Differential effects of short-term fasting on pulsatile thyrotropin, gonadotropin, and alpha-subunit secretion in healthy men--a clinical research center study.

In healthy subjects, short term fasting suppresses the hypothalamic-pituitary-thyroid and hypothalamic-pituitary-gonadal (HPG) axes, with decreased serum levels of TSH and LH. However, effects of fasting on pulsatile release of TSH, LH, FSH, and alpha-subunit are less clear. Eleven healthy young men each underwent two 2-day studies: a baseline study during normal caloric intake and a fasting study during 56 h of caloric deprivation. During the final 24 h of each study, blood samples were drawn every 15 min for measurement of serum TSH, LH, FSH, and alpha-subunit pulses. Fifty-six hours of fasting caused a 50% suppression of mean TSH levels and TSH pulse amplitude, without altering TSH pulse frequency. Nocturnal TSH pulse amplitude decreased by 60%, with abolition of the usual nocturnal TSH surge. Fasting suppressed mean LH levels and LH pulse amplitude by 30%, without affecting LH pulse frequency. In contrast, mean FSH levels only decreased by 13%, without changes in FSH pulse parameters, whereas mean alpha-subunit levels and pulse amplitude decreased by 20%. These data show that short term fasting has a greater suppressive effect on the hypothalamic-pituitary-thyroid axis than on the HPG axis. Within the HPG axis, FSH is more resistant to fasting-induced suppression than LH, implying discordant regulation of the two gonadotropins during nutritional deprivation. alpha-Subunit suppression during fasting appears to parallel that seen for LH.