¹³¹I treatment for thyroid cancer and risk of developing primary hyperparathyroidism: A cohort study

Purpose: To evaluate the association between ¹³¹I therapy for thyroid cancer and risk of developing primary hyperparathy roidism. Methods: This was a nationwide population-based cohort study of patients with thyroid cancer diagnosed during the period 1997-2008. The data were obtained from the Taiwan National Health Insurance Research dataset. The cumulative ¹³¹I dose in each patient was calculated. Hazard ratios (HRs) were calculated using a proportional hazards model to estimate the effect of ¹³¹I therapy on the risk of developing primary hyperparathyroidism in the cohort. Results: A total of 8,946 patients with thyroid cancer were eligible for the final analysis. Among these patients, 8 developed primary hyperparathyroidism during the follow-up period that represented 38,248 person-years giving an incidence rate of 20.9 per 10⁵ person-years. ¹³¹I was used in the treatment of 6,153 patients (68.8 %) with a median cumulative dose of 3.7 GBq. The adjusted HRs were 0.21 (95% CI 0.02-1.86) and 0.46 (95% CI 0.10-2.10) for those receiving a cumulative ¹³¹I dose of 0.1-3.6 GBq and ≥3.7 GBq, respectively, compared to no therapy. The risk of developing primary hyperparathyroidism did not increase with increasing ¹³¹I dose (test for trend p =0.51). No interaction was found between ¹³¹I dose and age (p =0.94) or ¹³¹I dose and sex (p = 0.99). Conclusion: ¹³¹I treatment for thyroid cancer did not increase risk of primary hyperparathyroidism during a 10-year follow-up in this study population. Further research with a longer follow-up period is needed to assess late adverse effects beyond 10 years.