Thyroid hormone levels are a major determinant of energy balance and are thought to modify body composition by their effects on metabolism of lipids, carbohydrate and protein. The present study evaluates changes of body composition and basal energy expenditure (BEE) in thyroidectomized patients studied during short-term profound hypothyroidism while off all thyroid hormone before diagnostic whole-body (131)I-imaging and while on thyrotropin-suppressive thyroxine therapy. Basal energy expenditure was assessed by indirect calorimetry, and four-point body impedance analysis was used to estimate body composition. Patients were compared with healthy controls matched with respect to sex, age, height and weight. Compared to healthy controls the percentages of body water and body cell mass were significantly lower while the percentage of fat was significantly higher in patients during short-term hypothyroidism. Weight did not change significantly when patients were put on thyroxine treatment, but body fat (-0.95 +/- 2.25 kg, p < 0.01) decreased while body water (+0.94 +/- 1.31 kg, p < 0.01) and body cell mass (+0.9 +/- 2.5 kg, p < 0.05) increased. With thyroxine replacement, body composition was not significantly different between patients and controls. Compared to healthy controls, BEE was significantly lower in patients without thyroxine replacement (5265 +/- 766 kJ/24h vs 6362 +/- 992 kJ/24h; p < 0.001). With thyroxine treatment, BEE increased (6492 +/- 967 kJ/24h) but was not significantly different from the controls (p > 0.05). Neither body composition nor BEE was significantly different in a subgroup of thyroxine-treated patients with free triiodothyronine or thyroxine values above the normal range. In conclusion, both body composition and energy expenditure showed significant changes when patients were deprived of thyroid hormone. However, no evidence of excess metabolic effects of thyroid hormone during thyrotropin-suppressive thyroxine therapy was found.