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Low-normal thyroid function increases life expectancy vs. high-normal function

In comparison to individuals with high-normal thyroid function, those with low-normal thyroid function lived up to 3.5 years longer overall and up to 3.1 years longer without CVD, according to findings published in JAMA Internal Medicine.

“Variations in thyroid function within reference ranges are associated with an increased risk of CVD and mortality,” Arjola Bano, MD, MSc, DSc, from the department of internal medicine at Erasmus Medical Center, and colleagues wrote. “However, the impact of thyroid function on life expectancy and the number of years lived with and without CVD remains unknown.”

Bano and colleagues reviewed data from the Rotterdam Study to determine if there are differences in total life expectancy and life expectancy with and without CVD in individuals aged 50 years and older (n = 7,785; mean age, 64.7 years; 52.5% women) using multistate life tables. Participants did not have known thyroid disease, and had thyrotropin and free thyroxine (FT4) levels within the reference ranges.

The researchers estimated life expectancy using prevalence, incidence rates and HRs for transitions from healthy to CVD, healthy to death, and CVD to death. They adjusted for sociodemographic and cardiovascular risk factors.

During follow-up (median, 8.1 years), 789 incident CVD events and 1,357 deaths occurred.

Men in the highest thyrotropin tertile lived 1.5 (95% CI, 0.2-2.6) years longer without CVD, 0.5 (95% CI, –0.5 to 1.4) years longer with CVD and 2 (95% CI, 1-2.8) years longer overall, compared with those in the lowest tertile. Women in the highest thyrotropin tertile lived 0.9 (95% CI, –0.2 to 2) years longer without CVD, 0.5 (95% CI, –0.5 to 1.2) years longer with CVD and 1.4 (95% CI, 0.2-2.4) years longer overall compared with those in the lowest tertile.

Among men in the highest FT4 tertile, the difference in life expectancy was –3.1 (95% CI, –4.9 to –1.4) years without CVD, –0.1 (95% CI, –1.7 to 1.6) years with CVD and –3.2 (95% CI, –5 to –1.4) years overall compared with those in the lowest tertile. Among women in the highest FT4 tertile, the difference in life expectancy was –2.5 (95% CI, –4.4 to –0.7) years without CVD, –1 (95% CI, –2.4 to 0.4) years with CVD and –3.5 (95% CI, –5.6 to –1.5) years overall compared with those in the lowest tertile.

“Our findings support a reevaluation of the current reference ranges of thyrotropin and FT4 measurements, implying the possibility of an upward shift of thyrotropin and a downward shift of FT4 current limits in middle-aged and elderly people,” Bano and colleagues concluded. “Future research is needed to replicate our findings and elucidate the exact mechanisms underlying the [life expectancy] differences within the reference range of thyroid function.” – by Alaina Tedesco

Disclosure: Bano reports no relevant financial disclosures. Please see the study for all other researchers’ relevant financial disclosures.