Neuroendocrine effects of caffeine in normal subjects.

In double-blind crossover experiments, we examined the effects of oral caffeine (250 or 500 mg) added to decaffeinated coffee on plasma hormone levels in adults who normally consume one to three cups of coffee a day. In one experiment, 250 mg (about 4 mg/kg) caffeine was given to men; in two other experiments, 500 mg (8 mg/kg) was given to both sexes. Caffeine, 500 mg, elevated plasma levels of beta-endorphin-like immunoreactivity in both men and women but had no significant effect on plasma levels of cortisol, thyroid-stimulating hormone, growth hormone, prolactin, or triiodothyronine in men nor on plasma levels of prolactin or cortisol in women. The 250-mg dose induced no significant changes in plasma levels of any of the hormones measured. We conclude that the threshold for caffeine's endocrine effects is higher than that for its behavioral effects.