Decreased serum testosterone and free triiodothyronine levels in healthy middle-aged men indicate an age effect at the pituitary level.

In an attempt to study further the age-specific influence on the hypothalamo-pituitary-gonadal axis as well as the hypothalamo-pituitary-thyroid axis, we have now investigated young and middle-aged men, considering possible confounding factors. Both serum total testosterone, free testosterone and the total ratio of testosterone to sex-hormone binding globulin were significantly lower among middle-aged men as compared with young men (p = 0.02, p = 0.002 and p = 0.0003, respectively). In accordance with these findings there was also a decrease in the luteinizing hormone response to gonadotrophin-releasing hormone in the middle-aged men (p = 0.02). Free testosterone was correlated significantly with the luteinizing hormone response (r = 0.32, p = 0.02). Serum free triiodothyronine was significantly higher among young men as compared with middle-aged men (p = 0.002) and the thyrotrophin-releasing hormone-stimulated thyrotrophin response was also higher in the young group compared with the middle-aged group. The present results may indicate that the age effect on serum levels of testosterone and free triiodothyronine is mediated at the pituitary level.