Triiodothyronine (T3) Suppression Test

Open Access
Publisher Website
Google Scholar
Abstract
The purpose of this study was to develop a T3suppression test to help in the diagnosis of mild hyperthyroidism in cats. We evaluated the response in circulating T4concentrations 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 T4and T3determinations, administering liothyronine at an oral dosage of 25 μg three times daily for seven doses, and, on the morning of the third day, again collecting serum samples for T4and T3determinations 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 T4values and 55 (71%) had T3values that were within the established normal ranges. After administration of liothyronine, mean serum T4concentrations fell much more markedly in the normal cats and the cats with nonthyroidal disease than in the hyperthyroid cats. The mean post‐liothyronine serum T4concentrations were significantly higher (P< 0.001) in the hyperthyroid cats (48.5 nmol/L) than in the normal cats (9.5 nmol/L) and the cats with nonthyroidal disease (9.9 nmol/L). The mean percent decrease in T4concentrations after administration of liothyronine in the hyperthyroid cats (9.1%) was also significantly (P< 0.001) lower than the decrease in either the normal cats (63%) or cats with nonthyroidal disease (67%). The results of this study indicate that the T3suppression test is a useful aid in the diagnosis of hyperthyroidism in cats when basal serum T4concentrations are high‐normal or only slightly high. The T3suppression test appears to be of particular value in differentiating between hyperthyroid cats and cats with nonthyroidal disease that have basal serum T4values in the normal to high‐normal range, since there was very little overlap in the amount of T4suppression between these two groups of cats.