Thyroxin Use Is Associated With Increased Risk of Thyroid Cancer in Patients With Hypothyroidism.

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.