Treatment with human insulin does not increase thyroid cancer risk in patients with type 2 diabetes

Background Whether human insulin therapy may increase thyroid cancer risk in patients with type 2 diabetes mellitus (T2DM) has not been investigated. Materials and methods The reimbursement databases of all Taiwanese diabetic patients from 1996 to 2009 were retrieved from the Bureau of National Health Insurance. The entry date was set at 1 January 2004, and 968 384 patients with T2DM were followed up for thyroid cancer incidence until the end of 2009. Ever-users, never-users and subgroups of human insulin exposure (using tertile cut-offs of time since starting insulin, duration of therapy and cumulative dose) at entry date were calculated for thyroid cancer incidence. Insulin glargine was not marketed until after the entry date. Therefore, to exclude the potential contamination of insulin glargine, patients who happened to use insulin glargine were censored at the time of its initiation when calculating the period of follow-up. Hazard ratios were estimated by Cox regression. Results There were 111 121 ever-users and 857 263 never-users of human insulin, with respective numbers of incident thyroid cancer of 118 (0.11%) and 1047 (0.12%), and respective incidences of 23.9 and 23.8 per 100 000 person-years. The overall hazard ratios (95% confidence intervals) did not show a significant association with human insulin in either the age-sex-adjusted or the fully adjusted model: 0.942 (0.778-1.141) and 1.096 (0.888-1.353), respectively. When categorized into tertiles of the dose-response parameters, none of the hazard ratios was significant. Conclusions This study does not support the role of human insulin therapy in increasing the risk of thyroid cancer in patients with T2DM.