Collet et al combined individual data on 52 674 participants from 10 prospective cohort studies in the United States, Europe, Brazil, and Australia. At baseline, 2188 (4.2%) participants had subclinical hyperthyroidism. During follow-up, 8527 participants died (including 1896 from coronary heart disease [CHD]), 3653 had CHD events, and 785 developed atrial fibrillation (AF). In age- and sex-adjusted analyses (hazard ratio; 95% CI), subclinical hyperthyroidism was associated with increased total mortality (1.24; 1.06-1.46), CHD mortality (1.29; 1.02-1.62), CHD events (1.21; 0.99-1.46), and AF (1.68; 1.16-2.43). Risks for CHD mortality and AF increased with lower thyrotropin levels, and were highest among those with a thyrotropin level lower than 0.10 mIU/L (for both, P value for trend, ≤.03). These findings of increased risks of total mortality, CHD mortality, and AF associated with subclinical hyperthyroidism, with greater risks of CHD mortality and AF among those with a thyrotropin level lower than 0.10 mIU/L, are consistent with recent treatment guidelines for subclinical hyperthyroidism.