FELINE T3 SUPPRESSION TEST

PROTOCOL AND GUIDE TO INTERPRETATION.

The T3 suppression test is indicated for cats with clinical signs suggestive of hyperthyroidism but with equivocal repeated basal T4 results. A TRH stimulation test protocol is available for cats which are difficult to dose orally with liothyronine.

PROTOCOL

1. Take a basal 2ml clotted or heparinised blood sample. If a centrifuge is available separate the serum and store at 4° C. If not then allow to clot and dispatch to the laboratory.
2. Administer liothyronine (T3) tablets (Tertroxin - Pitman-Moore) orally at a dose of one 20mg tablet three times daily for two days commencing a.m.
3. On the morning of the third day, give a seventh 20 mg dose and take a second 2ml clotted or heparinised blood sample 2 to 4 hours later. Dispatch all serum or heparin blood samples to the laboratory being careful to label them correctly i.e. "basal" sample and "post T3" sample.

INTERPRETATION
 

T4 reduction post T3 administration	Interpretation
>50%	Non-thyroidal illness/normal cat
<50% but >35%	Suggests non-thyroidal illness/normal cat
<35%	Suggests hyperthyroid cat

It can be seen from the interpretation that this test is most useful in ruling out a diagnosis of hyperthyroidism. The test interpretation does not categorize the response to T3 suppression of cats with both hyperthyroidism and a concurrent non-thyroidal illness. Although this may only represent a relatively small proportion of hyperthyroid cats (Thoday and Mooney, 1992), a biochemical and haematological profile (EPH) would be advisable in all cats with clinical signs suggestive of hyperthyroidism but with a normal serum T4 level. The results of these investigations should be considered alongside the results of the T3 suppression test.

OWNER COMPLIANCE.

A potential problem with the test is difficulty in administering the liothyronine tablets. Failure to administer the tablets could lead to a false positive test result. In cases where failure to administer the liothyronine tablets is suspected, a T3 assay on the pre and post T3 serum samples can be requested. An elevation in T3 levels post T3 administration would indicate successful administration of the liothyronine tablets.
 

Test Code	Test Name and Description	Sample
T3S	T3 Suppression Test	2 x Serum/ Heparin Plasma
T3S+	T3 Suppression Test (With confirmation of correct T3 dosage)	2 x Serum/ Heparin Plasma

REFERENCES.

Peterson M.E., Graves T.K. and Gamble D.A. (1990). Triiodothyronine (T3) suppression test; an aid in the diagnosis of mild hyperthyroidism in cats. J. Vet. Int.Med 4 (5) p 233-238.

Thoday K.L. and Mooney C.T. (1992). Historical, clinical and laboratory features of 126 hyperthyroid cats. Vet Record 131 p 257-264.

Test Protocol Index