Plasma free thyroxine in the upper quartile is associated with an increased incidence of major cardiovascular events in older men that do not have thyroid dysfunction according to conventional criteria

Background The aim of this study was to assess the association of plasma free thyroxine (FT4) and serum thyroid stimulating hormone (TSH) concentrations with the past diagnosis of cardiovascular disease and incidence of new cardiovascular events in older men with no known thyroid disease. Methods This study involved a cohort of community-recruited older men without known thyroid disease. Plasma FT4 and serum TSH were measured by immunoassay. Past cardiovascular disease diagnosis was defined through questionnaire data. The incidence of major cardiovascular events were assessed using the Western Australian Data Linkage System. The associations of plasma FT4 and serum TSH with the past diagnosis of cardiovascular disease and the incidence of new major cardiovascular events (cardiovascular death, myocardial infarction or stroke) were examined using logistic regression and Cox proportional hazard analyses. Results 3712 men were followed for a mean of 9.5 years. Men with plasma FT4 in the upper quartile, compared to other men, were more likely to have been previously diagnosed with cardiovascular disease but this association did not persist after adjustment for other risk factors. Men with plasma FT4 in the upper quartile, compared to other men, had an increased incidence of major cardiovascular events (adjusted hazard ratio, HR, 1.15, 95% CI 1.00–1.31) and myocardial infarction alone (adjusted HR 1.29, 95% CI 1.06–1.54). Conclusion This study suggests that older men with higher levels of plasma FT4 not meeting current criteria for the diagnosis of hyperthyroidism are at increased risk of major cardiovascular events.