Importance: Variations in thyroid function within reference ranges are associated with an increased risk of cardiovascular disease (CVD) and mortality. However, the impact of thyroid function on life expectancy (LE) and the number of years lived with and without CVD remains unknown.
Objective: To investigate the association of thyroid function with total LE and LE with and without CVD among euthyroid individuals.
Design, setting, and participants: The Rotterdam Study, a population-based, prospective cohort study. We included participants without known thyroid disease and with thyrotropin and free thyroxine (FT4) levels within the reference ranges.
Main outcomes and measures: Multistate life tables were used to calculate total LE and LE with and without CVD among thyrotropin and FT4 tertiles. Life expectancy estimates in men and women aged 50 years and older were obtained using prevalence, incidence rates, and hazard ratios for 3 transitions (healthy to CVD, healthy to death, and CVD to death), adjusting for sociodemographic and cardiovascular risk factors.
Results: The mean (SD) age of the 7785 participants was 64.7 (9.8) years, and 52.5% were women. Over a median follow-up of 8.1 (interquartile range, 2.7-9.9) years, we observed 789 incident CVD events and 1357 deaths. Compared with those in the lowest tertile, men and women in the highest thyrotropin tertile lived 2.0 (95% CI, 1.0 to 2.8) and 1.4 (95% CI, 0.2 to 2.4) years longer, respectively, of which, 1.5 (95% CI, 0.2 to 2.6) and 0.9 (95% CI, -0.2 to 2.0) years longer without CVD. Compared with those in the lowest tertile, the difference in life expectancy for men and women in the highest FT4 tertile was -3.2 (95% CI, -5.0 to -1.4) and -3.5 (95% CI, -5.6 to -1.5) years, respectively, of which, -3.1 (95% CI, -4.9 to -1.4) and -2.5 (95% CI, -4.4 to -0.7) years without CVD.
Conclusions and relevance: At the age of 50 years, participants with low-normal thyroid function live up to 3.5 years longer overall and up to 3.1 years longer without CVD than participants with high-normal thyroid function. These findings provide supporting evidence for a reevaluation of the current reference ranges of thyroid function and can help inform preventive and clinical care.