Diurnal hormonal responses to thyrotropin-releasing hormone in normal men

Bolus injections of synthetic thyrotropin-releasing hormone (TRH) were administered to five young normal men in the morning (0900 hr) and the evening (1800 hr) on different days. Frequent blood samples for prolactin (PRL) and TSH analyses were collected before and after TRH infusion. Although there were no differences between the morning and the evening basal PRL levels, a significantly greater PRL response to TRH in the evening was observed (Δ PRL, a.m. vs p.m., p < 0.025). Since TRH stimulates PRL through a direct effect on the pituitary, our data suggest that there is a diurnal variation in the pituitary lactotroph responsiveness to TRH. On the other hand, a.m. and p.m. basal and TRH-stimulated TSH responses were virtually identical.