Cold adaptation and thyroid hormone metabolism.

Resting oxygen consumption and energy expenditure is sensitive to slight alterations in thyroid function. This means that timing and magnitude of cold adaptation would to some extent depend on thyroid function. Local thyroid hormone metabolism is important for energy expenditure and dissipation of heat in special tissues. Recruitment of brown adipocytes and upregulation of uncoupling protein 1 in mitochondria depends on high tissue T3 concentrations. Most of this T3 is derived from local 5' deiodination of T4. Brown fat is vital for cold exposed mice and rats, and may be important for temperature adaptation in human neonates. The role of thyroid hormone metabolism in adult human cold adaptation has not been finally clarified. Hypothetically, cold exposure may enhance T3 production by deiodination of T4 in skeletal muscle, which may enhance heat production in muscle via a change in muscle fiber type. Another hypothetical possibility is recruitment of brown adipocytes embedded in white adipose tissue in human adults. Understanding cold adaptation in human adults may lead to development of new drugs against obesity.