Rhythms in the secretion of gonadotropins and gonadal steroids

Abstract

The occurrence of short term variations in the blood levels of pituitary gonadotropins and sex steroids is reviewed. 1. 1. The oscillations of LH levels are regular in timing and form and are almost certainly the result of episodic secretion by the pituitary in response to pulses of LRH from the hypothalamus. 2. 2. Sleep modifies the pattern of LH secretion in pubertal subjects and in women during the follicular phase of the menstrual cycle. 3. 3. The frequency and amplitude of the LH pulses are altered during the menstrual cycle and following testosterone and estradiol administration suggesting that the gonadal steroids modulate the episodic secretion of LH. 4. 4. Some patients with hypogonadotropism have LH pulses of normal proportional amplitude even though the mean levels are low, others have diminished or absent pulses with normal or low mean levels. 5. 5. Although the levels of FSH are less varied than those of LH, the oscillations tend to coincide. 6. 6. Testosterone levels exhibit an irregular circhoral rhythm in men and the oscillations are not consistently related to the LH pulses. It is possible that there is a variable time lag in the Leydig cell response to LH and it seems clear that factors other than LH contribute to the variability of the testosterone levels. 7. 7. There is a small amplitude circadian rhythm of testosterone secretion, at least in young men but its mechanism is uncertain. 8. 8. There is some evidence for episodic fluctuations of estradiol levels in women and there may also be a circadian rhythm. 9. 9. The biological significance of the high frequency rhythms in the levels of these hormones must remain speculative at present. It may be that the oscillations allow the transmission of a signal more efficiently than would a constant hormone level.