In patients with hyperthyroidism, treatment with radioiodine is not associated with incident cancer in long-term follow-up, according to study results published in Thyroid.

The study included patients from the Clalit healthcare database in Israel who had a new diagnosis of hyperthyroidism between 2002 and 2015 that was newly treated with radioactive iodine or thionamides (propylthiouracil/thiamazole). Patients with previous malignancies were excluded from the study.

To calculate propensity score to receive radioiodine, the researchers used age, sex, smoking history, body mass index, Clalit district, socioeconomic status, history of diabetes and/or hypertension, use of aspirin and statins, and adherence to cancer screening procedures. They determined incidence rates for overall cancer occurrence and for each type of cancer using Poisson distribution. Using cause-specific hazard ratios (HRs) estimated by univariate and propensity score-adjusted multivariable Cox proportional hazard models, they analyzed associations between study variables and time to first cancer event.

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Overall, 16,637 patients contributed 123,166 person-years of follow-up. The results indicated that there was not an association between radioiodine treatment and increased risk for cancer (univariate HR, 0.99; 95% CI, 0.83-1.19; P =.91; multivariable HR, 1.01; 95% CI, 0.83-1.21; P =.95). However, after performing a sensitivity analysis, the researchers found that radioiodine exposure was associated with overall cancer risk during the first tertile of follow-up, up to 4.2 years (HR, 2.11; 95% CI, 1.59-2.80; P <.0001). There was no association with increased cancer risk during longer follow-up.

In univariate analysis, radioiodine exposure was associated with an increased risk for non-Hodgkin lymphoma (HR, 2.89; 95% CI, 1.12-7.46; P =.03), but this association was nonsignificant in multivariable analysis (HR, 2.32; 95% CI, 0.88-6.23; P =.09).

The study had several limitations, including the potential for differential bias and detection bias. The researchers also noted that they were unable to include radioiodine activities given a lack of radioiodine dosing data.

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Reference

Gronich N, Lavi I, Rennert G, Saliba W. Cancer risk following radioactive iodine treatment for hyperthyroidism: a cohort study [published online December 27, 2019]. Thyroid. doi:10.1089/thy.2019.0205