Serum thyroxine and triiodothyronine responses of hyperthyroid cats to thyrotropin

Objective-To document circulating total thyroxine (T4) and triiodothyronine (T3) responses after administration of thyrotropin (thyroid-stimulating hormone [TSH]) to hyperthyroid and healthy cats and assess the value of these responses as an additional diagnostic test for hyperthyroidism. Design-Prospective case series. Animals-21 healthy and 40 hyperthyroid cats. Procedure-Serum total T4 and T3 concentrations were measured by radioimmunoassay before and 6 hours after administration of 0.5 IU of bovine TSH/kg of body weight. Results-In healthy cats, serum total T4 concentration increased after administration of TSH (mean +/- SD, 114.0 +/- 36.4 nmol/L) representing a mean increment 3 times baseline concentration (mean +/- SD, 33.7 +/- 7.6 nmol/L). In hyperthyroid cats, the relative increase in serum total T4 concentration was significantly (P < 0.001) different; baseline values (mean +/- SD, 236.2 +/- 146.0 nmol/L) increased minimally after TSH administration (mean +/- SD, 308.1 +/- 178.9 nmol/L). There was a significant negative correlation (r(s) = -0.366) between relative increase in serum total T4 concentration after TSH administration and baseline concentration in hyperthyroid cats. In 3 cats with equivocal baseline serum total T4 concentration, the T4 response to TSH administration was indistinguishable from that in healthy cats. Serum total T3 response to TSH administration was significantly (P < 0.001) lower in hyperthyroid, compared with healthy, cats but the T3 response in healthy cats was more variable than that for T4. Conclusions-Thyrotoxic cats with high baseline serum total T4 concentration have a limited T4 response to TSH stimulation. Hyperthyroid cats with equivocal baseline serum total T4 concentrations have T4 responses after TSH stimulation similar to those of healthy cats. Measurement of serum total T3 concentration provides no additional information. Clinical Relevance-The TSH response test is of limited value in diagnosing hyperthyroidism in cats.