The plasma concentration of a pituitary hormone is determined by the rate of secretion, degradation, and the volume of distribution of that hormone. Using a radioimmunoassay for human thyrotropin (TSH) and human TSH-131I, we have estimated the rates of degradation and distribution of TSH in man and calculated the rate of secretion. Either 0.5 or 5 μg of TSH-131I with specific activities of 1 to 50 μc per μg was administered intravenously to 12 euthyroid subjects. Serial determinations were made of TSH-131I, and the half-time of disappearance (t½) was thus estimated. The average t½ in euthyroid subjects was 53.9 minutes with a volume of distribution averaging 5.8% of body weight. The mean endogenous plasma TSH concentration was 1.8 mμg per ml (2.7 μU per ml in terms of the human TSH reference standard A). The mean total TSH pool, excluding the pituitary, was 5.8 μg (8.7 mU). From these data the mean secretion rate of TSH in euthyroid man was calculated to be 110.1 μg per day (165.2 mU).
W. D. Odell, R. D. Utiger, J. F. Wilber, P. G. Condliffe