Circadian rhythms of anterior pituitary hormone secretion: effects of dexamethasone.

Circadian rhythms of hormone secretion are by now well recognized but there are limited data available to compare such rhythms of multiple hormones in individual subjects. Therefore concentrations of serum thyrotropin (TSH), cortisol, growth hormone (GH), triiodothyronine (T3) and prolactin (PRL) were determined in blood samples from six healthy males over periods of 24 hours. A circadian periodicity was identified for cortisol, GH, PRL and TSH; maximum concentrations for TSH and PRL occurred at dissimilar times, namely respectively before and after the GH peak. Maximum TSH coincided with nadir values forplasma cortisol. Dexamethasone administration (3 mg over 36 hours) lowered serum T3 concentration and abolished TSH periodicity without affecting the PRL rhythm. These data suggest that a) nocturnal TSH and PRL maximum levels do not have common mediation; b) there may be an inverse relationship between cortisol and TSH, and c) glucocorticoids at low doses preferentially affect secretion of thyrotropin.