Short- and Long-Term Impact of Exogenous Hormones on Thermoregulation in Women

How the body changes its methods of thermoregulation as part of the female ovarian cycle is a crucial aspect of human adaptive strategies to negotiate variable environments and tasks. The use of exogenous hormones to manage the ovarian cycle is a regular part of modern life but its long-term impact on thermoregulation, particularly while moving, continues to be understudied.

Here, we specifically track adults (N = 35) including men, women of known ovarian phases, and women on an estrogen-progesterone combination pill, doing the same walking and resting protocol while their core body temperature was monitored continuously and their blood perfusion was monitored regularly. Blood perfusion was normalized to maximum blood flow, and lines were fit to the blood flow during exercise and during rest; the slope of these lines was used to analyze blood flow of the different endocrine groups.

Women on the pill showed significantly higher core temperatures (x¯ = 38.16°C) than women not on the pill (luteal x¯ = 37.85°C; follicular x¯ = 37.69°C) and men (x¯ = 37.80°C) (p = 0.02). Women in the luteal phase showed the most shallow slopes of blood perfusion (between -0.14 and 0.07), whereas all other endocrine groups showed slopes ranging from -0.31 to 0.15. Endocrine group significantly impacted the slope of blood perfusion during resting and walking (p = 0.059).

Given the different core temperatures, the similarity in the amount and pattern of blood perfusion between women on the pill and women in the follicular phase was not enough to substantially lower the core temperature of women on the pill. Another mechanism besides blood flow is necessary to explain the substantial difference in core body temperature maintained by women on the pill throughout the experimental period. These results suggest that core temperature does not automatically trigger blood perfusion as a mechanism of cooling for women on the pill. Additionally, women in the luteal phase must use other, non-vascular, mechanisms to cool the body during exercise. Finally, the thermoregulatory responses of people on a combination pill do not match the thermoregulatory responses of people in the luteal phase; studies which conflate these two should use caution in their interpretation.