Despite evidence linking thyroxin use and breast cancer, little is known regarding the risk of other cancers with thyroxin use. The purpose of this case-control study was to evaluate the association of thyroid cancer with primary hypothyroidism based on a population-based database. The data for this case-control study were retrieved from the Taiwan Longitudinal Health Insurance Database 2005. We included 1285 patients with thyroid cancer as cases and 3855 sex- and age-matched subjects as controls. We used conditional logistic regression to examine the association of thyroid cancer with previously diagnosed hypothyroidism. We found that a prior hypothyroidism diagnosis was found among 37 cases (2.88%) and 33 controls (0.86%). Conditional logistic regression analysis revealed that the cases were more likely to have been previously diagnosed with hypothyroidism than controls (adjusted OR, 3.01; 95%CI, 1.85-4.89). We also found that thyroid cancer was significantly associated with hypothyroidism patients who were regular thyroxin users (adjusted OR, 8.68; 95%CI, 4.34-17.34). However, we failed to observe a significant association between thyroid cancer and hypothyroidism patients who were irregular thyroxin users. This investigation detected a novel association between thyroid cancer and thyroxin use in patients with primary hypothyroidism.
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