T

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Taurine Blood Levels



DEFINITION


Amino acid with important role in retinal and myocardial metabolism



TYPICAL NORMAL RANGE



• Cats: <20 nmol/mL (plasma) is considered diagnostic of cardiomyopathy; cats with <60 nmol/mL should receive supplementation.

• Dogs: <40 nmol/mL (plasma) is considered low and has been associated with dilated cardiomyopathy in the cocker spaniel, golden retriever, and Newfoundland breeds (see p. 1075).

• Whole blood concentrations (and the whole blood reference ranges) are approximately double the plasma concentrations.


PHYSIOLOGY


Low blood levels of taurine have been associated with dilated cardiomyopathy in cats and certain breeds of dogs, such as cocker spaniels, retrievers, and many other breeds. Determination of plasma and/or whole blood levels of taurine may be useful in determining role in dilated cardiomyopathy and need for supplementation. Taurine-deficient cats (but not dogs) may develop degenerative retinal lesions.



CLINICAL APPLICATIONS


CAUSES OF ABNORMALLY LOW LEVELS: Dietary deficiency, poor dietary bioavailability, excessive loss of metabolic precursors (cysteine), and/or familial predisposition


NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS LOW: Correlate with cardiac and/or ophthalmic evaluation findings.


IMPORTANT INTERSPECIES DIFFERENCES: Fasting may decrease plasma levels of taurine in feline samples; therefore, whole blood is the preferred sample for cats. Ideally, both whole blood and plasma taurine levels should be determined for dogs.



SPECIMENS AND PROCESSING CONSIDERATIONS


LAB ARTIFACTS THAT MAY INTERFERE: Bacterial contamination may falsely decrease blood taurine levels. Delay in separating and chilling plasma or poor blood collection technique may result in falsely increased levels of plasma taurine, because taurine is released from leukocytes and activated platelets.


SPECIMEN: Collect 2 ml fresh heparinized (green-top tube) whole blood and freeze; freezing heparinized whole blood lyses RBCs and releases taurine (desirable). Alternatively, collect 1 mL of frozen heparinized plasma. Chill sodium heparin collection tube (green-top tube) and separate plasma immediately after collection in a refrigerated centrifuge. After centrifugation, transfer plasma—without disturbing the buffy coat—to red-top tube (without serum separator plug) for submission.


RELATIVE COST: $$$



PEARLS



• Dogs do not need to be eating a taurine-deficient diet to develop taurine-deficient dilated cardiomyopathy; poor bioavailability of nutritional taurine (e.g., binding by rice husks) and excess urinary excretion of taurine precursors (e.g., cystine) have been proposed as mechanisms. Reference intervals in dogs are not well established. They are suspected to be significantly higher than those in cats. Clinically normal dogs have been reported to have 77 + 2.1 (plasma) and 266 + 5.1 (whole blood) nmol/mL taurine.

• Oral taurine supplementation is inexpensive and safe.

• Low taurine levels may be used to confirm a diagnosis of cardiomyopathy but are generally not used as a screening test.

AUTHOR: SHERRY J. MORGAN



Testosterone



DEFINITION


Hormone responsible for development of male characteristics



TYPICAL NORMAL RANGE


Usually, serum levels > 1 ng/mL indicate an intact male.



PHYSIOLOGY


Derived from cholesterol, primarily produced by testicular interstitial cells; ovarian theca cells, zona reticularis of the adrenal cortex, and placenta produce lesser amounts. Hematogenous distribution to target tissue; affects bone growth and muscle mass, development of spermatogenic tissue in testes, and behavior. Extratesticular production accounts for sexual behavior in castrated males or animals with ovarian or adrenal tumors.



CLINICAL APPLICATIONS


CAUSES OF ABNORMALLY HIGH LEVELS: Cryptorchid or intersex patients may have concentrations consistent with intact male.


NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS HIGH: Investigate for presence of abdominal testicle(s) or abnormal gonadal tissue.


CAUSES OF ABNORMALLY LOW LEVELS: Infertility


NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS LOW: Resample because cyclically low levels may occur in fertile patients; pulses of testosterone may occur throughout the day. Increased concentration over basal levels following administration of human chorionic gonadotropin (hCG) or gonadotropin-releasing hormone (GnRH) indicates functional testicular tissue.


IMPORTANT INTERSPECIES DIFFERENCES: Male cats have pulsatile secretion of testosterone and may have undetectable levels. Therefore, evaluation should be done following intramuscular injection of either GnRH or hCG as follows:


• 25 μg GnRH IM, then draw sample 1 hour later (reference interval: 17.3-41.6 nmol/L) or

• 250 IU hCG IM, then draw sample 1 hour later (reference interval: 10.4-31.2 nmol/L)


SPECIMEN AND PROCESSING CONSIDERATIONS


DRUG EFFECTS ON LEVELS: False increases due to administration of androgenic drugs such as methyltestosterone; administration of radioisotopes falsely increases values measured by radioimmunoassay; chronic administration of androgenic drugs suppresses natural production.


LAB ARTIFACTS THAT MAY INTERFERE: Delayed separation from red blood cells, collection in EDTA (lavender-top tube), and storage at room temperature falsely decrease values.


SPECIMEN: Heparinized (green-top tube) plasma or serum; separate plasma or serum from red blood cells as soon as possible; store specimen at 2○C-8○C.


RELATIVE COST: $$


PEARLS:


• Main indications are assessment of infertile male patients and to help determine whether a male without external testicles is cryptorchid or has been neutered (see p. 268).

• Presence or absence of penile spines in adult male cats helps support or refute testosterone exposure, respectively.

AUTHOR: LOIS ROTH-JOHNSON



Thyroglobulin Antibody



DEFINITION


Autoantibodies against thyroglobulin, the major storage form for thyroxine and tri-iodothyronine in the thyroid follicle



SYNONYM


TgAA



TYPICAL NORMAL RANGE


Results reported as negative, positive, or inconclusive.



PHYSIOLOGY


The presence of thyroglobulin antibody (TgAA) is an indicator of immune-mediated thyroiditis and homozygous carrier state for the disease.


CAUSES OF ABNORMALLY HIGH LEVELS: Immune-mediated thyroiditis; low-grade false-positive results may occur with routine vaccinations in the previous 30-40 days.



CLINICAL APPLICATIONS


NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS HIGH: Free T4 (equilibrium dialysis). Repeat TgAA test in 2-4 months.



SPECIMEN AND PROCESSING CONSIDERATIONS


DRUG EFFECTS ON LEVELS: Thyroid supplement associated with false-negative test. Treatment should be discontinued for at least 90 days prior to testing.


SPECIMEN: Serum (red-top tube) or EDTA plasma (lavender-top tube). Separate serum or plasma from red blood cells as soon as possible. Store at 2○C-8○C (refrigeration).


RELATIVE COST: $$



PEARLS


Dogs that test positive are not recommended for breeding, owing to the hereditary nature of immune-mediated thyroiditis and resultant hypothyroidism. Many dogs may develop TgAA prior to clinically evident immune-mediated thyroiditis and hypothyroidism. Carriers (heterozygotes) are negative. Dogs with chronic disease with thyroid destruction may have negative results. Occasionally dogs may have TgAA without having hypothyroidism; TgAA should always be interpreted in conjunction with other thyroid hormone parameters.


AUTHOR: LOIS ROTH-JOHNSON


1ST EDITION AUTHOR: FONZIE QUANCE-FITCH



Thyroid-Stimulating Hormone



DEFINITION


Pituitary-derived hormone that acts on the thyroid gland to produce and secrete thyroid hormones



SYNONYMS


Canine TSH (cTSH), thyrotropin, TSH



TYPICAL NORMAL RANGE


Dogs: 0.1-0.5 ng/mL (μg/dL × 10 = ng/mL). Be sure test method has been validated for dogs.



PHYSIOLOGY


TSH is released from the pituitary from stimulation by thyrotropin-releasing hormone (TRH), and it acts on the thyroid gland to stimulate synthesis and release of thyroxine (T4) and triiodothyronine (T3). T3 and T4 act as negative feedback to reduce TSH and TRH secretion.


CAUSES OF ABNORMALLY HIGH LEVELS: Hypothyroidism (primary), thyroxine supplementation


NEXT DIAGNOSTIC STEP TO CONSIDER IF LEVELS HIGH: Free T4 by equilibrium dialysis (free T4 ED) and total T4


CAUSES OF ABNORMALLY LOW LEVELS: Sensitivity of the test is not sufficient to differentiate normal from low concentrations.


IMPORTANT INTERSPECIES DIFFERENCES: Species-specific immunoassay required. No valid measurement of feline TSH available; there may be some cross-reactivity with the canine TSH assay. Canine test may detect elevated levels in cats but can not document decreased levels.



SPECIMEN AND PROCESSING CONSIDERATIONS


SPECIMEN: Serum (red-top tube) or EDTA plasma (lavender-top tube). Separate serum or plasma from red blood cells as soon as possible. Store at 2○C-8○C (refrigeration).


RELATIVE COST: $$



PEARLS



• Canine patients with low free T4 (ED) and increased cTSH concentrations are extremely likely to have hypothyroidism.

• May increase ability to assess thyroid function when evaluated in conjunction with thyroid hormone concentrations

AUTHOR: LOIS ROTH-JOHNSON


1ST EDITION AUTHOR: FONZIE QUANCE-FITCH



Thyroxine (Total), Free Thyroxine by Equilibrium Dialysis



DEFINITION


Thyroxine (T4 is the major storage form of iodine-containing thyroid hormone. Its primary function is to increase the rate of cell metabolism. Free T4 (fT4 is the biologically available, non-protein-bound form of the hormone able to enter cells.



SYNONYM


T4: tetraiodothyronine

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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on T

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