Subclinical Hypothyroidism and the Risk of Cardiovascular Disease and All-Cause Mortality: A Meta-Analysis of Prospective Cohort Studies

Objectives: To determine the impact of subclinical hypothyroidism (SCH) on the risk of cardiovascular disease (CVD) and all-cause mortality, a comprehensive meta-analysis was performed according to the age or coexisting CVD risk status of the participants. Methods: Studies regarding the association of SCH with all-cause mortality from PubMed and Embase databases were included. The pooled relative risk (RR) of CVD and all-cause mortality was calculated using the Mantel–Haenszel method. A subgroup analysis of participants with high CVD risk was conducted, including history of coronary, cerebral, or peripheral artery disease; dilated cardiomyopathy; heart failure; atrial fibrillation; venous thromboembolism; diabetes mellitus; or chronic kidney disease. Results: In total, 35 eligible articles incorporating 555,530 participants were included. SCH was modestly associated with CVD and all-cause mortality (RR for CVD = 1.33 [confidence interval (CI) 1.14–1.54]; RR for all-cause mortality = 1.20 [CI 1.07–1.34]). However, the association was not observed in participants aged ≥65 years. Subgroup analysis showed that participants with SCH and high CVD risk showed a significantly higher risk of all-cause mortality (RR for CVD = 2.20 [CI 1.28–3.77]; RR for all-cause mortality = 1.66 [CI 1.41–1.94]), whereas those with SCH and low CVD risk did not. Additional subgroup analysis of six studies with a mean participant age of ≥65 years and high CVD risk showed a significant high risk of all-cause mortality in the SCH group (RR = 1.41 [CI 1.08–1.85]; I 2 = 0%). Conclusions: SCH is associated with an increased CVD risk and all-cause mortality, particularly in participants with high CVD risk.