Subclinical thyroid dysfunction occurs when peripheral thyroid hormone levels are within the normal laboratory reference range and the serum thyroid-stimulating hormone (TSH or thyrotropin) level is greater than normal (subclinical hypothyroidism) or less than normal (subclinical hyperthyroidism; TSH normal laboratory reference range: 0.3-5.0 mIU/l). For patients with subclinical hypothyroidism (serum TSH levels >10 mIU/l), thyroxine therapy is prescribed if other causes of TSH elevation and transient conditions have been excluded. For serum TSH levels between 5.0 and 10.0 mIU/l, selective therapy should be considered. For patients with sustained subclinical hyperthyroidism (serum TSH levels <0.1 mIU/l), therapy is recommended, especially in older patients. Observation or selective therapy should be considered for patients with serum TSH levels between 0.1 and 0.3 mIU/l.