Safety of the Use of Radioactive Iodine in Patients With Hyperthyroidism

Does Radioactive Iodine Therapy for Hyperthyroidism Cause Cancer?

Radioactive iodine has been considered a safe and effective therapeutic option for hyperthyroidism secondary to Graves disease and autonomously functioning thyroid nodules since the mid-20th century. The question of whether I-131 at the doses used for hyperthyroidism might increase the risk of cancer has been investigated in a number of observational cohort studies over the years, with the preponderance of evidence being reassuring as to its safety. In particular, the 1998 Cooperative Thyrotoxicosis Therapy Follow-up Study (CTTFUS) has been widely cited as compelling evidence that I-131 is safe in hyperthyroidism therapy with respect to carcinogenesis. However, in 2019, a study by Kitahara and colleagues re-analyzed the CTTFUS cohort, extending the follow-up time and applying a novel dosimetric model for estimating tissue absorbed doses of radiation. This new analysis concluded that radioactive iodine was associated with an increased risk for mortality from overall cancer, breast cancer, and non-breast solid cancers. Reaction to this study was vociferous and particularly negative in the nuclear medicine literature. This mini-review was inspired by the 2019 CTTFUS controversy, and it is intended to provide the necessary context for clinicians to provide nuanced advice to their patients on the subject. To that end, the pre-2019 literature is surveyed, the 2019 CTTFUS study and a 2020 follow-up are discussed, and lessons from the literature and critical commentaries are considered.

Cancer incidence and mortality following treatment of hyperthyroidism with radioactive iodine

PURPOSE OF REVIEW

Hyperthyroidism is a commonly encountered clinical issue. Radioactive iodine is one of the treatment modalities employed over the last 80 years. Prior studies are conflicting as to whether radioactive iodine is associated with an increased risk of subsequent malignancy and associated mortality. The present article reviews recent publications on this subject.

RECENT FINDINGS

Two recent studies make meaningful contributions to the existing literature; however, data remain inconsistent. The first, conducted using the Clalit Health Services database, evaluated solid tumor incidence after radioactive iodine and found no association with increased risk of solid tumor malignancy. The second, which is an updated analysis of the Cooperative Thyrotoxicosis Therapy Follow-up Study, concluded that there is a dose-dependent increased risk of solid tumor mortality using a novel method of estimating organ-specific radiation exposure.

SUMMARY

In patients with hyperthyroidism, radioactive iodine is a popular and effective treatment option. Prior studies reach conflicting conclusions on the potential relationship between radioactive iodine and both subsequent cancer incidence and mortality. We review recent publications that add to our understanding of this important clinical question.

Association of Radioactive Iodine, Antithyroid Drug, and Surgical Treatments With Solid Cancer Mortality in Patients With Hyperthyroidism

IMPORTANCE

The long-term health effects of radioactive iodine (RAI) and antithyroid drug (ATD) treatments compared with surgery for hyperthyroidism remain uncertain.

OBJECTIVE

To compare solid cancer mortality rates associated with RAI and ATD treatments vs surgical management for hyperthyroidism.

DESIGN, SETTING, AND PARTICIPANTS

This multicenter cohort study assessed patients treated for hyperthyroidism from January 1, 1946, to December 31, 1964, with follow-up through December 31, 2014. Data analysis was performed from August 1, 2019, to April 23, 2020.

EXPOSURES

Management with RAI, ATDs, surgical intervention, or combinations of these treatments.

MAIN OUTCOMES AND MEASURES

Comparisons of solid cancer mortality rates in each treatment group with expected rates from the general population were assessed using standardized mortality ratios (SMRs), and internal comparisons were assessed using hazard ratios (HRs) adjusted for age, sex, and underlying diagnosis (Graves disease or toxic nodular goiter).

RESULTS

Of 31 363 patients (24 894 [79.4%] female; mean [SD] age, 46.9 [14.8] years) included in the study, 28 523 (90.9%) had Graves disease. The median follow-up time was 26.0 years (interquartile range, 12.3-41.9 years). Important differences in patient characteristics existed across treatment groups at study entry. Notably, the drug-only group (3.6% of the cohort) included a higher proportion of patients with prior cancers (7.3% vs 1.9%-4.0%), contributing to an elevated SMR for solid cancer mortality. After excluding prior cancers, solid cancer SMRs were not elevated in any of the treatment groups (SMR for surgery only, 0.82 [95% CI, 0.66-1.00]; SMR for drugs only, 0.90 [95% CI, 0.74-1.09]; SMR for drugs and surgery, 0.88 [95% CI, 0.84-0.94]; SMR for RAI only, 0.90 [95% CI, 0.84-0.96]; SMR for surgery and RAI, 0.66 [95% CI, 0.52-0.85]; SMR for drugs and RAI, 0.94 [95% CI, 0.89-1.00]; and SMR for drugs, surgery, and RAI, 0.85 [95% CI, 0.75-0.96]), and no significant HRs for solid cancer death were observed across treatment groups. Among RAI-treated patients, HRs for solid cancer mortality increased significantly across levels of total administered activity (1.08 per 370 MBq; 95% CI, 1.03-1.13 per 370 MBq); this association was stronger among patients treated with only RAI (HR, 1.19 per 370 MBq; 95% CI, 1.09-1.30 per 370 MBq).

CONCLUSIONS AND RELEVANCE

After controlling for known sources of confounding, the study found no significant differences in the risk of solid cancer mortality by treatment group. However, among RAI-treated patients, a modest positive association was observed between total administered activity and solid cancer mortality, providing further evidence in support of a dose-dependent association between RAI and solid cancer mortality.