Triiodothyronine (T3) suppression test. An aid in the diagnosis of mild hyperthyroidism in cats

The purpose of this study was to develop a T3 suppression test to help in the diagnosis of mild hyperthyroidism in cats. We evaluated the response in circulating T4 concentrations to exogenous T3 (liothyronine) administration in 44 clinically normal cats, 77 cats with hyperthyroidism, and 22 cats with nonthyroidal disease. The test was performed by first collecting blood samples for basal serum T4 and T3 determinations, administering liothyronine at an oral dosage of 25 micrograms three times daily for seven doses, and, on the morning of the third day, again collecting serum samples for T4 and T3 determinations 2 to 4 hours after the seventh dose of liothyronine. The mean basal serum concentrations of T4 (53.1 nmol/L) and T3 (1.8 nmol/L) were significantly higher in the cats with hyperthyroidism than in the normal cats (T4 = 25.3 nmol/L, T3 = 1.3 nmol/L) and the cats with nonthyroidal disease (T4 = 29.5 nmol/L, T3 = 1.4 nmol/L); however, there was a great deal of overlap of basal values between the three groups of cats. Of the 77 cats with mild hyperthyroidism, 41 (53%) had serum T4 values and 55 (71%) had T3 values that were within the established normal ranges. After administration of liothyronine, mean serum T4 concentrations fell much more markedly in the normal cats and the cats with nonthyroidal disease than in the hyperthyroid cats.(ABSTRACT TRUNCATED AT 250 WORDS)