Endocrine Abstracts

Background: Radioactive iodine (RAI) therapy is well-established in the management of benign thyroid disorders associated with thyrotoxicosis, but the optimal dose remains controversial. Guidelines recommend 500–800 MBq, though it is thought that Graves’ thyrotoxicosis may respond more readily to lower doses (400–600 MBq).

Methods: We studied all local cases receiving RAI over four calendar years (2010–2013; n=447). Demographic and clinical data were compiled from case records. Cohort and subgroup analyses of cases receiving an initial dose of ~400 MBq (n=373) or ~600 MBq (n=74) RAI were conducted using SPSS v21.

Results: Our patient cohort was female-dominated (78.2%) and largely Caucasian (74.4%). The vast majority (89.0%) had received thionamide treatment prior to RAI. We observed 20 cases (4.5%) of RAI-associated acute ophthalmopathy, one case (0.2%) of stroke and no fatalities over the first three months following RAI. The overall cure rate was 91.3%, with 63.5% of cases manifesting permanent iatrogenic hypothyroidism. Among patients receiving an initial dose of ~400 MBq RAI (range 197–420 MBq, mean 398 MBq) the single-dose cure rate was 79.1% and incidence of permanent hypothyroidism was 52.8%. Among patients receiving an initial dose of ~600 MBq RAI (range 550–651 MBq, mean 614 MBq) the single-dose cure rate was 71.7% (P<0.001) and incidence of permanent hypothyroidism was 59.5% (P<0.001). Graves’ disease was equally represented (53.0% vs 55.0%) among patients who were cured after one dose of RAI. Conversely, it was over-represented (71.8%; P<0.001) in the group of patients requiring multiple RAI administrations (n=68). Overall cure rate in this latter group was 88.2%, with 63.2% of cases developing permanent hypothyroidism.

Conclusions: A RAI dose at the lower end of current recommendations for thyrotoxicosis is more efficacious and better tolerated than higher doses, regardless of underlying pathology. Definitive control of autoimmune thyrotoxicosis is more likely to require multiple RAI administrations.