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1386 Thyroid-Stimulating Hormone (TSH) Thyroxine (Total T 4 ), Free Thyroxine by Equilibrium Dialysis

• Increased TgAA increases suspicion of • Patients with increased TgAA but normal AUTHOR: Angela Royal, MS, DVM, DACVP
hypothyroidism, but is not confirmatory thyroid hormone status may proceed to EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
clinical hypothyroidism in the future, but
VetBooks.ir some do not.
for diagnosis.

Thyroid-Stimulating Hormone (TSH)

Definition Next Diagnostic Steps to Consider centrifugation and removal of serum. Store
Pituitary-derived hormone that acts on the if Levels are High refrigerated up to 7 days.
thyroid gland, causing production and secretion Generally measured with total T 4 ; consider free
of thyroid hormones T 4 (by equilibrium dialysis). See p. 525. Relative Cost: $$

Synonyms Causes of Abnormally Low Levels Pearls
Thyrotropin, TSH Test sensitivity is insufficient to differentiate • 75% of hypothyroid dogs have increased
normal from low concentrations. TSH. High TSH and concurrently low T 4
Physiology or free T 4 are diagnostic for hypothyroidism.
Thyroid-stimulating hormone (TSH) is Important Interspecies Differences However, hypothyroidism is not ruled out
released from the pituitary after stimulation Species-specific immunoassay. No specific feline if TSH is within the reference interval.
by thyrotropin-releasing hormone (TRH). TSH TSH assay is available, but the canine assay • Although not well standardized, an expected
acts on the thyroid to stimulate synthesis and detects increased levels (due to cross-reactivity) decrease in TSH levels can be used along with
secretion of thyroxine (T 4 ) and triiodothyronine in cats. T 4 levels to monitor thyroid replacement
(T 3 ). In turn, T 3 and T 4 provide negative therapy in canine hypothyroidism.
feedback to reduce TSH and TRH secretion. Drug Effects • Using the canine TSH assay for cats can
Mild increases may be seen with corticosteroids, be helpful to detect high concentrations in
Reference Interval phenobarbital, and potentially other drugs. iatrogenic hypothyroid states.
Dogs: 0-0.5 ng/mL
Specimen Collection and Handling AUTHOR: Shelley Burton, DVM, MSc, DACVP
Causes of Abnormally High Levels Serum (red top tube), 1 mL. To avoid interfer- EDITOR: Lois Roth-Johnson, DVM, PhD, DACVP
Hypothyroidism (primary); mild increases with ence by fibrin, allow full clot formation before
some nonthyroidal illnesses or drugs

Thyroxine (Total T 4 ), Free Thyroxine by Equilibrium Dialysis

Definition Causes of Abnormally High Levels Important Interspecies Differences
Total thyroxine (T 4, TT 4) is the major storage Hyperthyroidism; presence of autoantibodies • Spontaneous hypothyroidism is extremely
form of thyroid hormone. Free T 4 (fT 4) is the to T 4; exogenous T 4, TSH, or TRH; rarely, rare in cats, so a low T 4 or fT 4 is typically due
biologically available non–protein-bound form. administration of iodine-containing compounds to nonthyroidal illness, drug administration,
or raw food diet or iatrogenic hypothyroidism.
Synonym • Hyperthyroidism is uncommon in dogs.
T 4, tetraiodothyronine Next Diagnostic Steps to Consider Consider thyroid neoplasia, thyroid-rich
if Levels are High diet (such as some raw diets), or T 4
Physiology • If hyperthyroidism suspected: see p. 503. autoantibodies.
Thyroxine is highly protein bound in circulation • If autoantibodies suspected: measure T 4
and not biologically active. In cells, thyroxine is autoantibody levels. Drug Effects
deiodinated to the active form of the hormone • Evaluate diet, supplements for iodine content. Many drugs, including but not limited to
(T 3), which increases metabolic rate. glucocorticoids and phenobarbital, may decrease
Causes of Abnormally Low Levels total T 4 concentration, with less (or no) effect
Reference Interval Spontaneous or iatrogenic hypothyroidism; on fT 4.
• Approximate; check specific laboratory nonthyroidal illness; drugs
reference interval. Common unit (mcg/dL) Lab Artifacts
× 12.87 = SI unit (nmol/L) Next Diagnostic Steps to Consider • Artifactual increase of free and total T 4 if
○ Dogs: T 4 : 1.2-3.9 mcg/dL (15-50 nmol/L); if Levels are Low stored in glass tubes at 37°C
fT 4: 0.8-3.2 mcg/dL (10-41 pmol/L) Measure serum TSH levels (p. 525); • Severe hyperbilirubinemia can artifactually
○ Cats: T 4 : 0.8-3.5 mcg/dL (10-45 nmol/L); evaluate for nonthyroidal diseases or drug increase T 4 in some assays.
fT 4 : 0.6-2.2 mcg/dL (8-28 pmol/L) administration.

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