Diseases of the Endocrine System

Chapter 3 Diseases of the Endocrine System


The cells of the body must maintain homeostasis (equilibrium) with their internal environment because the chemical processes conducted at the cellular level can occur only under specific conditions. The endocrine system works together with the nervous system to achieve a stable internal environment; each system may work independently or the systems may act together to accomplish this task. The hypothalamus and the pituitary gland regulate the release of chemical messengers called hormones (chemicals secreted directly into the bloodstream) to signal target cells to perform certain functions in response to changes in homeostasis. As hormone concentrations in the blood increase, the system is signaled to reduce hormone production; this negative feedback system works much the same way as a thermostat controls the heating system in your home.


Ideally, the glands that regulate the release and balance of hormones work continuously to maintain homeostasis. When one or more of these glands work ineffectively, incorrectly, or not at all, the potential for problems develops. The endocrine glands most commonly involved in diseases of small animals are the thyroid gland, adrenal gland, pancreas, parathyroid gland, and gonads (testes and ovaries) (Fig. 3-1).


image

Figure 3-1 Locations of major endocrine glands in the cat.


(From McBride DF: Learning veterinary terminology, ed 2, St Louis, 2002, Mosby, by permission.)


Diseases that affect the pituitary gland or the hypothalamus cause multiple irregularities in the body; they are discussed in this chapter only in the context of their relation to other syndromes.



THYROID GLAND


The thyroid gland is located in the ventral cervical region along the lateral margins of the trachea. The gland is not usually palpable in a healthy animal. The gland is composed of follicles that produce the thyroid hormones triiodothyronine (T3) and tetraiodothyronine (T4). These hormones are produced by the follicular cells and stored in the gland until they are needed by the body. A third hormone, calcitonin, is produced by the parafollicular cells in the thyroid gland. Calcitonin acts to increase calcium deposition within bone to decrease blood calcium concentration, whereas T3 and T4 function to control all of the body’s metabolic processes.


Two common diseases of small animals involve the thyroid gland (Fig. 3-2): hypothyroidism (insufficient thyroid hormone) and hyperthyroidism (excessive thyroid hormone). Hypothyroidism is most commonly seen in the canine (1/156-500 animals). Hyperthyroidism is seen primarily in the cat.


image

Figure 3-2 Canine thyroid and parathyroid glands.


(From Christenson DE: Veterinary medical terminology, Philadelphia, 1997, WB Saunders, by permission.)



Hypothyroid Disease


Primary acquired hypothyroidism is the most common type of hypothyroid disease seen in the dog and usually follows thyroid atrophy or lymphocytic thyroiditis. Central acquired hypothyroidism is rare and develops when the pituitary or hypothalamus is diseased.



CLINICAL SIGNS



• Breeds predisposed to this disease: golden retrievers, Dobermans, Irish setters, schnauzers, cocker spaniels, and dachshunds

• Common in middle-aged dogs (4-10 years of age); female/male ratio is 2.5:1

• Weight gain with no change in diet

• Bilaterally symmetric alopecia; “rat tail” (loss of hair on the tail)



TECH ALERT


Hypothyroid disease and Cushing’s disease are the only known diseases that produce this alopecia pattern:


• Dry hair or excessive shedding

• Lethargy

• Anestrus (infertility, testicular atrophy)

• Hyperpigmentation of the skin

• Cold intolerance

• Anemia

• Hypercholesterolemia


DIAGNOSIS



• 66% to 75% of animals will have increased cholesterol level.

• 25% to 40% of animals have a mild, nonregenerative anemia.

• Perform radioimmunoassay for T3 and T4; normal T4 concentration is 1 to 4 μg/dl.

• Thyroid-stimulating hormone (TSH) tests are expensive and often unavailable. Recommended doses of TSH for this test are 0.396 U/kg body weight intravenously (IV), 0.099 U/kg intramuscularly (IM), or 1 unit IV to 10 units IV, IM, or subcutaneously (SQ). Take blood sample at 6 hours. The normal response shows an increase of T4 concentration greater than 2 μg/dl above baseline concentration.

• Measure thyroid hormone antibody concentration.


TREATMENT



• Life-long supplementation with thyroid replacement hormone

• Dogs: Initially use trade name products at a dose of 0.1 mg/4.5 kg twice daily; reevaluate in 4 to 8 weeks for clinical response and adjust the dose as needed; some dogs may need only one dose daily

• Cats: 0.05 to 0.1 mg daily; reevaluate in 4 to 6 weeks and adjust the dose


Products



• Levothyroxine sodium tablets: 0.1-, 0.2-, 0.3-, 0.4-, 0.5-, 0.6-, 0.7-, 0.8-mg sizes

• Animal-approved levothyroxine sodium products: Soloxine (Daniels), Thyro-Tabs (Vet-A-Mix), Thyro-Form (Vet-A-Mix) chewable tablets

• Human-approved levothyroxine sodium products: Synthroid (Knoll Pharmaceuticals), 0.025 to 0.3 mg; Levo-T (Lederle), Levothroid (Forest Laboratories)

• Thyroxyl: oral solution of levothyroxine sodium; allows for easy dosing at any level


INFORMATION FOR CLIENTS



• Oral supplementation will be required for the life of the animal.

• Daily dosing is required to maintain normal concentration of thyroid hormone.

• Excess medication can produce signs of hyperthyroidism: increased thirst and urination, nervousness, weight loss, excessive panting, weakness, and increased appetite. If any of these signs are noticed, stop the medication and call your veterinarian.

• Follow-up blood tests may be required to ensure adequate hormone concentration.

• Certain drugs may decrease thyroid hormone concentration. Inform your veterinarian if your animal is currently taking any of these medications: cortisone, aspirin, flunixin (Banamine), furosemide (Lasix), or phenobarbital.


Hyperthyroid Disease


Hyperthyroidism is the most commonly seen endocrine disorder in cats; however, it is rarely seen in the dog except as a result of neoplasia. The disease was first documented in the late 1970s to early 1980s, with incidence increasing steadily. The excess of both T3 and T4 results in a multisystemic disease seen primarily in older cats. Bilateral thyroid gland enlargement occurs in approximately 70% of cases and is the result of a functional thyroid adenoma. Thyroid carcinoma is rarely seen in the cat, comprising only 1% to 2% of all cases of hyperthyroidism.



CLINICAL SIGNS



• Middle-aged to older cat

• Weight loss

• Polyphagia

• Vomiting

• Increased appetite

• Tachycardia with or without murmurs

• Aggressive behavior

• Palpable enlarged thyroid

• Increased systolic blood pressure

• Blindness with retinal detachment


DIAGNOSIS



• Check for palpable enlarged thyroid.

• Test for increased serum T4 concentration.

• Perform serum chemistries to rule out other organ system failures that may be present. Increased alanine aminotransferase (ALT), alkaline phosphatase (ALP), and/or LDH levels are usually seen; increased blood urea nitrogen (BUN) and creatinine level may be seen.

• Packed cell volume is often in the high to normal range, stress leukogram.

• Check for abnormal thyroid radionuclide uptake imaging using either iodine-131 (131I) or 99mTc.


TREATMENT



• Control of excessive excretion of thyroid hormones can be accomplished by three methods. Choice of treatment depends on the severity of the disease, the age and physical status of the cat, and facilities available.


Surgery



• Removal of the thyroid gland provides a cure for the disease, but it is important to preserve the parathyroid glands when removing thyroid tissue.


Radioactive iodine-131



• Radioactive 131I is the treatment of choice. Diseased tissue in the thyroid gland will take up larger amounts of radioactive 131I than normal tissue. This uptake results in the destruction of the tissue, reducing thyroid hormone concentration. Cats should remain hospitalized until their wastes (especially urine) are below hazardous radiation levels (1-3 weeks).


Antithyroid drug therapy



• Antithyroid drugs inhibit the synthesis of thyroid hormone by disrupting the incorporation of iodine.

• Methimazole (Tapazole): Initially administer daily, adjusting the dose every 2 to 3 weeks (monitor T4 concentration). Gradually increase the dose until the desired effect is achieved.



TECH ALERT


Side effects include anorexia, vomiting, lethargy, and facial excoriation.


Calcium ipodate: administer daily orally (PO)



INFORMATION FOR CLIENTS



• Surgery or radioactive iodine is the only cure for hyperthyroid disease.

• The cause of this disease is unknown.

• Medical management produces side effects in many cats. Report negative effects to your veterinarian.

• Treatment to decrease thyroid hormone concentration may unmask concurrent diseases such as renal failure. This renal disease may be life threatening if not recognized.

• Concurrent diseases may need to be corrected before surgery.

• Bilateral removal of the thyroid glands may result in hypothyroidism, which will require daily treatment.

• All animals with hyperthyroid disease should have their blood pressure checked routinely. If Tapazole does not reduce blood pressure, then other antihypertensive agents should be added.


PANCREAS



Diabetes Mellitus


Cells need glucose as a fuel. Through the processes of glycolysis, the citric acid cycle, electron transport, and oxidative phosphorylation, glucose is chemically converted into energy in the form of adenosine triphosphate, carbon dioxide, and water. It is therefore important for the body to regulate the concentration of glucose in circulation. Levels must be kept within certain limits to assure that adequate fuel is always available for energy production. The endocrine pancreas aids in this regulation; β cells in the pancreatic islets (islets of Langerhans) produce the hormone insulin, which facilitates the entry of glucose into the cell for the process of glycolysis. Diabetes mellitus results when these β cells stop producing insulin in adequate amounts or when the cells in specific body tissues become resistant to the action of insulin. The incidence of diabetes mellitus in the dog and cat is reported to be between 1 in 100 and 1 in 500. The cause of the disease is unknown, although chronic pancreatitis, immune-mediated disease, and hereditary predisposition have been suggested as possible causes.


Almost 100% of dogs and about 50% of cats will have insulin-dependent diabetes (type I) at examination. As many as 50% of presenting cats will have non–insulin-dependent diabetes (type II), which does not require insulin therapy.


Therapy for diabetes mellitus in most animals includes dietary regulation (usually a high-fiber diet) and daily insulin replacement. In non–insulin- dependent cats, drug therapy and diet restriction are somewhat successful in managing the disease.


The type of insulin chosen for therapy depends on the severity of the disease and animal needs. Treatment should be tailored to the species of animal involved. (Feline insulin most resembles beef insulin, and canine insulin resembles pork and human insulin.) Although it would be ideal to match the structure of each species’ insulin, it is not easy to do this. Beef and pork insulin and their combinations have virtually disappeared from the market since the late 1990s; Vetsulin has taken their place as the most available insulin.


Animals with diabetes whose sugar concentration remains uncontrolled may become ketotic. Cells begin to use fat as fuel for energy production, yielding ketone bodies that accumulate in the blood. Acidosis, dehydration, and electrolyte imbalances can occur as a result of ketosis.



CLINICAL SIGNS



Nonketotic diabetes



• Dogs: 4 to 14 years of age, female dogs are twice as likely to be affected; Cats: all ages, with neutered male cats most affected

• Breeds predisposed to the disease: poodles, schnauzers, keeshonds, cairn terriers, dachshunds, cocker spaniels, and beagles

• Polyuria (Pu)/polydipsia (Pd)

• Weight loss (especially in cats)

• Polyphagia

• Sudden cataract formation

• Dehydration

• Plantigrade posture in cats (walking on their hocks)


Ketotic diabetes


Clinical signs include all of the preceding symptoms plus the following:


• Depression

• Weakness

• Tachypnea

• Vomiting

• Odor of acetone on the breath


DIAGNOSIS



• Evaluate clinical signs present in animal.

• Observe for documented fasting blood glucose concentration >200 mg/dl.

• Test urine for glycosuria.

• Complete serum chemistry to rule out other concurrent disease.


TREATMENT



Dietary management



• Restrict animal to a diet high in fiber and complex carbohydrates, such as Prescription Diet R/D or W/D (Hill’s), Fit and Trim (Purina), or Science Diet Light (Hill’s). This type of diet helps to avoid postprandial increases of glucose concentration and allows for better regulation of blood glucose concentration.


Insulin Therapy



• Two types of insulin are available: human (Humulin, Eli Lilly and Company) and beef/pork combination (Iletin I, Eli Lilly and Company). Most treatment regimens involve the use of neutral protamine Hagedorn (NPH, intermediate-acting) or Ultralente (long-acting) insulins.



TECH ALERT


These forms of insulin recently have been removed from the market by Eli Lilly and Company. Animals that must be switched to another form of insulin should be monitored carefully. It is recommended that doses be reduced when switching until the animal’s glucose levels stabilize.



• Oral therapy: Glipizide can be used in cats that do not require insulin to control their blood glucose concentration (type II). The exact mechanism of action is unknown; however, it is believed that the sulfonylureas may stimulate the existing β cells to secrete more insulin. Animals may become resistant to this therapy and need insulin.

• Recommended therapy doses:
Dogs: initially NPH insulin daily; can be increased to twice daily if necessary.

Cats: ultralente insulin (Humulin, Eli Lilly and Company) daily or beef/pork lente insulin (Iletin, Eli Lilly and Company) twice daily

• Insulin therapy should be monitored using improvement in clinical signs as a guide. Blood glucose concentration can be checked weekly to assist in adjusting the insulin dose. Animals whose glucose levels do not regulate will require a glucose curve to adjust their insulin therapy.

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Aug 31, 2016 | Posted by in GENERAL | Comments Off on Diseases of the Endocrine System

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