Assessment of thyroid functional reserve in the cat by the thyrotropin-stimulation test.

Serum thyroxine (T4) concentrations before and after various IV doses of bovine thyrotropin (TSH) were measured over a 48-hour period in 19 healthy cats. Base-line T4 values, as measured by radioimmunoassay, varied greatly. The peak T4 concentration occurred 6 hours after TSH injection, and there was an increase in post-TSH serum T4 concentration that was linearly related to the logarithm of the dose. Greatest stimulation was seen with the highest dose used (1 U of TSH/kg of body weight), and 6 hours after administration of this dose, the serum T4 concentration range was 4.1 to 8.4 micrograms/dl. The post-TSH serum T4 concentration and the absolute increase in serum T4 concentration after TSH administration correlated more closely with the TSH dose than did the ratio of post-TSH serum T4 concentration to base-line T4 concentration. Therefore, in cats with normal thyroid-binding protein concentrations, the former indices should represent the most reliable assessment of thyroid functional reserve.