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The 24-h mean plasma concentrations of androgens (dihydrotestosterone and total and free testosterone), estrogens (estrone and estradiol), and gonadotropins (LH and FSH) were measured in 35 healthy men, aged 21-85 yr, who were rigorously screened to exclude factors known or suspected to alter endocrine function. The plasma total testosterone concentration showed a slow continuous decline with age, decreasing about 35% between 21 and 85 yr of age; the free testosterone level was closely correlated with that of total testosterone over the entire observed concentration range. The concentrations of dihydrotestosterone, estrone, estradiol, and LH were age invariant. The concentration of FSH showed a continuous linear increase with age; the level at age 85 was about 2.5 times the level at age 21. The following conclusions were drawn. 1) Testosterone secretion appears to decline slowly and continuously throughout adult life in men. 2) Measurement of the plasma free testosterone level adds no independent information in healthy men, since its level is closely correlated with that of total testosterone at all concentrations. 3) The continuous rise with age in FSH concentration while LH is age invariant cannot be explained by changes in testosterone or estrogen production, but might be due to a decline of inhibin production with age.

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