Radioiodine treatment outcomes in thyroid glands previously irradiated for Graves' hyperthyroidism

Persistent or recurrent Graves' hyperthyroidism after an initial treatment dose of radioactive iodine (RAI) is not uncommon and usually necessitates additional administrations. The radiation sensitivity of the previously irradiated thyroid gland is unknown but is of importance in selecting the retreatment dose.

A retrospective analysis of patients receiving RAI for Graves' hyperthyroidism was undertaken. A first treatment dose was given to 1076 patients, and 168 of these patients subsequently required a second dose for persistent or recurrent hyperthyroidism (interval between RAI treatments, 8.5 +/- 17.1 mo).

Paired comparisons for retreated patients showed similar RAI doses (291 +/- 95 MBq and 283 +/- 129 MBq; p = ns) and treatment intensities (3.26 +/- 1.87 MBq g(-1) and 3.48 +/- 1.88 MBq g(-1); p = ns) for first and second treatments. Hypothyroidism occurred significantly earlier and more frequently after the first RAI dose (p = 0.002), but there was no difference for persistent or recurrent hyperthyroid events (p = 0.14). Multivariate regression established that the RAI treatment number (first or second) was a significant independent determinant of hypothyroid (p = 0.008) and combined (p = 0.001) events, whereas RAI dose and dose intensity were not.

We conclude that previous RAI treatment failure does not lessen the chance of successfully eradicating Graves' hyperthyroidism with additional RAI treatment. Furthermore, the previously irradiated thyroid gland may be less susceptible to early hypothyroidism than the RAI-naive thyroid gland.