The interaction effect between BMI, diabetes and age at diabetes onset on the risk of thyroid cancer: A population-based cohort study in Shanghai, China.

[AIM] To determine the association of the presence of diabetes and, among persons with diabetes, the age at type 2 diabetes mellitus (T2DM) onset, BMI and the interactive effect with the subsequent thyroid cancer risk.

[MATERIALS AND METHODS] We conducted a population register-based longitudinal cohort study in Shanghai, including 428 568 persons with new-onset T2DM matched with the general population. The risk of thyroid cancer among subgroups was calculated based on standardized incidence ratio (SIR), hazard ratio (HR) and Cox proportional hazards models.

[RESULTS] In total, 1142 thyroid cancer cases were identified during 8 years of follow-up, with an incidence rate of 59.01/100 000 person-years and a higher risk (SIR = 1.21) compared with the general population. The earlier age at T2DM onset and higher BMI were associated with an increasing risk of thyroid cancer independently (onset age <50, SIR: 1.46; BMI ≥30.0 kg/m, SIR: 1.93), with the highest risk in patients with both BMI ≥30.0 kg/m and onset age <50 years (SIR = 3.91, HR = 3.04). Among patients with T2DM onset age <60 years, SIR increased with higher BMI, while there were no trends when onset age ≥60 years. Among patients with BMI ≥25.0 kg/m, SIR increased with an earlier onset age, whereas no trends were shown in the BMI <24.9 kg/m groups. Obese (BMI ≥30.0 kg/m) patients had a significantly higher HR of thyroid cancer only when T2DM onset age <60 years.

[CONCLUSIONS] Both earlier age of T2DM onset (<50 years) and higher BMI (≥30 kg/m) contributed to the higher risk of thyroid cancer. Patients with young-onset T2DM and obesity are considered more vulnerable to thyroid cancer development.