Association Between Serum Thyrotropin Levels and Mortality Among Euthyroid Adults in the United States

Background: Low thyroid function within the euthyroid range, as well as overt and subclinical hypothyroidism, reportedly increases the risk of cardiovascular disease and mortality. However, the association between low normal thyroid function and mortality remains controversial. This study was performed to elucidate the association between low normal thyroid function and all-cause and/or cause-specific mortalities among U.S. adults. Methods: A prospective cohort study was conducted using a nationally representative sample of 12,584 U.S. adults aged 20 years with thyrotropin (TSH) levels within the reference range from the National Health and Nutrition Examination Survey (NHANES) III (1988-1994). Associations between TSH tertiles (high, medium, and low normal TSH groups) and mortalities (all-cause, cardiovascular, and cancer) were investigated using multivariable Cox models. Stratum-specific analyses were estimated within subgroups defined according to sex (male or female) and baseline age (age <60 years or age 60 years). Further, the same analysis was conducted using continuous NHANES 2001-2002, 2007-2008, and 2009-2010 cohorts, which included data on free thyroxine levels. Results: The median follow-up period was 19.1 years, with 3395 all-cause deaths. A significantly higher risk of all-cause mortality (adjusted hazard ratio [HR] 1.27; [confidence interval (CI) 1.10-1.47]), and cardiovascular mortality (HR 1.30 [CI 1.02-1.67]), and cancer mortality (HR 1.43 [CI 1.01-2.01]) was observed in the high normal TSH group than in the medium normal TSH group. Additionally, the low normal TSH group had an increased risk of all-cause mortality. In stratum-specific analyses, a significant association was found between high normal TSH levels and all-cause mortality among males, females, and participants <60 years. Continuous NHANES cohorts demonstrated a non-significant increase in the HR for all-cause mortality in the high normal TSH group. Conclusions: High normal TSH levels compared with medium normal TSH levels were associated with increased risk of all-cause, cardiovascular, and cancer mortalities over a long-term follow-up period among U.S. adults. This study indicates that the reference range for TSH levels may require re-evaluation.