Serum levels of prolactin, TSH and gonadotrophins following LRH/TRH - double stimulation tests during the menstrual cycle

The objective of this study was to re-investigate the capacity of pituitary prolactin (Prl) and thyrotrophin (TSH) secretion throughout the normal menstrual cycle to respond to repeated thyrotrophin releasing hormone (TRH) stimulation analogous to the double luteinizing hormone-releasing hormone (LRH) stimulation test. This test has been shown to be a sensitive parameter for oestrogenic effects on the gonadotrophs. In addition, the volunteers were selected carefully on the basis of ovulatory cycles and otherwise normal endocrine function.In 9 women a combined LRH/TRH double stimulation test was performed during the early follicular, periovulatory and mid-luteal phases. TRH (200 μg) and LRH (25 μg) each were given iv twice, 2 h apart.Basal and LRH stimulated luteinizing hormone (LH) and follicle stimulating hormone (FSH) were found to follow characteristic cyclic response patterns. The LH responses after both LRH stimulations were greatest in the periovulatory phase; Δ1and Δ2were higher in the mid-luteal phase than in the follicular phase. Maximum FSH response to LRH was found during the periovulatory phase, but the FSH response in the early follicular phase was greater than that found in the mid-luteal phase. In contrast, basal and TRH stimulated serum concentrations of TSH and Prl remained constant throughout the cycle.The gonadotrophin ratios Δ2:Δ1were generally greater than 1. They increased from 1.4 in the early follicular phase to 3.0 in the late follicular phase, concomitant with the rise in oestrogens. The Δ2: Δ1ratios for TSH and Prl were less than 1, ranging from 0.66 to 0.98 for TSH and from 0.26 to 0.99 for Prl. They did not show any cyclic changes.Thus, this study shows that after LRH/TRH double stimulation, the gonadotrophin but not the Prl and TSH responses vary with the physiological changes in oestrogens during the menstrual cycle.The supposed mechanism of oestrogen effects on pituitary hormone secretion and their possible clinical significance are discussed.