The T3 suppression test is the safest and most reliable dynamic evaluation of thyroid function available for use in cats. The purpose of the test is to demonstrate the presence of a negative feedback on the production of thyroid hormone after administration of exogenous administration of liothyroxine (T3) in subjects not affected by hyperthyroidism. The test is particularly useful in cases in which there is strong clinical suspicion of hyperthyroidism but equivocal results have been obtained from measuring basal hormone concentrations (T4, free T4).
TEST EXECUTION
- Day 1: a sample of serum is taken for the baseline assays of T4 concentration [T4] and T3 concentration [T3]. The sample must be frozen if the assay is to be performed subsequently;
- Days 2 and 3: the owner of the cat administers 25 μg of liothyroxine (T3) per os every 8 hours, at home;
- Day 4: the cat is brought to the clinic 2-4 hours after the last dose of liothyroxine (T3) and a second sample of serum is taken for the post-T3 measurements of [T4] and [T3].
INTERPRETATION OF THE RESULTS
The post-T3 concentration of T3 must be higher than the basal T3 concentration. If this is not the case, it may be that the liothyroxine (T3) was not administered correctly. In this case the result of the test is compromised.
- If the concentration of T4 after suppression by the exogenous T3 is lowered by at least 50% with respect to the basal value, hyperthyroidism can be excluded;
- A post-T3 [T4] >20 nmol/L (>1.6 µg/dl) indicates a lack of suppression of secretion of T4 and, therefore, supports the diagnosis of hyperthyroidism.
- A post-T3 [T4] <20 nmol/L (<1.6 µg/dl) demonstrates that the secretion of T4 has been suppressed and the cat is not, therefore, affected by hyperthyroidism.
Suggested readings
- Stockham L.S., Scott M.A.: Thyroid function. In: Stockham L.S. e Scott M.A. Foundamentals of Veterinary Clinical Pathology, II edizione, Blackell Publishing, pg.783-803, 2008.