TSH release during the estrous cycle of the rat: variations in responsiveness to cervicovaginal stimulation

Serum TSH, LH and FSH were measured at various times during the day, during the estrous cycle of the rat. Proestrous surges of FSH and LH were detected as previously reported. TSH values fell to low levels in the late afternoon on each day of the cycle. There was a significant elevation of TSH at 13.00 h on both proestrus and estrus. The elevation of TSH on proestrus was accompanied by a significant fall in pituitary TSH stores. LH stores were also significantly depleted by 19.00 h on proestrus. Cervicovaginal stimulation by a glass rod provoked a significant rise in serum TSH 3 h after stimulation at 10.00 h on estrus but not on other days of the cycle. Responsiveness to a test dose of TRH varied during the cycle. It was maximal on proestrus and declined to minimum values on diestrus day II. Values were further lowered after ovariectomy but were restored to diestrus day II levels by treatment with estradiol. It is concluded that there are important cyclic variations in TSH release in the female rat, that cervicovaginal stimulation can provoke TSH release on estrus and that these changes are associated with altered responsiveness to TRH. We conclude that ovarian steroids modulate the release of TSH from the pituitary and affect the responsiveness to cervicovaginal stimulation. These changes are presumably brought about by alterations in TRH release and in responsiveness of the pituitary to the neurohormone.