While classic hypothyroidism (myxedema) is easily recognized, subclinical hypothyroid cases may present no specific diagnostic symptoms. Yet borderline hypothyroidism is a common cause of general debility, retarded bone maturation, ovarian and uterine dysfunction, fatigue, and cardiac and mental disorders. The level of thyroid activity can be determined by the protein-bound iodine (PBI) test, a specific and convenient method of measuring serum protein-bound iodine. The optimal dosage of thyroid medication may be established simply by increasing the intake until a normal PBI level is obtained. Maintenance therapy is then continued through the patient's life, subject, of course, to any dosage modifications indicated by periodic laboratory checks. The written review of this film appears on page 1310 of this issue.