It has long been accepted that thyroid hormone is an important determinant of overall energy expenditure and the basal metabolic rate. Indeed, regulating thermogenesis is one of the major tasks of thyroid hormone in adult humans. A wealth of data have demonstrated the effects of thyroid hormone on cellular processes involved with energy expenditure, yet in spite of this body of work it remains unclear which 3,3′-triiodothyronine–responsive energetic processes are most relevant for the determination of the basal metabolic rate. Recently, a novel metabolic role for thyroid hormone has been recognized based on the observation that bile acids can activate local production of thyroid hormone via induction of the type 2 deiodinase. Nevertheless, more work must be done before it can be fully explained how thyroid hormone determines the metabolic rate.