Open AccessTriiodothyronine (T3) Suppression Test
Author(s) -
Peterson Mark E.,
Graves Thomas K.,
Gamble David A.
Publication year - 1990
Publication title -
journal of veterinary internal medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.356
H-Index - 103
eISSN - 1939-1676
pISSN - 0891-6640
DOI - 10.1111/j.1939-1676.1990.tb03114.x
Subject(s) - cats , medicine , basal (medicine) , triiodothyronine , endocrinology , morning , serum concentration , physiology , thyroid , insulin
The purpose of this study was to develop a T 3 suppression test to help in the diagnosis of mild hyperthyroidism in cats. We evaluated the response in circulating T 4 concentrations to exogenous T 3 (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 T 4 and T 3 determinations, 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 T 4 and T 3 determinations 2 to 4 hours after the seventh dose of liothyronine. The mean basal serum concentrations of T 4 (53.1 nmol/L) and T 3 (1.8 nmol/L) were significantly higher in the cats with hyperthyroidism than in the normal cats (T 4 = 25.3 nmol/L, T 3 = 1.3 nmol/L) and the cats with nonthyroidal disease (T 4 = 29.5 nmol/L, T 3 = 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 T 4 values and 55 (71%) had T 3 values that were within the established normal ranges. After administration of liothyronine, mean serum T 4 concentrations fell much more markedly in the normal cats and the cats with nonthyroidal disease than in the hyperthyroid cats. The mean post‐liothyronine serum T 4 concentrations 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 T 4 concentrations 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 T 3 suppression test is a useful aid in the diagnosis of hyperthyroidism in cats when basal serum T 4 concentrations are high‐normal or only slightly high. The T 3 suppression test appears to be of particular value in differentiating between hyperthyroid cats and cats with nonthyroidal disease that have basal serum T 4 values in the normal to high‐normal range, since there was very little overlap in the amount of T 4 suppression between these two groups of cats.