Subclinical Hyperthyroidism and the Risk of Coronary Heart Disease and Mortality

Abstract

Subclinical hyperthyroidism, defined by low thyrotropin level with normal concentrations of free thyroxine (FT₄) and triiodothyronine (T₃),^1-4 has been associated with several biological effects on cardiovascular system, such as increased heart rate, left ventricular mass, carotid intima-media thickness, and plasma fibrinogen levels.^3,5 Observational studies have reported an association between subclinical hyperthyroidism and coronary heart disease (CHD),^6-8 incident atrial fibrillation (AF),^9-12 and cardiac dysfunction.^13,14 Results from prospective cohort studies are conflicting,^6,10 and study-level meta-analyses have reached contradictory conclusions, for example, regarding the association between subclinical hyperthyroidism and cardiovascular mortality.^15-17 In fact, interpretation of these studies is hampered by several methodological factors: population heterogeneity, different thyrotropin cutoff levels for subclinical hyperthyroidism definition, different use of covariates, and different CHD definitions.¹⁶ Although no large randomized controlled trials have examined the effects of treating subclinical hyperthyroidism on clinically relevant outcomes, a consensus statement² and recent guidelines⁴ advocate treatment of subclinical hyperthyroidism, particularly when thyrotropin level is lower than 0.10 mIU/L, to avoid long-term complications.