This study examined the effect of prolonged physical activity in a cold environment upon circulating thyroid hormone levels. A secondary focus of the study involved the role of nocturnal habitat upon the thyroidal responses to the physical activity and cold exposure. Military personnel exposed to 10 d of field-based operations in the arctic region of Norway were studied. Blood samples were collected before (day 1), and at days 5 and 10 of the operations. Levels of total T4, free T4, total T3, free T3, and thyroid binding globulin were assessed in all blood samples. The results indicated considerable, significant (p less than 0.05) decreases in total thyroid hormone levels, while relative increases of free fractions of the hormones occur with the 10-d operations in the Arctic. However, no significant influence on thyroidal responses were observed due to the nocturnal habitat that the subjects lived in. The hormonal alterations noted are possibly brought about by the combined effects of physical activity and cold exposure acting synergistically to alter thyroid physiology (e.g., most likely the protein carrier binding affinity).