1. In this prospective cohort study, men and women with TSH levels in the highest tertile lived a few years longer than those in the lowest tertile, and lived more years without cardiovascular disease.
2. This relationship also existed with free T4 levels – the lower the T4 level the higher the life expectancy.
Evidence Rating Level: 2 (Good)
Study Rundown: There have been previous studies that look at variation of thyroid function and its association with cardiovascular disease (CVD) and mortality, however its relationship with life expectancy (LE) and number of years lived with and without CVD remains unclear. This prospective cohort study aimed to investigate the association of thyroid function with total LE and LE with and without CVD among euthyroid individuals.
Among middle-aged patients, over a follow-up of 8 years, men and women in the highest TSH tertile lived 2.0 and 1.4 years longer than those in the lowest TSH tertile. Of this, men in the lowest tertile lived 1.5 years longer without CVD, and women lived 0.9 years longer without CVD. Also, higher levels of free T4 were associated with lower life expectancy and fewer years without CVD. Strengths of this study included its population-based, prospective design. Limitations included the predominantly Caucasian population, which limits generalizability.
In-Depth [prospective cohort]: The Rotterdam Study was a population-based, prospective cohort study that was conducted from 1989 to 2006 using participants without known thyroid disease and with thyrotropin and free T4 levels within reference range. They were followed for a median of 8 years for the outcomes of cardiovascular events and death. Using life expectancy charts, the total life expectancy and life expectancy with and without CVD were calculated between TSH and free T4 tertiles.
Of 7785 participants where the median age was 64.7 (9.8) years, and 52.5% were women, there were 789 incident CVD events and 1357 deaths over the 8-years of follow-up. Men and women in the highest TSH tertile had longer life expectancies than those in the lowest TSH tertile: men lived 2.0 (95%CI 1.0-2.8) years longer of which 1.5 (95%CI 0.2-2.6) were years longer without CVD, women lived 1.4 (95%CI 0.2-2.4) years longer of which 0.9 (95%CI -0.2-2.0) were years longer without CVD. Men and women with lower free T4 levels also had higher differences in life expectancy than those with the highest free T4 levels.
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