting of accelerated bone resorption, the kidneys become the principal defense against hypercalcemia (Bilezikian, 1992a; Hruska and Teitelbaum, 1995).

Associated with accelerated osteocytic and osteoclastic bone resorption, mineral is removed and replaced by immature fibrous connective tissue. The bone lesion of fibrous osteodystrophy in afflicted dogs is reported to be generalized throughout the skeleton but accentuated in certain areas, such as the cancellous bone of the skull (Capen and Martin, 1983). Iliac crest bone biopsies from virtually all humans with primary hyperparathyroidism reveal histomorphometric evidence of the effects of excess PTH (Arnaud and Kolb, 1991; Hruska and Teitelbaum, 1995). These findings may include increased bone resorption surfaces, increased numbers of osteoclasts, osteocytic osteolysis, and marrow fibrosis. Bone histomorphometric findings in dogs with primary hyperparathyroidism were similar to those reported in humans (i.e., parallel increases in osteoblastic and osteoclastic activity). The increased rate of bone formation in these dogs was reflected in an increased rate of mineral apposition and a greater extent of tetracycline-labeled bone-forming surface (Meuten et al, 1983a; Weir et al, 1986).

KIDNEYS.

When renal and endocrine functions are normal, any tendency for a rise in serum calcium is attenuated by increased urinary excretion of calcium. This process is inhibited by PTH excess, the foundation of primary hyperparathyroidism. Severe hypercalcemia (defined as a serum calcium concentration >14 mg/dl in humans and, arbitrarily, >15 mg/dl in dogs) is the result of humoral factors (PTH or PTHrP) that induce osteoclast-mediated bone resorption and stimulate intestinal absorption of calcium and renal tubular resorption of calcium (Mundy, 1988; Hruska and Teitelbaum, 1995). This impairs the kidneys’ ability to excrete the increased filtered load of calcium. Thus animals with an excess of PTH lack the first lines of defense against hypercalcemia.

The hypercalcemic condition also interferes with renal mechanisms for resorption of sodium and water, leading to polyuria. This is due to an acquired inability to respond to antidiuretic hormone (ADH, vasopressin, AVP). This represents a reversible form of nephrogenic diabetes insipidus. Increased serum calcium concentrations reduce the concentration of cyclic adenosine monophosphate (cAMP) in the renal tubular cells that normally respond to ADH, either through alteration of ADH receptors or through inactivation of adenylate cyclase, the enzyme necessary for the generation of intracellular cAMP. Compensation for the resultant polyuria by commensurate intake of fluid may not occur in people because of hypercalcemia-induced anorexia or nausea. Polyuria and resultant polydipsia are also recognized in hypercalcemic dogs and cats. Although hypercalcemia in dogs may cause “inappetence,” it does not commonly result in anorexia or lack of water intake. Rather, most hypercalcemic dogs exhibit mild to moderate polydipsia and polyuria. A small number of hypercalcemic dogs have a urine specific gravity less than 1.007 and thus are arbitrarily classified as having “severe” polyuria.

Depletion of the extracellular fluid volume and a reduced glomerular filtration rate (GFR) as a result of dehydration may further increase the serum calcium concentration (Bilezikian, 1991). Hypercalcemia alters the GFR by causing (1) volume contraction of the extracellular fluid space secondary to fluid losses; (2) alterations in glomerular capillary permeability; and/or (3) sustained vasoconstriction that may result in ischemic injury to the kidneys, directly potentiating the toxic effects of hypercalcemia on renal tubular cells. Furthermore, mineralization of tissues may begin in the basement membrane of the ascending loop of Henle, the distal convoluted renal tubule, and the collecting ducts. This damages the basement membrane and impairs renal tubular repair. At the organelle level, mineralization of mitochondria occurs in renal tubular epithelial cells, potentially leading to disrupted cellular function, death, and sloughing of mineralized cell debris into renal tubular lumina. Casts composed of renal tubular cells may contribute to obstruction of urine flow along affected nephrons.

Early in the course of the hyperparathyroidism, when hypercalcemia is mild, urinary calcium excretion is relatively low. PTH action, at the renal level, enhances tubular resorption of calcium. When serum calcium concentrations are greater than 12 to 14 mg/dl, the renal tubular mechanism for reabsorbing calcium becomes overwhelmed. Here, the kidney’s adaptive mechanism for correcting hypercalcemia becomes operative despite excessive concentrations of PTH. This hypercalciuria, however, is not sufficient to correct the hypercalcemia or the resultant nephrocalcinosis.

Nephrocalcinosis may be mild and reversible or severe and progressive, leading to continued renal damage and uremia. Severe mineralization of the kidneys is characteristically located subjacent to the corticomedullary junction and has the appearance of a white gritty band (Meuten, 1984). Histologic findings include interstitial infiltration of mononuclear cells, varying degrees of renal tubular atrophy, calcification, necrosis, and sloughing of the epithelium. In humans, renal failure may initially result from prerenal factors, but if hypercalcemia persists, the failure may be the result of acquired intrinsic renal lesions. It is extremely important to point out that this series of physiologic events described in humans seems to be much less worrisome in dogs with PHPTH. Almost all dogs with PHPTH have normal blood urea nitrogen (BUN) and serum creatinine concentrations. Furthermore, as we have monitored a considerable number of untreated dogs for periods longer than 12 to 18 months, it is safe to say that we have observed no evidence of progressive renal damage. To the contrary, the kidneys in dogs appear to be “protected” by PHPTH. As has been noticed in untreated humans, virtually no dramatic changes in serum biochemical profiles (aside from increasing calcium and decreasing phosphate concentrations) are noted over time (Silverberg et al, 1999).

Uroliths and Urinary Tract Infections

Hypercalciuria resulting from primary hyperparathyroidism may contribute to the urolithiasis and urinary tract infections that are “common” with this disorder. The incidence of urinary tract infection approaches 25% of afflicted dogs and may be related to reduction of the urine specific gravity. This is not known for certain, however, and other factors may contribute to the development of infection.

Reports on dogs with primary hyperparathyroidism (PHPTH) have also included a percentage of dogs (˜30%) with urinary calculi (Berger and Feldman, 1987; Klausner et al, 1987). Hypercalciuria results from increased glomerular filtration of calcium, which we have always presumed to predispose these animals to urolithiasis. However, other hypercalcemic conditions have not been associated with urolithiasis. This discrepancy may be explained by the chronic nature of the hypercalcemia in dogs with PHPTH, because many tend to be asymptomatic or to have only mild signs, as opposed to the short-term nature of hypercalcemia in dogs with neoplasia, toxin exposure, and other such conditions. In these other conditions, dogs tend to develop obvious or severe clinical signs quickly and, therefore, do not have chronic hypercalcemia. Additional factors may play a role in urolith formation or urinary solute saturation, including pH, ionic strength, and crystal aggregation inhibitors. Calcium phosphate, for example, is markedly less soluble in alkaline urine than in acidic urine.

Vitamin D Excesses and Deficiencies

An adaptive mechanism to increases in the serum calcium concentration is a decrease in the circulating vitamin D concentration, assuming that the hypercalcemia is not a result of a primary renal or endocrine disorder. Decreases in the vitamin D concentration should result in reduced calcium absorption from the intestine. By contrast, the response to mild decreases in the serum calcium concentration is secretion of both PTH and vitamin D. PTH-stimulated bone resorption and vitamin D–stimulated intestinal absorption of both calcium and phosphorus then stimulate a rise in the serum calcium and phosphate concentrations (see Fig. 16-1). The renal tubular effects of PTH aid mineral homeostasis by stimulating synthesis and secretion of 1,25[OH]₂D₃ (vitamin D). PTH also functions at the renal tubular level to increase phosphate excretion into the urine. This is important for preventing hyperphosphatemia after phosphate is removed from bone with calcium and/or absorbed from the intestine with calcium.

In primary hyperparathyroidism, hypercalcemia is aggravated by the increased production of vitamin D and by a decrease in the amount of serum phosphate available to form complexes with serum ionized calcium. The result is decreased tubular resorption of phosphate, hyperphosphaturia, and hypophosphatemia. These actions are responsible for the development of the biochemical triad classic for primary hyperparathyroidism: hypercalcemia, hypophosphatemia, and hyperphosphaturia.

Vitamin D deficiency, after chronic PTH-stimulated use of vitamin D stores, is a possible natural adaptive mechanism for coping with primary hyperparathyroidism. This process seems unlikely in dogs with PHPTH because humans with vitamin D deficiency caused by chronic hyperparathyroidism typically have severe osteomalacia, a problem not yet reported in animals (Meuten et al, 1983a; Weir et al, 1986). Furthermore, in a study that included four dogs with primary hyperparathyroidism, three had calcitriol concentrations that were mildly increased or in the high normal range (Rosol et al, 1992a). If the results in these three dogs accurately reflect the physiology of most dogs with PHPTH, osteomalacia would be uncommon, and the previously discussed physiologic triad is better understood.

Renal Effects of Hypercalcemia and Soft Tissue Mineralization

Several natural adaptive mechanisms are initiated in an attempt to control and possibly correct the hypercalcemia associated with primary hyperparathyroidism. One mechanism is the previously described hypercalciuria. Another occurs once the renal tubular transport of calcium is overwhelmed. When the solubility product of the serum calcium (Ca⁺⁺) and phosphate (PO₄) concentrations exceed 60 to 80 mg/dl, deposition of calcium in soft tissue ensues (Meuten et al, 1981). The result of this trade-off to lower the serum calcium concentration is the development of progressive organ dysfunction, especially in the kidneys as a result of nephrocalcinosis. Soft tissue calcification may occur in a variety of other organs or tissues (or both), but none is quite as worrisome as that which occurs in the kidneys.

As discussed previously, azotemia occurs commonly in dogs with hypercalcemia of malignancy, hypoadrenocorticism, chronic renal failure (CRF), and hypervitaminosis D (Kruger et al, 1996). Hypercalcemia can contribute to azotemia with any combination of the following: prerenal reduction in extracellular fluid volume (anorexia, hypodipsia, vomiting, and polyuria); renal vasoconstriction; decreased permeability coefficient of the glomerulus (K_f); acute tubular necrosis from the ischemic and toxic effects of hypercalcemia; and CRF caused by nephron loss, nephrocalcinosis, tubulointerstitial inflammation, and interstitial fibrosis (Rosol et al, 2000).

The frequency of azotemia was higher in dogs with malignancy (71%) than in those with PHPTH (11%) (Kruger et al, 1996). It is our impression, however, that CRF is almost never caused by PHPTH in dogs. The incidence of CRF in our series of dogs with PHPTH is approximately 4%. It is likely that a similar incidence of CRF would be identified in any population of dogs in which the average age is 10 to 11 years. The most common cause of CRF in hypercalcemic dogs would be related to a calcium × phosphorus product above 60 to 80 mg/dl. Such increases in this product would be extremely unusual in PHPTH because of the low-normal or low serum phosphorus concentrations caused by PTH-induced excesses in vitamin D. Therefore, although a typical normal dog has a total serum calcium (Ca) concentration of about 10.5 mg/dl, a serum phosphate of about 4.5 mg/dl, and a product of 47, the typical dog with PHPTH has a total serum Ca concentration of about 15 mg/dl, a serum phosphate of about 3.0 mg/dl, and a product of 45. This is in contrast to a dog with CRF or vitamin D toxicosis, with a total serum Ca concentration of about 10.5 mg/dl (or higher), a serum phosphate of about 10.0 mg/dl (or higher), and a product in excess of 100.

Rapid Metabolism of PTH

Stimulation, via hypercalcemia, of rapid tissue degradation of the biologically active forms of PTH by parathyroid and nonparathyroid cells may represent a physiologic mechanism to curtail worsening hypercalcemia in PHPTH (Arnaud and Kolb, 1991). Plasma calcium may regulate PTH secretion and also determine the circulating quantities of biologically active PTH molecules versus inactive hormone fragments. This phenomenon may be more theoretical than practical because the disappearance rate of PTH from serum after the removal of a parathyroid tumor is no different from that of dogs with nonparathyroid disease in which the parathyroids are surgically removed.

Calcitonin

Calcitonin, a 32–amino acid polypeptide hormone produced by parafollicular or “C cells” located in the thyroid, is secreted in response to increases in serum calcium (Mol et al, 1991). This hormone has the primary responsibility of limiting postprandial hypercalcemia in normal mammals. Calcitonin decreases bone resorption by reducing the size of osteoclast brush borders, the number of osteoclasts, and osteoclast motility. Calcitonin does not affect the kidney or intestine. It may, however, influence the satiety center, decreasing appetite. The role of calcitonin in dogs with hypercalcemia secondary to PHPTH is not known. Increased calcitonin secretion in response to hypercalcemia seems a reasonable physiologic response. Parafollicular cells are markedly hyperplastic and appear as small white foci in the thyroid gland of dogs with primary hyperparathyroidism (Capen and Martin, 1983). Hyperplastic parafollicular cells may displace colloid-containing follicles lined by thyroid follicular cells, implying the presence of an adaptive response by the parafollicular cells. Studies in humans suggest that this adaptive mechanism is inconsistent (Arnaud and Kolb, 1991).

ASSAYS OF SERUM PARATHYROID HORMONE

History

The development of an assay for serum PTH in 1963 by Berson and coworkers was a major breakthrough in the diagnosis of human parathyroid disorders. Indirect tests of parathyroid function, such as the renal clearance of calcium, phosphate, or cAMP, had not proved satisfactory for differentiating parathyroid from nonparathyroid causes of hypercalcemia or hypocalcemia. In the 1960s and 1970s it became clear that an immunoassay for PTH would provide much more consistent information but that development of such an assay presented far more complex problems than routine immunoassays established for other peptide hormones.

Complete (84–amino acid) molecules of PTH and numerous incomplete “pieces” of PTH are found in the circulation of all animals. These circulating PTH and partial-PTH peptides have different metabolic half-lives. The half-lives of intact PTH and the N-terminal fragments are measured in minutes, whereas the half-lives of middle and carboxyl (C)-terminal PTH fragments are tenfold to twentyfold longer. Bioactivity resides in the intact molecule, the major secretory product of the parathyroid glands. The intact molecule is present in relatively low serum concentrations in dogs and cats. The N-terminal fragment, a product of PTH metabolism by the liver and kidneys, contributes little or not at all to the circulating pool of immunoreactive PTH. The C-terminal and midmolecule fragments are biologically inactive but are present in rather large concentrations. These biologically inactive fragments are excreted primarily by the kidneys, and they accumulate in the serum of patients with renal failure.

PTH Assay Systems

Development of the “two-site” PTH assay system currently used by most human and veterinary laboratories depends on the production of two different polyclonal antibodies. The two-site immunoradiometric (IRMA) system for intact human PTH (Allegro Intact PTH; Nichols Institute, San Juan Capistrano, Calif.) has been demonstrated to be valid for measurement of dog and cat PTH (Torrance and Nachreiner, 1989a; Flanders and Reimers, 1991; Barber et al, 1993). One antibody binds only the midregion and C-terminal 39-84 amino acids of human PTH. The other antibody binds only the N-terminal 1-34 amino acids of human PTH. Samples being assayed are incubated simultaneously with both antibodies, with a “sandwich” created composed of the 1-34 antibody plus the patient’s intact hormone plus the 39-84 antibody. Only these “sandwiches” are detected by the assay system, eliminating any interference by midregion or C-terminal fragments, even if present in large concentrations. Two-site immunochemiluminometric assays for intact PTH in humans use antibodies similar to those in the intact PTH IRMA assays and have the same range of applicability to animal sera (Michelangeli et al, 1997).

Clinical Usefulness

BACKGROUND.

Determination of the serum PTH concentration is a useful diagnostic aid in the evaluation of dogs and cats suspected of having parathyroid disease. As with any assay, the clinician must be certain that the assay used is valid for dogs or cats. Furthermore, laboratories must develop their own reference values for normal conditions, as well as values typical for most disease states. The need for development of independent reference ranges cannot be overstated, even when the assay system used has been validated elsewhere.

Experience with the new PTH two-site assays has been extremely positive. Samples obtained for this assay should be stored and shipped frozen to prevent degradation of intact PTH. Stability is best achieved in plasma collected with ethylenediamine tetra-acetic acid (EDTA), but serum is adequate if stored frozen after separation from red blood cells. Normal values for the serum PTH concentration are 2 to 13 pmol/L in dogs and 0 to 4 pmol/L in cats. Samples should be shipped to the appropriate laboratory with two or three frozen gel packs (Torrance and Nachreiner, 1989a and 1989b).

As with most hormone assays, evaluation of the serum hormone concentration “out of context” is not as consistently informative as evaluation in the proper context. With this in mind, the serum PTH concentration must be evaluated relative to the total and, ideally, ionized serum calcium concentrations. Decreases in the serum calcium concentration are normally associated with increases in the serum PTH concentration. Increases in the serum calcium concentration are normally associated with decreases in the PTH concentration (Fig. 16-2). As with any laboratory value, a single PTH result may not provide correct information. Therefore practitioners are encouraged to complete a thorough evaluation of hypercalcemic dogs or cats. If test results do not appear logical, some tests may need to be repeated, including measurement of serum hormone concentrations, which may fluctuate.

FIGURE 16-2 Graph showing the serum parathyroid hormone (PTH), parathyroid hormone–related peptide (PTHrP), ionized calcium, and vitamin D concentrations in the most common causes for hypercalcemia of dogs.

ASSAYS OF IONIZED SERUM CALCIUM CONCENTRATIONS

The biologically active form of calcium in the circulation is the ionized fraction. Measurement of the serum ionized calcium concentration has become widely available and cost-effective (normal or “reference” concentrations for our laboratory are 1.12 to 1.42 mmol/L; see Table 16-1). Although samples can be obtained routinely rather than according to the previously described anaerobic requirements, anaerobically obtained, handled, and stored serum is stable for a longer time and therefore preferred (Schenck et al, 1995). Anaerobic collection ensures that no increase in pH occurs as a result of loss of carbon dioxide (CO₂). An acidic pH favors dissociation of calcium from protein and increases the amount of ionized calcium in the sample. An alkaline pH occurs with loss of CO₂ and favors calcium binding to protein, thus reducing the amount of ionized calcium. Mixing of serum and air increases the pH, decreasing the ionized calcium concentration. Ionized calcium and pH are more stable in serum than in whole or heparinized blood. The analysis of serum eliminates potential interference by heparin and allows longer storage periods. Serum may be stored for subsequent ionized calcium analysis as long as it has been handled anaerobically. Silicone separator tubes should not be used. Measurement of ionized calcium in canine serum was stable after storage at 23°C for 72 hours and for 7 days at 4°C (Schenck et al, 1995).

The use of automated equipment with calcium ion–selective electrodes has resulted in easy and accurate measurement that minimizes interference caused by magnesium, potassium, protein, hemolysis, and other factors (Gouget et al, 1988). However, differences among analyzers exist, and it is recommended that each laboratory establish reference ranges for the analyzer they use (Hristova et al, 1995). Some instruments mathematically manipulate the ionized calcium concentration and actual pH value of the sample and yield an “adjusted” value. Formulas have also been developed to “adjust” results for samples exposed to air. These formulas have not been validated in animals, and their use is not recommended (Rosol et al, 2000).

The addition of this diagnostic tool can be valuable in the evaluation of hypercalcemia and hypocalcemia as well as renal insufficiency. An effect of aging has been observed in both dogs and cats: young dogs and cats (up to 2 years of age) have serum ionized calcium concentrations slightly higher than those reported in “adult” animals (Deniz and Mischke, 1995; Mischke et al, 1996).

ASSAY OF VITAMIN D CONCENTRATIONS

Measurement of vitamin D metabolites, although not commonly used in veterinary medicine, would occasionally be helpful in the diagnosis of calcium disorders. 25[OH] vitamin D (calcidiol) and 1,25[OH]₂ vitamin D (calcitriol) are the metabolites of greatest clinical interest for detection of hypovitaminosis or hypervitaminosis D syndromes and in renal failure (Carothers et al, 1994). These metabolites are stable during refrigeration and freezing, but samples should not be exposed to light for any length of time. Both metabolites are chemically identical in all species, therefore receptor-binding assays or radioimmunoassays (RIAs) used for people are satisfactory for dogs and cats (Hollis et al, 1996). Young growing individuals have higher calcitriol concentrations than adults.

Calcitriol assays have demonstrated genetic errors of vitamin D metabolism and normal-to-increased concentrations in PHPTH, whereas patients with hypercalcemia of malignancy have levels that may be low, normal, or high. Dogs and cats with possible exposure to rat and mouse poisons containing vitamin D (e.g., Rat-B-Gone, Mouse-B-Gone, Rampage, Quintox) may present challenging diagnostic problems. Serum concentrations of vitamin D are increased in these toxicity states (Dougherty et al, 1990).

ASSAY OF PARATHYROID HORMONE–RELATED PROTEIN

Background

Parathyroid hormone–related protein (PTHrP) was identified in the 1980s as a tumor product that had the ability to activate PTH receptors and cause hypercalcemia. PTHrP resembles PTH not only in terms of its genetic sequence but also in terms of structure (Fig. 16-3), thus PTHrP is a second member of the PTH family of hormones (Broadus and Stewart, 1994). The first clues to the function of PTHrP came from studies of its expression in different tissues. In marked contrast to PTH, which is found only in the parathyroid glands, PTHrP is found in many tissues in both fetuses and adults, including epithelia, mesenchymal tissues, endocrine glands, and the central nervous system (Table 16-2). This widespread distribution suggests a diversity of roles for PTHrP (Fig. 16-4). Insights into these roles are now emerging from gene-knockout experiments and other approaches (Strewler, 2000). However, the key importance of this peptide is its central role in the development of humoral hypercalcemia of malignancy (HHM). It appears that the 1-36–amino acid peptide portion of PTHrP has an effect on cartilage, breast, and skin; the 38-94 midregion of PTHrP has an effect on placental calcium transport and perhaps on renal bicarbonate transport; the 107-139–amino acid portion of the molecule has an effect on the brain and on bone resorption (Strewler, 2000).

FIGURE 16-3 Structural and functional domains of parathyroid hormone–related protein (PTHrP). The 1-13 region of PTHrP is 70% homologous with the corresponding region of PTH and is believed to be involved in activation of adenylate cyclase and other second-messenger systems in target tissues. The 14-34 region shares no homology with the 14-34 region of PTH but has been shown to bind effectively to the PTH receptor. The 35-108 region of PTHrP is unique, sharing no homology with any other known peptide. This region is extraordinarily highly conserved among species, which suggests that it has a crucial but as yet unknown function or functions. The 88-108 region of the peptide is rich in potential proteolytic cleavage sites and contains several amidation signals and is therefore presumed to be the site of posttranslational processing, at least in some tissues. The 112-141 region of the peptide is poorly conserved among species. Preliminary evidence suggests that at least in some situations, C-terminal fragments derived from this region enter the circulation. The functional consequences of this are unknown.

(Broadus AE, et al: N Engl J Med 319:556, 1988.)

TABLE 16-2 SITES AND PROPOSED ACTIONS OF PARATHYROID HORMONE-RELATED PROTEIN (PTHrP)

Site		Proposed Actions
Mesenchymal Tissues
Cartilage		Promotes proliferation of chondrocytes; inhibits terminal differentiation and apoptosis of chondrocytes
Bone		Stimulates or inhibits bone resorption
Smooth muscle		Released in response to stretching; relaxes smooth muscle
	Vascular system
	Myometrium
	Urinary bladder
Cardiac muscle		Positive chronotropic stimulus; indirect positive inotropic stimulus
Skeletal muscle		Unknown
Epithelial Tissues
Mammary		Induces branching morphogenesis; secreted in milk; possible roles in lactation
Epidermis		Unknown
Hair follicle		Inhibits anagen
Intestine		Unknown
Tooth enamel		Induces osteoclastic resorption of overlying bone
Endocrine Tissues
Parathyroid glands		Stimulates placental transport of calcium?
Pancreatic islets		Stimulates insulin secretion and somatic growth
Pituitary		Unknown
Placenta		Calcium transport?
Central nervous system		Released from cerebellar granular neurons in response to activation of L-type calcium channels; receptors in cerebellum, hippocampus, hypothalamus

From Strewler, 2000.

FIGURE 16-4 Actions of parathyroid hormone–related protein (PTHrP).

(Rosol TJ, et al: In DiBartola SP [ed]: Fluid Therapy in Small Animal Practice, 2nd ed. Philadelphia, WB Saunders, 2000, pp 108-162.)

PTHrP Assay Systems

Two-site IRMA and N-terminal RIAs are available for the measurement of human PTHrP, and these same assay systems have been validated for the dog because of the high degree of sequence homology between species (Brown et al, 1987; Burtis, 1992). PTHrP is susceptible to degradation by serum proteases, therefore these assays should be run from fresh or frozen plasma using EDTA as anticoagulant. The EDTA complexes with plasma calcium, limiting the action of most proteases. The addition of protease inhibitors, such as aprotinin, may provide further protection. Use of serum is not recommended (Rosol et al, 2000).

Clinical Usefulness

Simultaneous measurement of PTH and PTHrP in the plasma may be especially valuable when attempting to distinguish primary hyperparathyroidism (PHPTH) from hypercalcemia of malignancy (HHM) (Burtis et al, 1990). In several studies, the plasma PTHrP concentration was increased in most people with HHM, including those with metastatic disease. PTHrP has been detectable in only a small percentage of humans with primary hyperparathyroidism, this hypercalcemia being attributed to increased PTH concentrations. Impaired renal function is associated with increased immunoreactivity to PTHrP fragments (Budayr et al, 1989; Henderson et al, 1990).

There seems to be little doubt that valid assays for dog and cat PTHrP serve as an excellent tumor marker. In other words, simultaneous assaying for PTH and PTHrP in hypercalcemic dogs and cats has the potential to be a quick, reliable, noninvasive, and inexpensive means of distinguishing between malignant and nonmalignant hypercalcemia. However, animals with renal insufficiency may also have increased concentrations of PTHrP without malignancy (see Fig. 16-2).

DIFFERENTIAL DIAGNOSIS OF HYPERCALCEMIA

See Table 16-3.

TABLE 16-3 DIFFERENTIAL DIAGNOSES OF HYPERCALCEMIA

Primary Hyperparathyroidism

See next section.

Hypercalcemia of Malignancy

GENERAL OVERVIEW.

Malignancy-associated hypercalcemia is the most common cause of increased serum calcium concentrations in dogs and cats. Neoplasms can cause hypercalcemia by three recognized physiologic processes (Fig. 16-5): (1) humoral hypercalcemia of malignancy (HHM); (2) hematologic malignancies growing in the bone marrow; and (3) hypercalcemia induced by metastases of solid tumors to bone (Rosol et al, 2000). Dogs with lymphosarcoma, apocrine gland carcinomas of the anal sac, and multiple myeloma frequently have hypercalcemia. Some of the tumors less commonly associated with hypercalcemia include malignant mammary tumors, nasal adenocarcinomas, thyroid carcinoma, thymoma, squamous cell carcinoma of the gastrointestinal system or vagina, and melanoma.

FIGURE 16-5 Pathogenesis of cancer-associated hypercalcemia. Humoral and local forms of cancer-associated hypercalcemia increase circulating concentrations of calcium by stimulating osteoclastic bone resorption and increased renal tubular resorption of calcium.

(Rosol TJ, et al: In DiBartola SP [ed]: Fluid Therapy in Small Animal Practice, 2nd ed. Philadelphia, WB Saunders, 2000, pp 108-162.)

HUMORAL HYPERCALCEMIA OF MALIGNANCY

Humoral “Factors.”

Tumor tissue at a site distant from bone may synthesize PTHrP, which is released into the circulation, stimulating bone resorption and causing hypercalcemia. Excessive secretion of biologically active PTHrP plays a central role in the pathogenesis of hypercalcemia in most forms of HHM. Cytokines, such as interleukin-1 (IL-1), tumor necrosis factor-α (TNF-α), transforming growth factor-α (TGF-α), transforming growth factor-β (TGF-β), or calcitriol, can have synergistic or cooperative actions with PTHrP (Fig. 16-6) (Rosol et al, 2000). PTHrP binds to the N-terminal PTH-PTHrP receptor in bone and kidney but does not cross-react immunologically with native PTH. The PTHrP stimulates osteoclastic bone resorption, increases renal tubular calcium resorption, and decreases renal tubular phosphate resorption. IL-1 and the transforming growth factors also have the potential to stimulate bone resorption (McCauley et al, 1991; Rosol et al, 2000). Not only does PTHrP provide the physiologic explanation for humoral hypercalcemia, it also explains why some tumors are not associated with hypercalcemia. Furthermore, assays for PTHrP are highly touted as tumor markers in that abnormal concentrations are obtained only from individuals with malignancies and renal failure (see Fig. 16-2).

FIGURE 16-6 Humoral factors such as parathyroid hormone–related protein (PTHrP), interleukin-1, tumor necrosis factors (TNFs), and transforming growth factors (TGFs) produced by tumors induce humoral hypercalcemia of malignancy (HHM) by acting as systemic hormones and stimulating osteoclastic bone resorption or by increasing tubular resorption of calcium.

(Rosol TJ, et al: In DiBartola SP [ed]: Fluid Therapy in Small Animal Practice, 2nd ed. Philadelphia, WB Saunders, 2000, pp 108-162.)

Serum PTH concentrations in dogs with malignancy-associated hypercalcemia are typically low or undetectable. This should be seen as a normal response by the parathyroid glands to hypercalcemia. In an extremely small percentage of these dogs, PTH concentrations may be normal or increased. Obviously such results would be misleading, therefore it is important to evaluate each hypercalcemic dog or cat completely. Assay of PTH and PTHrP concentrations should not be viewed as a replacement for a complete physical examination, thoracic radiography, abdominal ultrasonography, or any other key parameter used to identify neoplasia. These assays complement other necessary components of an evaluation.

Hematopoietic Neoplasia.

Lymphosarcoma is the most common hematopoietic tumor and the most common cause of hypercalcemia in dogs. It afflicts dogs of any age and either sex. Clinical signs may be subtle, moderate, or severe, or these dogs can be terminally ill. Approximately 20% to 40% of dogs with lymphosarcoma are hypercalcemic (Weller et al, 1982; Matus et al, 1986; Rosol et al, 2000). Hypercalcemia has been documented in cats with lymphosarcoma, but the incidence is much lower than in dogs. Studies on affected dogs have demonstrated parameters consistent with influence by PTHrP, including increased fractional excretion of phosphorus, increased nephrogenous cAMP, and increased osteoclastic bone resorption. PTHrP appears to differ from natural PTH in its inability to stimulate renal formation of 1,25 [OH]₂ vitamin D and its lack of cross-reactivity with specific, two-site, intact PTH assays. A significant percentage of these dogs have increased serum concentrations of PTHrP (Fig. 16-7) (Weir et al, 1988a, 1988b, and 1988c).

FIGURE 16-7 Circulating N-terminal parathyroid hormone–related protein (PTHrP) concentrations in normal dogs (control); dogs with hypercalcemia (>12 mg/dl) and anal sac adenocarcinoma (CAC), lymphoma, or miscellaneous tumors (MISC TUMOR); and dogs with normocalcemia (<12 mg/dl) and anal sac adenocarcinoma, lymphoma, or miscellaneous tumors.

(Rosol TJ, et al: In DiBartola SP [ed]: Fluid Therapy in Small Animal Practice, 2nd ed. Philadelphia, WB Saunders, 2000, pp 108-162.)

Lymphomas can synthesize both PTHrP and vitamin D, in which case the dog would not only have the expected increases in serum PTHrP but also would have increased vitamin D concentrations (Seymour and Gagel, 1993). Some lymphocytes contain the 1α-hydroxylase (similar to that found in renal tubules) that converts 25-hydroxyvitamin D to the active metabolite, 1,25-[OH]₂ vitamin D (Rosol et al, 2000). Therefore lymphomas that retain this capability may synthesize excessive amounts of calcitriol, which would increase calcium absorption from the intestinal tract and exacerbate the hypercalcemia caused by PTHrP (see Fig. 16-2).

In contrast to the persistent hypercalcemia observed in dogs with PHPTH, dogs with malignancy-associated hypercalcemia may have fluctuating serum concentrations of calcium and sometimes are normocalcemic. Although the clinical signs associated with lymphoma are variable, it is safe to state that dogs with a combination of lymphoma and HHM, as a group, are considerably more ill than dogs with PHPTH. However, until proven otherwise, lymphoma must remain a possible explanation for hypercalcemia in any dog or cat. Lymphosarcoma may or may not be apparent to the veterinarian during the physical examination. At least 40% of dogs with both lymphoma and HHM do not have peripheral lymph node, liver, spleen, or renal enlargement. Most of these dogs have an anterior mediastinal mass that is usually obvious on thoracic radiographs. The finding of an anterior mediastinal mass in a dog with hypercalcemia should suggest a diagnosis of lymphoma, although other forms of neoplasia (e.g., thymoma) may account for both the mass and the hypercalcemia (Foley et al, 2000).

Apocrine Gland Adenocarcinoma of the Anal Sac.

Adenocarcinomas of the anal sac represent another classic example of a cancer commonly associated with hypercalcemia. Like lymphosarcoma, this neoplasm is known to synthesize PTHrP (Matus and Weir, 1989). This is a well-defined but relatively uncommon tumor, and the diagnosis is usually made in older dogs; the mean age at presentation is 10 to 11 years, but the range is 3 to 17 years (Bennett et al, 2002). Although early reports describe this neoplasia primarily in females, no obvious gender predilection was noted in recent studies. In a review of 238 dogs with this condition, numerous breeds were represented; in another report, mixed breed dogs were most commonly afflicted (Ross et al, 1991; Goldschmidt and Shofer, 1992; Bennett et al, 2002). Clinical signs associated with this condition include (among many owner observations): recognition of a mass near the rectum, tenesmus, poor appetite or anorexia, polyuria/polydipsia, and lethargy.

In dogs with polyuria/polydipsia, hypercalcemia was usually present. However, not all hypercalcemic dogs had evidence of polyuria/polydipsia (Bennett et al, 2002). The incidence of hypercalcemia has varied in different studies from about 25% of afflicted dogs to about 50% (Ross et al, 1991; Bennett et al, 2002). In one study, the serum calcium concentrations (upper reference value of 12.6 mg/dl) at the time of diagnosis ranged from 12.7 to 21.7 mg/dl (Bennett et al, 2002). Tumor resection results in normocalcemia. Local reappearance of the cancer or metastasis causes recurrence of hypercalcemia. Dogs afflicted with this form of cancer have increases in urinary cAMP and fractional phosphorus excretion. Serum PTH concentrations have been suppressed, and bone histomorphometry has revealed increased bone resorption with no compensatory increase in formation. These observations are consistent with the biologic actions of PTHrP (Meuten et al, 1983b). Apocrine gland adenocarcinoma cell lines established in mice have also demonstrated that of the potential factors responsible for HHM, only PTHrP is increased in the serum (Grone et al, 1998). Successful treatment of this neoplasia results in rapid normalization of serum calcium, phosphorus, calcitriol, and PTH concentrations (Rosol et al, 1992b).

With recognition of hypercalcemia in any dog, a rectal examination and careful palpation of the anal sac areas should be routine. In these dogs, the rectal examination commonly demonstrates a space-occupying mass that may be invasive and occasionally ulcerated. Careful digital rectal palpation is necessary to identify the presence of sublumbar lymph node enlargement. Although radiography may also be used to evaluate the sublumbar area, abdominal ultrasonography has been our most sensitive diagnostic aid. Radiography of the thorax and abdomen can be used in searching for pulmonary metastases and/or bony metastases (lytic areas).

Other Nonneoplastic and Solid Tumors that May Synthesize PTHrP.

Interstitial cell tumors of the testicle, squamous cell carcinoma, thymoma, melanoma, and thyroid adenocarcinoma are less commonly encountered solid tumors that cause hypercalcemia without bone metastasis (Grain and Walder, 1982; Meuten et al, 1983a; Pressler et al, 2002). This underscores the value of PTHrP assays in dogs with hypercalcemia. The remission of hypercalcemia after tumor excision and the return of hypercalcemia after tumor recurrence or growth of metastases support the presence of a hypercalcemic factor (PTHrP) secreted by these tumors, but studies are limited. In humans, other tumors that may produce PTHrP include bronchogenic non–small-cell carcinomas, breast cancer, squamous cell carcinoma of the esophagus, renal-cell carcinoma, and hepatoma (Harris et al, 2002). PTHrP has also been associated with hypercalcemia in nonneoplastic diseases, such as schistosomiasis (Fradkin et al, 2001).

HEMATOLOGIC MALIGNANCIES GROWING IN THE BONE MARROW: OSTEOLYTIC HYPERCALCEMIA

Background.

Some types of hematologic malignancies in the bone marrow produce hypercalcemia by inducing bone resorption locally (Rosol et al, 2000). This condition is usually associated with lymphoma and multiple myeloma. In humans, metastatic breast cancer is another example, although this association is not common in dogs or cats. A number of paracrine factors, or cytokines, may be responsible for the stimulation of bone resorption in dogs with such tumors. The cytokines most often implicated in the pathogenesis of local bone resorption are IL-1 and IL-6, TNF-α and TNF-β, TGF-α and TGF-β, and PTHrP (Black and Mundy, 1994). Production of small amounts of PTHrP by a tumor in bone may stimulate local bone resorption without inducing a systemic response. Prostaglandins (especially prostaglandin E₂) may also contribute to local stimulation of bone resorption (Rosol et al, 2000). Together, these cytokines and prostaglandins comprise the osteoclast-activating factors.

Multiple Myeloma.

Multiple myeloma is a tumor of the B-lymphocyte or plasma cell line that may be associated with the development of osteolytic bone lesions and, occasionally, hypercalcemia. The hypercalcemia develops secondary to production of osteoclast-activating factors. In addition, there is a correlation between the extent of bone destruction, the amount of tumor cell burden, and the amount of osteoclast-activating factor produced by myeloma cell cultures (Durie et al, 1981). Approximately 17% of dogs afflicted with this cancer are hypercalcemic (Matus et al, 1986), and 50% of dogs with multiple myeloma have radiographic evidence of bone lysis.

Bone pain is usually associated with the lytic areas. The initial database from these dogs often reveals abnormal increases in the total serum protein concentration as a result of a monoclonal spike in the globulin fraction. This monoclonal gammopathy can be demonstrated specifically with serum protein electrophoresis. Bone marrow aspiration confirms the diagnosis in most cases, demonstrating an infiltration of malignant plasma cells. Analysis of urine for light chains of myeloma protein (Bence Jones protein) has not been of much value.

HYPERCALCEMIA INDUCED BY METASTASES OF SOLID TUMORS TO BONE.

Certain malignant neoplasms with osseous metastasis may cause hypercalcemia and hypercalciuria. Primary bone tumors, by contrast, do not commonly induce hypercalcemia. For example, hypercalcemia would be rare in a dog with osteosarcoma. In contrast, malignant mammary adenocarcinoma or squamous cell carcinoma is infrequently associated with both bone metastasis and hypercalcemia. Several different types of cells may be involved in the actual destruction of bone at sites of metastasis, including osteoclasts, tumor cells, lymphocytes, and monocytes. Lymphocytes and monocytes may accumulate as part of the cell-mediated immune response to a tumor (Mundy et al, 1984). Osteolysis is a result of the physical disruption of bone by proliferating neoplastic cells, but it also can be caused by secretion of cytokines or prostaglandins that stimulate local bone resorption (Garrett, 1993).

Epithelial tumors, especially squamous cell carcinomas, are the most likely neoplasms to metastasize to bone in dogs and cats (Quigley and Leedale, 1983). Common metastatic bone sites in the dog include the humerus, femur, and vertebrae, whereas in the cat, local invasion of bone rather than distant metastasis is more common. Although reported, these tumors are not commonly associated with hypercalcemia (Grain and Walder, 1982).

Hypervitaminosis D and Rodenticide Toxicosis

BACKGROUND.

Vitamin D is cumulative in its action and may require 1 to 2 weeks before its maximum effects on mineral metabolism occur. However, acute toxicity after massive ingestion of cholecalciferol appears to be the more common experience when dealing with this problem in dogs. Hypercalcemia and hyperphosphatemia are the anticipated electrolyte abnormalities in animals with vitamin D toxicity, although normophosphatemia and transient periods of normocalcemia have been reported (Figs. 16-8 and 16-9) (Harrington and Page, 1983). Hypercalcemia begins as soon as 12 to 18 hours after ingestion, and peak concentrations are usually demonstrated by 48 to 72 hours, coinciding with increases in the BUN and creatinine (Rumbeiha, 2000). Increased resorption of bone, coupled with increased gastrointestinal absorption of calcium and phosphorus, is responsible for these abnormalities. Skeletal disease is usually not detectable radiographically, probably because of the acute nature of this form of toxicosis. The osteoclastic phase of bone resorption occurs early and is followed by osteoid deposition and hyperosteoidosis (Boyce and Weisbrode, 1983). Extensive soft tissue mineralization of the endocardium, blood vessels, tendons, kidney, and lung is frequently associated with vitamin D toxicity (Meuten, 1984).

RODENTICIDE TOXICOSIS.

Hypercalcemia that develops secondary to cholecalciferol rodenticide toxicosis in dogs and cats is a recognized concern (Gunther et al, 1988; Moore et al, 1988; Fooshee and Forrester, 1990; Dougherty et al, 1990). Rat bait products containing cholecalciferol include Quintox, Rampage, and Muritan, among other proprietary names (Rumbeiha, 2000; Morrow, 2001). Dogs studied after being given this type of poison became weak, lethargic, and anorexic within 48 hours. Within 60 to 70 hours of consumption, all dogs became recumbent, exhibited hematemesis, and progressed into shock before dying or being euthanized (Gunther et al, 1988). Although the median lethal dose of cholecalciferol in dogs is widely reported to be 43 to 88 mg/kg, studies have shown that as little as 10 mg/kg given once orally can be lethal. Dogs that ingest as little as 4 to 6 mg/kg once become ill. Clinically healthy dogs that ingest single doses of 2 mg/kg develop hypercalcemia (Rumbeiha, 2000).

Most dogs and cats exposed to these toxins have had rapid increases in the serum calcium and phosphate concentrations (see Fig. 16-8). Diffuse hemorrhage in the stomach and small intestine was obvious. Histologic lesions consisting of hemorrhage or mineralization or both were identified in the gastrointestinal tract, kidneys, and myocardium and in the blood vessels of many organs (Gunther et al, 1988). The incidence of acute and/or severe renal failure was variable. Three exposed cats survived (Moore et al, 1988).

FIGURE 16-8 Serum calcium (A) and phosphorus (B) concentrations of dogs given vitamin D₃ at a dosage of 10 mg/kg (circles) and 20 mg/kg (triangles).

(Gunther R, et al: J Am Vet Med Assoc 193:211, 1988.)

OTHER CAUSES OF VITAMIN D TOXICOSIS.

Since 1997, perhaps the most common accidental cause of vitamin D toxicosis in pets has been ingestion of human psoriasis medications containing the vitamin D analogs calcipotriol or calcipotriene, and marketed as Davionex, Dovonex, or Psorcutan. Additional sources of vitamin D are dietary supplementation and overzealous administration of vitamin D by veterinarians to dogs or cats with hypoparathyroidism. This has become recognized as a problem in hyperthyroid cats as a result of the frequent use of surgery and the potential for iatrogenic disease. Removal of parathyroids is a concern during any major surgery involving the neck, especially when thyroid tissue is resected. Cestrum diurnum (day-blooming jessamine) is a popular houseplant that should be considered a possible source of vitamin D toxicity in pets because it contains the active metabolite of vitamin D. Jasmine, an indoor climbing plant without active vitamin D metabolites, should not be confused with day-blooming jessamine. Other plants containing glycosides of vitamin D include Solanum malacoxylon and Trisetum flavescens.

DIAGNOSIS.

The diagnosis of vitamin D toxicosis is based on a history of exposure and the presence of dark, bloody feces, azotemia, oliguria or polyuria, proteinuria, and sometimes glucosuria. An additional clue to this diagnosis would be hyperphosphatemia in a hypercalcemic dog or cat. Most other causes of hypercalcemia are associated with hypophosphatemia or normal serum phosphate concentrations. Cholecalciferol-induced renal failure can be differentiated from ethylene glycol toxicosis or soluble oxalate toxicosis because those two conditions usually cause moderate hypocalcemia, as opposed to the severe hypercalcemia often associated with vitamin D toxicity. However, the history and signs of vitamin D toxicosis are strikingly similar to those seen in dogs with hypoadrenocorticism, acute renal failure, and chronic renal failure.

Hypoadrenocorticism

Hypoadrenocorticism (Addison’s disease) is one of the more common causes of hypercalcemia in dogs, accounting for approximately 10% to 50% of cases (Uehlinger et al, 1998; Rosol et al, 2000). Serum calcium concentrations have been reported to be increased in as many as 33% of dogs with adrenocortical insufficiency (hypoadrenocorticism) (Peterson and Feinman, 1982; Peterson et al, 1996) as well as in a smaller percentage hypoadrenal cats (Johnessee et al, 1982; Peterson et al, 1989). In our series of hypoadrenal dogs, a correlation has been noted between the degree of hyperkalemia and the level of hypercalcemia (see Chapter 8). If the serum potassium concentration exceeds 6.0 to 6.5 mEq/L, a large percentage of these animals have serum calcium concentrations of 12 to 13.5 mg/dl. The hypercalcemia is not restricted to the extremely ill hypoadrenal dog. It is not common, however, for the serum calcium concentration to exceed 13.5 mg/dl, and it rarely exceeds 15 to 16 mg/dl. Despite the increased total serum calcium concentrations, serum ionized calcium levels usually remain in the reference range. Serum phosphate concentrations also correlate with serum calcium concentrations, with the hyperphosphatemic animal more likely to exhibit hypercalcemia (see Fig. 16-9).

FIGURE 16-9 The range in serum calcium and phosphorus concentrations for the more common causes of hypercalcemia and/or hyperparathyroidism in the dog. HP, Hyperparathyroidism; 2° HP, secondary hyperparathyroidism.

Clinical signs and laboratory abnormalities associated with hypoaldosteronism (a primary component of Addison’s disease) usually are striking and overshadow concerns related to hypercalcemia (see Chapter 8). Most dogs and cats with hypoadrenocorticism have hyperkalemia, hyponatremia, azotemia, hyperphosphatemia, and related problems. The only differential diagnoses for this combination of serum abnormalities are hypoadrenocorticism, renal failure, and vitamin D (rodenticide) toxicosis. Furthermore, the hypercalcemia rapidly resolves after treatment for the adrenal insufficiency, frequently within a few hours of initiation of saline resuscitation.

The pathogenesis of the hypercalcemia associated with hypoadrenocorticism is probably multifactorial. Any combination of the following may be involved: (1) hyperproteinemia resulting from dehydration and hemoconcentration; (2) increased plasma protein– binding affinity for calcium; (3) increased concentrations of calcium-citrate complexes; and (4) increased renal tubular resorption of calcium (Peterson and Feinman, 1982).

Chronic Renal Failure

PATHOGENESIS OF HYPERCALCEMIA.

The pathogenesis of hypercalcemia associated with chronic renal failure is complicated. Diffuse hyperplasia of the parathyroid glands is characteristic of chronic renal failure. Parathyroidectomy decreases the serum calcium and PTH concentrations in dogs with hypercalcemia and renal failure (Meuten and Armstrong, 1989). The development of hypercalcemia in experimentally nephrectomized dogs depends on the presence of the thyroparathyroid complex (Tuma and Mallette, 1983). The kidneys excrete PTH and its metabolic byproducts, a process impeded by renal failure. Additional possible mechanisms for hypercalcemia in renal failure include (1) decreased renal excretion of calcium due to a reduction in the GFR; (2) increased concentrations of PTH due to both excessive secretion and reduced renal tubular hormone degradation, which could enhance bone resorption; (3) renal failure or PTH-induced increased concentrations of organic cations (e.g., citrates), which would increase complexed calcium concentrations; (4) an exaggerated response to vitamin D, with increased intestinal absorption of calcium; and (5) autonomous parathyroid gland activity (tertiary hyperparathyroidism), causing enhanced secretion rates (Meuten and Armstrong, 1989). The actual presence or incidence of a syndrome involving “autonomously functioning” parathyroid glands that are the result of chronic stimulation due to the renal disease (i.e., tertiary hyperparathyroidism) is not known. Tertiary hyperparathyroidism is the name given to the syndrome of chronic renal secondary hyperparathyroidism, in which one or more of the parathyroid glands begin to autonomously secrete PTH (“tertiary” disease).

Transient hypercalcemia may develop after administration of oral phosphate-binding agents, presumably as a result of shifts in calcium from bone to extracellular fluid in response to the rapid lowering of the circulating phosphorus concentration (Capen and Martin, 1983).

< div class='tao-gold-member'>

Only gold members can continue reading. Log In or Register to continue