Background: It is still discussed if subclinical hyperthyroidism and high-normal thyroid function are risk-factors for cardiovascular mortality.
Objectives: To examine the risk of cardiovascular and all-cause mortality in relation to subclinical hyperthyroidism.
Methods: Patients consulting their general practitioner from 2000 to 2009 in Copenhagen, Denmark, who underwent thyroid blood tests, were identified by individual-level linkage of nationwide registries. Patients with a history of thyroid disease or related medication were excluded. Risk of cardiovascular mortality was analyzed using KaplanMeier curves and Poisson regression models were applied to estimate Incidence Rate Ratios (IRR).
Results: Of 574 595 included individuals (mean age 48.7 years (S.D. ±18.3); 39.1% males) 550 927 (95.9%) were euthyroid, 1603 (0.3%) had overt hypothyroidism, 11 834 (2.1%) subclinical hypothyroidism, 3967 (0.7%) overt hyperthyroidism and 6264 (1.1%) subclinical hyperthyroidism. Increased risk of cardiovascular mortality was found in two levels of subclinical hyperthyroidism (TSH <0.1, 0.10.2 mU/l and normal free thyroixine): IRR 1.24 (95% CI: 1.091.41), IRR 1.21 (1.091.34) and in high-normal levels of euthyroidism (TSH 0.20.4 mU/l): IRR 1.21 (1.131.29).
Conclusions: Subclinical hyperthyroidism and high-normal thyroid function are significant risk-factors for cardiovascular and all-cause mortality.