Exposure to antithyroid drugs and ethylenethiourea and risk of thyroid cancer : a systematic review of the epidemiologic evidence

Introduction: The thyroid peroxidase inhibiting compounds methimazole, methylthiouracil, propylthiouracil, thiouracil (i.e. 'antithyroid' drugs) and ethylenethiourea have been associated to thyroid tumours in rodents. According to a systematic review by the International Agency for Research on Cancer (IARC) published in 2000, evidence for the human carcinogenicity was inadequate. Methods: We performed an up-to-date systematic review of human epidemiological studies on the association between such compounds and thyroid cancer incidence or mortality. Results: The literature research (1999-March 2020) identified four relevant articles. Considering also reports from the previous IARC review, this systematic review considered seven reports (five distinct studies) on antithyroid drugs and two on ethylenethiourea. As for antithyroid drugs, three reports based on different follow-ups gave results from a cohort of patients treated for hyperthyroidism in 1946-1964. In the earlier report, thyroid cancer incidence was higher in patients primarily treated with antithyroid drugs (3.2/1000) than in those originally treated with thyroidectomy (0.34/1000) or radioactive iodine (0.88/1000), which can be explained by the higher frequency of subsequent thyroidectomy, and hence the higher chance of cancer detection, in that group (30 vs. 0.5 and 1.2%). The two subsequent reports found no deaths from thyroid cancer among patients treated exclusively with antithyroid drugs through 1990 and 2014. A nested case-control study found an odds ratio (OR) of thyroid cancer of 2.79 [95% confidence interval (CI), 0.78-10.02, from a 2-year lag analysis] for ≥3 vs. no propylthiouracil prescriptions. The increased risk can be attributed to advanced diagnosis of an underlying cancer, as suggested by the stronger association observed in a no-lag analysis (OR, 8.03). In a historical cohort of newly diagnosed hyperthyroid patients, the hazard ratio for treatment with radioactive iodine vs. thionamides only was 0.45 (95% CI, 0.21-0.99), possibly due to the closer surveillance of patients receiving thionamides only. Two case-control studies did not find any association with the use of antithyroid drugs. As for ethylenethiourea, no thyroid cancer cases were found in a historical cohort of 1929 workers occupationally exposed in a 15-year period and no association with proxies of mancozeb exposure (a fungicide whose main metabolite is ethylenethiourea) was detected in a cohort of >236 000 farmers. Conclusion: There is no evidence for a relevant role of either antithyroid drugs or ethylenethiourea on thyroid cancer.