The thyroid gland is required for reproductive neuroendocrine responses to photoperiod in the ewe.

A study was conducted to determine the role of the thyroid gland in three neuroendocrine responses to photoperiod; secretion of melatonin, PRL, and LH. Ewes were thyroidectomized (THX) in midsummer or left thyroid intact, and both groups were moved indoors to artificial short days (8 h of light, 16 h of darkness) for 90 days. Thereafter, a subset of both THX and thyroid-intact ewes was challenged with long days (16 h of light, 8 h of darkness) for 120 days. The other ewes remained in short days so that neuroendocrine responses to the photoperiodic shift could be distinguished from hormonal changes that occur spontaneously. Blood was sampled twice weekly for determination of serum concentrations of LH and PRL and hourly for 48 h surrounding the photoperiodic switch for assay of melatonin. All ewes were ovariectomized and treated with constant release implants of estradiol, so that PRL and LH secretion would not be influenced by alterations in gonadal steroid secretion. There was no effect of thyroidectomy on the circadian pattern of circulating melatonin or on the change in this pattern after the shift from short to long days. Similarly, thyroidectomy did not alter the PRL response to this photoperiodic shift; long days caused PRL to increase whether the thyroid was present or absent. In marked contrast, thyroidectomy blocked the effect of long days on circulating LH, a hormone indicative of reproductive neuroendocrine activity. Specifically, long days induced a precipitous drop in LH in thyroid-intact ewes, but not in THX ewes. Thus, although the thyroid plays an obligatory role in photoperiodic inhibition of the reproductive neuroendocrine axis of ewes, it may not be required for photoneuroendocrine responses in terms of melatonin and PRL secretion. Our findings suggest that in the absence of the thyroid, the reproductive neuroendocrine axis is uncoupled from the photoperiodic influence between the pineal and the GnRH neurosecretory system.