Abstract Purpose Although thyroid hormones are irrefutably implicated in cardiovascular physiology, the impact of within-reference range variations of thyroid function on cardiovascular disease (CVD) remains unclear. Elucidating this is important, since it could foster preventive treatment and reduce global CVD burden. We therefore investigated the impact of within-reference range variations of thyroid function on all-cause and cardiovascular mortality. Methods We included community-dwelling individuals aged 28–75 years from a prospective cohort study, without known use of thyroid-affecting therapy and with thyrotropin within reference range. Associations of thyroid function with mortality were quantified using Cox models and adjusted for sociodemographic and cardiovascular risk factors. Results Mean (SD) age of the 6,054 participants (52.0% male) was 53.3 (12.0) years. During 47,594 person-years of follow-up, we observed 380 deaths from all causes and 103 from CVDs. Although higher thyrotropin was not associated with all-cause mortality (adjusted HR 1.02, 95% CI 0.92–1.14), point estimates for cardiovascular mortality diverged toward increased risk in younger (<72 years) participants (1.31, 1.00–1.72) and decreased risk in elderly (≥72 years) (0.77, 0.56–1.06). Higher free thyroxine (FT₄) was associated with all-cause mortality (1.18, 1.07–1.30) and with cardiovascular mortality only in elderly (1.61, 1.19–2.18), but not in younger participants (1.03, 0.78–1.34). Higher free triiodothyronine (FT₃) was associated with all-cause mortality in females only (1.18, 1.02–1.35). FT₃ was not associated with cardiovascular mortality (0.91, 0.70–1.18). Conclusions Community-dwelling elderly individuals with high-normal thyroid function are at increased risk of all-cause and cardiovascular mortality, reinforcing the need of redefining the current reference ranges of thyroid function.