“Independent pilot study that investigated the hormonal changes experienced by warfighters training in extremely cold weather and harsh terrain, and the correlation of such changes with cognitive performance. Objectives: Prior research has shown a relationship between hormone dysregulation, impaired cognitive and physical performance, and injury. The goal of this study was to quantify the hormonal responses and adaptations that occur in elite military personnel during winter warfare training (WWT). Methods: 14 Army Special Forces operators (13 males; 1 female; mean age of 31 years) engaged in 2 separate WWT weeks in Montana (MT) and Alaska (AK), separated by 1 month. The participants underwent cognitive testing (via DANA), which consisted of the following tests: simple reaction time (SRT), procedural reaction time (PRT), and Go/No-Go (GNG), at their home base in Colorado prior to engagement (baseline), and again on the first and last day of WWT in both MT and AK. Sleep quality was assessed (via the Pittsburg Sleep Quality Index (PSQI)) and blood samples were taken at baseline and again on the first and last day of WWT in both MT and AK. Plasma samples were then analyzed for (i) stress-related biomarkers (cortisol and adrenocorticotropic hormone (ACTH)), (ii) metabolic hormones (thyroid-stimulating hormone (TSH), triiodothyronine (T3), thyroxine (T4), and leptin), and (iii) growth-related hormones (total testosterone (TT), free testosterone (fT), dehydroepiandrosterone sulfate (DHEA-S), thyroid-stimulating hormone (TSH), insulin-like growth factor 1 (IGF-1), insulin-like growth factor binding protein 3 (IGFBP-3), and sex hormone-binding globulin (SHBG)). Results: Findings from the study showed alterations in stress-related and growth-related hormones, as well as cognitive performance throughout WWT, while metabolic hormones were unchanged. The variation in hormones between training locations may be reflective of travel time and distance from home station. The increase in stress hormones (i.e., ACTH and cortisol) and associated decrease in cognitive performance observed in AK suggest heightened physiological strain in AK compared to MT, despite similarities in training routine, protocol, and sleep quality. These data suggest that training in cold environments, similar to the WWT locations, may exacerbate the physiological strain experienced by operators, which may negatively impact warfighter health and performance. Therefore, an acclimatization period and additional recovery time may be warranted for cold-weather trainings.”