LABORATORY TESTING FOR THYROID DISEASE

37 LABORATORY TESTING FOR THYROID DISEASE



1 What is the best diagnostic test for feline hyperthyroidism?


The majority of hyperthyroid cats have serum total thyroxine (T4) concentration above reference ranges. For about 90% of hyperthyroid cats, measuring serum total T4 will be diagnostic.



2 How useful is measuring serum total triiodothyronine in diagnosis of feline hyperthyroidism?


Although usually elevated in feline hyperthyroidism, serum total triiodothyronine (T3) may be within reference ranges in up to 33.5% of hyperthyroid cats and thus is less diagnostically useful than measurement of total T4. In virtually all cats with elevated total T3, total T4 is also elevated.



3 Why do some hyperthyroid cats have total T4 concentration within the reference range?


Total T4 concentration may be within the reference range in some hyperthyroid cats for the following reasons:


a. A very early stage of hyperthyroidism is present, with overproduction of thyroxine just beginning.

b. Normal fluctuation in T4 levels over time bring total T4 within reference range at the time of sample collection.

c. Concurrent nonthyroid illness in hyperthyroid cats may decrease total T4 to within reference range.


4 How can one diagnose the 10% of hyperthyroid cats with total T4 concentration within reference range?


Three methods can be used to diagnose hyperthyroid cats with total T4 within the reference range:


a. Retest total T4 concentration a few weeks later.

b. Measure free T4 concentration.

c. Use one of the dynamic thyroid function tests, either T3 suppression or thyrotropin-releasing hormone (TRH) stimulation.


5 Why would repeating the total T4 assay a few weeks later help with the diagnosis of hyperthyroidism?


Concentrations of total T4 in hyperthyroid cats may vary significantly over a 2-week period and may be within reference range at one or more times during that period. The peak of T4 does not occur at any particular time of day. In a cat with clinical signs consistent with hyperthyroidism and total T4 in the reference range, repeating the total T4 on a different day may be diagnostic.



6 How accurate is measurement of free T4 in diagnosis of hyperthyroidism?


Free T4 concentration appears to be a more sensitive diagnostic test for feline hyperthyroidism than total T4 concentration. Unfortunately, free T4 is also less specific. In other words, some euthyroid cats with nonthyroid illness will have high free T4 concentrations but total T4 values below the reference range. It is recommended that free T4 assay results in cats be interpreted in the context of total T4 results as well as clinical signs and physical examination findings. A high free T4 along with a total T4 that is within or above the reference range is most consistent with hyperthyroidism.



7 Does the method used to measure free T4 affect test sensitivity?


The method used does affect test sensitivity; free T4 concentration should be measured by equilibrium dialysis.



8 When should dynamic tests of thyroid function be performed?


If repeated total T4 testing and measurement of free T4 are not diagnostic in a cat with clinical signs of hyperthyroidism, the T3 suppression or the TRH stimulation test might be used.



9 How does the T3 suppression test work?


In a normal cat, exogenous T3 should suppress thyroid-stimulating hormone (TSH) secretion from the pituitary and thus decrease T4 production by the thyroid. T4 is typically suppressed by more than 50% in euthyroid cats after administration of T3. In hyperthyroid cats, T4 concentration does not decrease to this degree, primarily because TSH is already suppressed due to negative feedback by the excess circulating T4.


To administer the T3 suppression test, a baseline blood sample is taken for measurement of serum T3 and T4, then 15 to 25 μg of liothyronine sodium is given orally to the cat every 8 hours for the next 2 days. On the morning of the third day, a seventh dose is given, and a blood sample taken 2 to 4 hours later for T3 and T4 assay. An increase in the T3 level above baseline indicates that the synthetic T3 was successfully administered and absorbed. This test is not associated with adverse clinical effects.



10 How is the thyrotropin-releasing hormone (TRH) stimulation test performed and interpreted?


Blood for baseline serum T4 is obtained, then 0.1 mg of TRH/kg body weight is given intravenously. A second blood sample for serum T4 is obtained 4 hours after TRH administration. Adverse reactions seen in cats include salivation, urination, defecation, vomiting, tachycardia, and tachypnea. Results after stimulation are generally interpreted as follows:


• Increase of less than 50% over baseline serum T4 is consistent with hyperthyroidism.

• Increase of greater than 60% over baseline is normal.

• Increase of 50% to 60% over baseline T4 is nondiagnostic.


11 Can one measure endogenous thyroid-stimulating hormone (TSH) to assist in the diagnosis of feline hyperthyroidism?


There is currently no validated assay for feline TSH, although there seems to be some cross-reactivity between feline TSH and the reagents used for assay of canine TSH.



12 If a laboratory reports T3 and T4 in different units from those used in another laboratory, how is nmol/L converted to mg/dl?


Table 37-1 lists conversion factors for units used in T3 and T4 assays.


Table 37-1 Unit Conversions in Thyroxine (T4) and Triiodothyronine (T3) Assays































ANALYTE (UNITS) CONVERSION FACTOR (DIVIDE BY) UNITS OBTAINED
Total T4 (nmol/L) 12.87 μg/dl
Total T3 (nmol/L) 1.536 ng/ml
Free T4 (pmol/L) 1.287 pg/ml
Free T4 (pmol/L) 12.87 ng/dl
Free T3 (pmol/L) 1.536 pg/ml
Free T3 (pmol/L) 15.36 ng/dl

Units: nmol, nanomoles; pmol, picomoles; μg, micrograms; ng, nanograms; pg, picograms Per (/) liter (L), deciliter (dl), or milliliter (ml).

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Aug 26, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on LABORATORY TESTING FOR THYROID DISEASE

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