We studied plasma concentrations of TSH (basal and after TRH), thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (reverse T3; rT3), free T4 and free T3 in thirty obese subjects, twenty patients with anorexia nervosa, fifteen malnourished subjects and twenty normal weight subjects. Total serum T4 values were similar for the four groups of subjects while serum free T4 values were slightly increased in anorexia nervosa and normal in the other groups. Serum total and free T3 levels were both significantly decreased in anorexia nervosa and malnutrition, and within normal limits in obesity. The mean serum rT3 level was increased in anorexia nervosa and malnutrition while was reduced in obesity. A delay in peak response of TSH to TRH stimulation (30' rather than 20') was noted in anorexia and malnourished patients. The results suggest that these alterations of serum iodothyronines are due to a different peripheral conversion of T4 to T3 according to nutritional status.