Estrogen modifies the temperature effects of progesterone

To test the hypothesis that progestin-mediated increases in resting core te mperature and the core temperature threshold for sweating onset are counter acted by estrogen, we studied eight women (24 +/- 2 yr) at 27 degrees C res t, during 20 min of passive heating (35 degrees C), and during 40 min of ex ercise at 35 degrees C. Subjects were tested four times, during the early f ollicular and midluteal menstrual phases, after 4 wk of combined estradiol- norethindrone (progestin) oral contraceptive administration (OC ESP), and a fter 4 wk of progestin-only oral contraceptive administration (OC P). The o rder of the OC P and OC E + P were randomized. Baseline esophageal temperat ure (T-es) at 27 degrees C was higher (P < 0.05) in the luteal phase (37.08 +/- 0.21 degrees C) and in OC P (37.60 +/- 0.31 degrees C) but not during OC E + P (37.04 +/- 0.23 degrees C) compared with the follicular phase (36. 66 +/- 0.21 degrees C). T-es remained above follicular phase levels through out passive heating and exercise during OC P, whereas T-es in the luteal ph ase was greater than in the follicular phase throughout exercise (P < 0.05) . The T-es threshold for sweating was also greater in the luteal phase (38. 02 +/- 0.28 degrees C) and OC P (38.07 +/- 0.17 degrees C) compared with th e follicular phase (37.32 +/-: 0.11 degrees C) and OC E + P (37.46 +/- 0.18 degrees C). Progestin administration raised the T-es threshold for sweatin g during OC P, but this effect was not present when estrogen was administer ed with progestin, suggesting that estrogen modifies progestin-related chan ges in temperature regulation. These data are also consistent with previous findings that estrogen lowers the thermoregulatory operating point.