Body Temperature Manipulation Increases Salivary Cortisol Independent of Exercise

The vast majority of circulating cortisol in the blood is bound and inactivated by binding proteins like corticosteroid-binding globulin (CBG) and albumin. The cortisol that is free of these binding proteins is secreted via saliva and other body fluids. Ex-vivo studies have shown that CBG binding to cortisol is temperature-dependent within the physiological range of human core body temperatures. Exercise is a standard experimental paradigm for inducing cortisol responses. However, these studies rarely consider that exercise-induced changes in core body temperature might increase free cortisol independent of psychological stress or physical exertion. We tested the effect of heat exposure without exertion or other stressors on saliva cortisol. 26 Participants (18 healthy and 8 with Major Depression MDD) sat motionless in a dry infrared sauna set to 65.5 degrees C (150 F). They had their tympanic membrane temperature and salivary cortisol measured as their body temperature increased. Exposure to exogenous heat significantly increased core body temperature from 36.93 C/98.48 F (95% CI 36.79-37.07 C, 98.23-98.73 F) to 38.42 C /101.16 F (95% CI 38.24-38.6 C, 100.83-101.48 F) after 45 minutes of heat exposure, F(15,42.33)=43.509, p<0.001. Concurrently saliva cortisol also increased significantly from 4.00 nmol/l (95%CI 1.89-6.11) to 9.87 nmol/l (95% CI 5.58-14.17), F(1,33.17)=6.64, p=0.015. Elevations in core body temperature can significantly increase free cortisol levels even in the absence of exercise or overt psychological stressors.