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Background: In 2004, the U.S. Preventive Services Task Force found insufficient evidence to recommend thyroid screening. Purpose: To update the 2004 U.S. Preventive Services Task Force review on the benefits and harms of screening and treatment of subclinical and undiagnosed overt hypothyroidism and hyperthyroidism in adults without goiter or thyroid nodules. Data Sources: MEDLINE and Cochrane databases through July 2014. Study Selection: Randomized, controlled trials and observational studies of screening and treatment. Data Extraction: One investigator abstracted data, and a second investigator confirmed; 2 investigators independently assessed study quality. Data Synthesis: No study directly assessed benefits and harms of screening versus no screening. For subclinical hypothyroidism (based on thyroid-stimulating hormone levels of 4.1 to 11.0 mIU/ L), 1 fair-quality cohort study found that treatment of subclinical hypothyroidism was associated with decreased risk for coronary heart disease events versus no treatment. No study found that treatment was associated with improved quality of life, cognitive function, blood pressure, or body mass index versus no treatment. Effects of treatment versus no treatment showed potential beneficial effects on lipid levels, but effects were inconsistent, not statistically significant in most studies, and of uncertain clinical significance (difference, -0.7 to 0 mmol/L [-28 to 0 mg/dL] for total cholesterol levels and -0.6 to 0.1 mmol/L [-22 to 2 mg/dL] for low-density lipoprotein cholesterol levels). Treatment harms were poorly studied and sparsely reported. Two poor-quality studies evaluated treatment of subclinical hyperthyroidism but examined intermediate outcomes. No study evaluated treatment versus no treatment of screen-detected, undiagnosed overt thyroid dysfunction. Limitation: English-language articles only, no treatment study performed in the United States, and small trials with short duration that used different dosage protocols. Conclusion: More research is needed to determine the clinical benefits associated with thyroid screening. [ABSTRACT FROM AUTHOR]