Many differences in thyroid function exist between elderly and younger populations. Although serum T4 levels probably do not change with age, serum T3 levels appear to decline. Hyperthyroidism in the geriatric population may be atypical and is characterized by anorexia and constipation. The pulse rate is often slower than in younger patients. Apathetic hyperthyroidism mainly occurs in older patients. The cause of hyperthyroidism is usually toxic multinodular goiter. Isolated T3 or T4 elevations may be seen. RAIU is often normal. Hypothyroidism is common in the elderly. TSH is a reliable indicator, but the significance of mild elevations (less than 20 microU/ml) is unclear. Serum antithyroid antibodies are unreliable in the definitive diagnosis of hypothyroidism. Acute and chronic illnesses occur frequently in older patients and have varied and important effects on thyroid function tests. Low T3 and both low T4 and low T3 are seen. High T4 syndrome may be more common in older patients than in younger patients. It is also seen in psychiatric populations. TSH levels are usually normal but may be mildly elevated.