Estimation of the Secretion Rate of Thyrotropin in Man*

Abstract

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 (t_½) was thus estimated. The average t_½ 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). Similar data were estimated for 3 mildly hypothyroid patients. The t_½ were 75.1, 97.1, and 83.6 minutes, with a mean of 85.3 minutes (1.6 times normal). The mean TSH pool was 58.1 μg (10 times normal). The secretion rate was 688.7 μg per day (1,033.1 mU). In other hypothyroid patients, plasma TSH levels ranging from 6 to 230 mμg per ml (9 to 345 μU) have been found. If similar half-times and a normal distribution volume are assumed, the secretion rate of TSH in hypothyroid patients can be estimated to range from about 260 to 15,350 μg per day (390 to 23,025 mU) or from about 2 to 307 times normal. Therefore, the elevated plasma TSH levels found in hypothyroidism are a result of both slower degradation and increase in rate of secretion.