A B S T R A C T Metabolic clearance rates (MCR) ofthe alpha and beta subunits of human thyrotropin (hTSH-a and hTSH-,8) were determined by a constant infusion to equilibrium method. In 15 normal individuals (six men, six premenopausal women, and three postmenopausal women), the mean MCR of hTSH-a (68 ml/min per m2) was significantly faster than that of hTSH-,B (48 ml/min per M2) was significantly faster than that of hTSH-,B (48 ml/min per m2); both were two to three times more rapid than the previously determined MCR of hTSH. In patients with primary hypothyroidism, MCR were significantly slower with a mean value of 55 ml/min per m2 for hTSH-a and 37 ml/min per m2 for hTSH-,B. However, MCR of subunits were not significantly faster than normal in hyperthyroid patients.