Metabolic Clearance Rate and Interconversion of Androgens and the Influence of the Free Androgen Fraction

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Abstract
Using the continuous infusion technique, the conversion ratios (CR) of testosterone (T) to androstenedione (A) and dihydrotestosterone (DHT) and of A to T and DHT were determined in 12 normal males (aged 31-72 yr), 10 normal postmenopausal women, and 4 amenorrheic women with idiopathic hirsutism; in 4 additional males these studies were performed during infusion of cold T to increase plasma T to supraphysiological levels. It was observed that besides the MCR of T and DHT, the blood conversion ratios (CRBB) of T in A and to a lesser extent of T in DHT were also significantly correlated with either the free or the nontestosterone-estradiolbinding globulin-bound T fraction but not with total plasma T. In postmenopausal women, plasma A was by far the most important precursor of plasma DHT; the CRA/DHTBB was significantly higher than CRT/DHTBB. It is suggested that total plasma A, but only nonspecifically bound T, freely gains access into the cells where these conversions occur and that plasma A might be an important parameter of androgenicity. Less than 50% of plasma DHT could be accounted for by peripheral conversion of either A or T. Whereas in males this may be explained by direct DHT secretion, in (postmenopausal) women conversion of other precursors to plasma DHT should be considered. (J Clin Endocrinol Metab48: 320, 1979)