Triiodothyronine has been recommended in the treatment of "metabolic insufficiency," a syndrome characterized by a low basal metabolic rate, a normal level of serum protein-bound iodine, and a normal thyroidal uptake of radioactive iodine. Only 22 patients fulfilling the foregoing criteria were found in a busy out-patient department during the year of investigation; 18 (11 females and 7 males) were available for detailed study. The average age was 35 years. The most common complaints were nervousness, fatique and weakness. In 10 patients, the Minnesota Multiphasic Personality Inventory test showed a high degree of abnormality, with responses suggesting depressive, hypochondriacal and schizoid traits. The basal metabolic rate averaged - 21per cent, the serum protein-bound iodine level 5.2 [mu]g per 100 ml., and the thyroidal I131 uptake 23 per cent. Thyroxine (0.1 mg) tri-iodothyronine (.025 mg) and lactose were given in the form of coded tablets which were identical in appearance. There identity was not disclosed until after completion of the study. Patients assigned to treatment groups by rotation received one tablet daily for one week, two tablets daily for the next two weeks. No significant differences in basal metabolic rates or in symptoms were observed during treatment. Subsequently, the medication in 8 patients was changed, without their knowledge, to the second and then the third drug. The correlation between the type of therapeutic agent used, the basal metabolic rate, and the patient s complaints was poor. Therefore, no support for the specific use of triiodothyronine in the metabolic insufficiency syndrome was demonstrated.