Hyperthyroidism and hypothyroidism are common conditions that have lifelong effects on health. About 5% of U.S. adults report having thyroid disease or taking thyroid medication. In a cross-sectional study of 2,799 well-functioning adults aged 70-79, 9.7% of black women, 6% of white women, 3.2% of black men, and 2.2% of white men reported a history of hyperthyroidism. In the same study, 6.2% of black women, 16.5% of white women, 1.7% of black men, and 5.6% of white men reported a history of hypothyroidism. Hyperthyroidism has several causes. Graves' disease, the most common intrinsic cause, is an autoimmune disorder associated with the development of long-acting thyroid stimulating antibodies (LATS). Single or multiple thyroid nodules that produce thyroid hormones can also cause hyperthyroidism. The use of excessive doses of the thyroid hormone supplement levothyroxine is also a common cause. The most common cause of hypothyroidism is thyroiditis due to antithyroid antibodies, a condition called "Hashimoto's thyroiditis." Another common cause of hypothyroidism is prior treatment for Graves' disease with surgery or radioiodine. Consequences of untreated hyperthyroidism include atrial fibrillation, congestive heart failure, osteoporosis, and neuropsychiatric disorders. Both hyperthyroidism and hypothyroidism cause symptoms that reduce functional status and quality of life. Subclinical thyroid dysfunction, which can be diagnosed by thyroid function tests before symptoms and complications occur, is viewed as a risk factor for developing hyperthyroidism and hypothyroidism complications. The goal of screening is to identify and treat patients with subclinical thyroid dysfunction before they develop these complications. This report focuses on whether it is useful to order a thyroid function test for patients who have no history of thyroid disease when they are seen by a primary care clinician for other reasons. The review was used by 2 expert panels: the U.S. Preventive Services Task Force (USPSTF), which will make recommendations regarding screening in the general adult population, and the Institute of Medicine, which will focus on the Medicare population.