The cost-effectiveness of thyroid function tests (serum thyroxine concentration, triiodothyronine [T3]-resin uptake, free thyroxine index, serum T3, and serum thyrotropin concentration) was assessed in 135 ambulatory patients suspected of hypothyroidism or hyperthyroidism who did not have a history of thyroid disease requiring medication or thyroid surgery within the preceding two years. Of patients with five or more signs and symptoms compatible with thyroid dysfunction, 50.0% had biochemical abnormalities substantiating hypothyroidism or hyperthyroidism, while only 1.5% of patients with fewer than two signs and symptoms had either disease. The cost of thyroid function tests was twice as much per patient evaluated by residents as for those evaluated by faculty physicians. These results suggest that interventions to reduce the number and type of tests in patients without multiple signs and symptoms of thyroid disease could improve the cost-effective use of these tests.