We found biochemical evidence of hypothyroidism (low T3 and T4 and elevated TSH) in azotemic patients. Hemodialysis did not correct the hypothyroidism but renal transplantation did. The hypothyroidism is probably a consequence of impaired thyroidal elaboration of thyroid hormone, as well as decreased peripheral conversion of T4 to T3. The impaired thyroidal gland function may be secondary to the increased concentration of aryl acids found in chronic renal disease.