Polycystic ovary syndrome and risk of differentiated thyroid cancer: A nationwide, register-based cohort study based on Danish health data

Objectives: To evaluate the association between polycystic ovary syndrome (PCOS) and the risk of differentiated and papillary thyroid cancer. Methods: We included all individuals assigned female at birth and born during 1962–1996 in Denmark. Information on vital status, PCOS- and cancer diagnoses, and covariates were attained from various Danish nationwide registers. Personal identification numbers assigned to all individuals at birth are used throughout all registers and allows accurate individual-level data linkage. Using Cox regression analysis, we estimated hazard ratios (HRs) and 95 % confidence intervals (CI) for differentiated thyroid cancer (overall and separately for papillary thyroid cancer) according to diagnosis of PCOS. Results: The final study cohort comprised 990 850 individuals. During follow-up, we identified 980 individuals with incident differentiated thyroid cancer, of whom 15 were previously diagnosed with PCOS. We found no increased rate of differentiated thyroid cancer for individuals with PCOS (HR=1.52, 95 % CI:0.91–2.53). However, a positive association for women diagnosed with thyroid cancer within 10 years after PCOS diagnosis (HR=3.81, 95 % CI:1.90–7.66) compared to women without PCOS was seen. Of note, none of the individuals were diagnosed with differentiated thyroid cancer within the first three years following the PCOS diagnosis. Results were similar for papillary thyroid cancer. Conclusion: We found no association when investigating the association between PCOS and differentiated and papillary thyroid cancer overall, though we did find a positive association in the first 10 years after PCOS diagnosis. Even this large study was limited by the low number of cancer cases in exposed individuals.