PURPOSE: This article focuses on whether it is useful to order a thyroid function test in patients who have no history of thyroid disease and have few or no signs or symptoms of thyroid dysfunction. DATA SOURCES: A MEDLINE(r) search, supplemented by searches of EMBASE(r) and the Cochrane Library, reference lists, and a local database of thyroid-related articles. STUDY SELECTION: We selected controlled studies of treatment that used thyroid-stimulating hormone (TSH) levels as an inclusion criterion and reported quality of life, symptoms, or lipid level outcomes. We also reviewed observational studies of the prevalence, progression, and consequences of subclinical thyroid dysfunction. DATA EXTRACTION AND SYNTHESIS: Using preset criteria, we assessed the quality of each trial and abstracted information about its setting, patients, interventions, and outcomes. RESULTS: The prevalence of unsuspected thyroid disease is lowest in men and highest in older women. Evidence regarding the efficacy of treatment in patients found by screening to have subclinical thyroid dysfunction is inconclusive. Several small, randomized trials of treatment for subclinical hypothyroidism have been done, but the results are inconclusive except in patients who have a history of treatment for Graves' disease, a subgroup that is not a target of screening in the general population. No trials of treatment for subclinical hyperthyroidism have been done. Data on the adverse effects of broader use of levothyroxine is sparse. CONCLUSION: Large randomized trials of treatment are needed to determine the likelihood that treatment will improve quality of life in otherwise healthy patients who have abnormal TSH levels and normal thyroxine (T4) levels.