Metabolic Clearance of Human Follicle-Stimulating Hormone Assessed by Radioimmunoassay, Immunoradiometric Assay, andin VitroSertoli Cell Bioassay*

We measured the equilibrium MCR of purified human FSH via continuous infusion and its half-life by bolus injection in gonadotropin-deficient men. Serum FSH concentrations were determined by RIA, immunoradiometric assay, and an in vitro FSH bioassay (rat Sertoli cell). At steady state, the MCR of FSH averaged 5.4 +/- 0.4 mL/min.m2, which was not statistically different in the three assays at the three different infusion rates. The half-life of FSH after bolus injection averaged 274 +/- 45 min (4.6 +/- 0.75 h) when analyzed as a single exponential, and 1.8 and 10 h for biexponential kinetics. The distribution volume of FSH was 3.1 L (immunoradiometric assay) and 2.1 L (bioassay). In summary, the MCR of human FSH is invariant across a range of physiological gonadotropin concentrations and quantitatively similar in three different immunological/biological assays. These results support the hypothesis that removal of circulating FSH molecules proceeds in parallel for immunoreactive and biologically active glycoprotein hormone.