Hypothyroidism is not rare in aged people. Hashimoto thyroiditis and radiation to the neck region are main causes of hypothyroidism in aged subjects. The symptoms are slowly progressing, and are similar to those of the aged subjects free from thyroid disease. Thus, it is difficult to make a diagnosis of hypo-thyroidism in the elderly. Administration of T4 (not T 3) to patients with hypothyroidism completely releases them from symptoms, indicating that supplement with T4 is important in maintaining QOL. However, a risk of acute coronary syndrome is usually associated with the supplement. Rapid decrease in serum levels of TBG is frequently associated with acute coronary syndrome. Slowly increasing the dose of T4 and monitoring serum thyroxine-binding globulin (TBG) and electrocardiogram (ECG) are important during the period of increasing the dose of T4.