Background: Subclinical thyroid dysfunction is associated with increased mortality and cardiovascular risk. It is unknown whether this association remains within normal thyroid function range.
Methods: The study was conducted using the computerized database of the Sharon-Shomron district of Clalit Health services. Included were subjects aged ≥40 years with normal thyroid function. Patients with a history of thyroid or cardiovascular diseases or diabetes were excluded. The primary end points were all-cause mortality and the need for coronary revascularization with either percutaneous coronary intervention or coronary artery bypass grafting.
Results: The 42,149 participants were stratified into 3 groups of equal thyrotropin intervals (0.35-1.6, 1.7-2.9, and 3-4.2 mIU/L). During a mean follow-up of 4.5±2.1 years, 4239 (10.1%) participants died and 1575 (3.7%) underwent coronary revascularization. For both women and men, the lowest mortality rates were observed in the intermediate thyrotropin group. Nevertheless, only for the low thyrotropin group, mortality risk remained significantly higher as compared with the intermediate thyrotropin group, even following multivariate model adjusted for the conventional cardiovascular risk factors, in both women (odds ratio 1.22; 95% confidence interval, 1.09-1.36 for the low thyrotropin group, compared with the intermediate group) and men (odds ratio 1.14; 95% confidence interval, 1.01-1.3 for the low thyrotropin group, compared with the intermediate group). There was no significant difference in the need for coronary revascularization among the 3 thyrotropin groups in both men and women.
Conclusions: Low thyrotropin level within the reference range is associated with increased risk for all-cause mortality.
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