Thyroid function and cancer risk: a prospective population study.

BACKGROUND It has been hypothesized that thyroid function may influence cancer risk, but few studies with adequate statistical power have investigated this question, and the results have not been consistent. METHODS In a prospective study of 29,691 people (19,710 women and 9,981 men) without previously known thyroid disease, thyrotropin was measured at baseline, and cancer incidence was recorded during 9 years of follow-up. Using Cox regression analysis, we studied the associations (hazard ratios) of thyrotropin categories with total cancer risk, and specifically, with risk of lung, colon, prostate, and breast cancer adjusted for age, sex, and smoking status. RESULTS Low thyrotropin levels (<0.50 mU/L) were associated with increased cancer risk [adjusted hazard ratio (HR), 1.34; 95% confidence interval (CI), 1.06-1.69] compared with the euthyroid reference group. The higher risk was driven by lung cancer (adjusted HR, 2.34; 95% CI, 1.24-4.40) and prostate cancer (adjusted HR, 1.97; 95% CI, 1.04-3.76). After excluding the first 2 years of follow-up, the associations were strengthened to 2.91 (1.49-5.70) for lung cancer and 2.60 (1.36-4.99) for prostate cancer. CONCLUSION Thyrotropin levels suggestive of hyperthyroid function are associated with increased cancer risk, and specifically, with increased risk of lung and prostate cancer, whereas hypothyroid function does not seem to be associated with cancer risk.