Effects of estrogen replacement therapy on the circadian rhythms of serum cortisol and body temperature in postmenopausal women

Estrogen replacement therapy (ERT) seems to enhance longevity in women. Bot h gender and aging have been shown to influence the regulation of circadian rhythms, yet little is known about the effect of ERT on circadian regulati on. The aim of this study was to determine the effects of ERT (oral conjuga ted estrogen: Premarin(R), 0.625 mg) for 6-8 weeks on circadian serum corti sol by continuous blood sampling every 15 min for 24 h with simultaneous me asurements of body temperature in six healthy postmenopausal women (range, 54-61 years). The results are presented as median values (range in quartile s). The circadian amplitude of cortisol increased during ERT from 20.20 (18 .35, 23.61) to 25.97 (24.94, 27.74) mu g/dL (p = 0.016), whereas the timing of nocturnal nadir and morning acrophase did not differ significantly. ERT lowered the 24-h body temperature from 37.03 degrees C (36.95 degrees C, 3 7.07 degrees C) to 36.90 degrees C (36.77 degrees C, 36.97 degrees C) (p = 0.038), but did not alter the peak and trough body temperatures significant ly. These findings are noteworthy because the increased circadian amplitude of serum cortisol during ERT contrasts with the reduction in circadian amp litude seen with normal aging. The reduction in body temperature confirms t he regulatory effect of ERT in thermoregulation and has implications regard ing the correlation between basal metabolic rate and life span. (C) 1999 El sevier Science Inc. AII rights reserved.