Thyroid hormone response to varying doses of TSH

Fifty-five normal subjects were studied following intravenous injection of increasing doses of bovine thyrotropin (b-TSH) from 0.5 to 250 mU/kg. Serum triiodothyronine (T3) and thyroxine (T4) were measured 1, 2, 3 and 4 h after the TSH injection. Dose-related increments in serum T3 and T4 were demonstrated with doses of b-TSH = 2.5 mU/kg and a maximum response was obtained after approximately 100 mU/kg. The fractional increase in serum T3 was greater than in serum T4, but the ratio between the increase in serum T4 and serum T4 and serum T3 (21.3/l ng/ng) was independent of the dose of b-TSH and time after TSH stimulation. The T3 response was reproducible and unaffected by sex and age.