I am interested in understanding the biochemical adaptations involved in energy expenditure that are mediated by thyroid hormone activation. Thyroxine (T4) is a pro-hormone secreted by thyroid gland, which must be activated by deiodination to T3 in order to initiate thyroid action. Its activation is catalyzed by deiodinase 1 and 2 (D1/D2). D2 is considered a critical homeostatic T3-producing deiodinase during adaptation to cold exposure and the enzyme activation seems to be part of a thermogenic program in mouse brown adipocytes. My current research aim is to investigate the biochemical adaptations that are dependent of the thyroid hormone activation. To address this point I am using two models: (i) Goldfish, a fish that is able to survive when exposed at low temperature and in which the importance of thyroid hormone activation is unclear; and (ii) mice, that are used to identify new activators of the D2 enzyme in brown adipocytes and also to search the physiological co-factors of D2 activity. The latter is useful to understand the participation of D2 in certain diseases characterized by disturbances in energy expenditure, such as diet-induced obesity, fasting and type 2 diabetes.