How Do Thyroid Levels Impact Life Expectancy?

BACKGROUND AND PURPOSE:

Thyroid dysfunction: Thyrotropin and free thyroxine (FT4) levels outside the reference ranges
Subclinical thyroid dysfunction: Thyrotropin levels outside the reference range but FT4 levels within the reference range
Cut-offs are arbitrary and set at 2.5^th and 97.5^th %tiles of normal populations but does not account for clinical outcomes
Both thyroid and subclinical thyroid dysfunction associated with increased mortality, coronary heart disease and heart failure
Clinical outcomes for patients who are considered ‘euthyroid’ remain unclear
Bano et al. (JAMA, 2017) investigated the association between thyroid function and life expectancy and CVD among euthyroid individuals

Population based, prospective cohort study
Participants had known thyroid disease and thyrotropin and free thyroxine (FT₄) levels within the reference range
Multistate life tables were used to calculate total life expectancy and life expectancy +/- CVD, stratified by 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

The mean (SD) age of the 7,785 participants was 64.7 (9.8) years, and 52.5% were women
Median follow-up of 8.1 years
Compared with those in the lowest tertile, those in the highest thyrotropin tertile lived longer
- Women
  - 1.4 (95% CI 0.2 to 2.4) years
  - 0.9 (95% CI −0.2 to 2.0) years without CVD
- Men
  - 2.0 (95% CI 1.0 to 2.8) years
  - 1.5 (95% CI 0.2 to 2.6) years without CVD
- Compared with those in the lowest tertile, those in the highest FT4 tertile did not live as long
  - Women
    - −3.5 (95% CI, −5.6 to −1.5) years
    - −2.5 (95% CI, −4.4 to −0.7) years without CVD
  - Men
    - −3.2 (95% CI, −5.0 to −1.4) years
    - −3.1 (95% CI, −4.9 to −1.4) years without CVD

Individuals 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
Authors suggest low-normal thyroid function may promote energy conservation or an association with a heritable phenotype associated with longevity
Known deleterious effects of high thyroid function may extend into the high-normal range
The authors recommend that guidance committees reevaluate current thyroid function reference ranges