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

METHODS:

• 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

RESULTS:

• 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

CONCLUSION:

• 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

Learn More – Primary Sources:

Association of Thyroid Function With Life Expectancy With and Without Cardiovascular Disease

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