Sixteen patients with typical signs and symptoms of anorexia nervosa were studied with measurement of serum thyroxine (T4), triiodothyronine (T3) and thyrotropin (TSH), both baseline and stimulated by thyrotropin-releasing hormone (TRH). The results of the patients were compared with those of 16 normal control subjects. Serum T4 (5.8 plus or minus 0.26 mug/100 ml, mean plus or minus SE) and T3 (82 plus or minus 5.7 ng/100 ml) of patients with anorexia nervosa were significantly (P less than 0.001) lower than those of control subjects (T4 7.7 plus or minus 0.32 mug/100 ml and T3 158 plus or minus 4.7 ng/100 ml respectively). Furthermore, the ratio of T3/T4 (1.48 plus or minus 0.243 x 10(-2)) in anorexia nervosa was also lower than that of control subjects (2.21 plus or minus 0.093 x 10(-2)) (P less than 0.001). Basal serum TSH was within normal or below the limits of detection. TSH and T3 rose after administration of TRH. The peak values of TSH were observed after 60 to 12o min, instead of 30 min normally seen after TRH injection.