Endocrine Abstracts

Background: Fixed-dose radioactive iodine therapy (RAI) is one of the standard therapy for Graves’ disease (GD). However, optimal RAI dose to achieve hypothyroidism in GD has been controversial.

Methods: The clinical outcome of 124 patients with GD treated with a RAI between Apr. 2004 and Dec. 2014 was analyzed retrospectively. Responder group was defined as patients who were rendered hypothyroid by the RAI. Thyroid volume (TV) was assessed with sonography using the ellipsoid formula.

Results: The median duration of GD was 3 years (0.1–18 y) and the mean TV was 40.7±25.8 g. Seventy five patients (60.5%) had single RAI. Fifty-four patients (43.5%) became hypothyroid after first RAI, and additional 31 patients needed 2 to 4 times of RAI to achieve hypothyroidism. In 54 patients who became hypothyroid after first RAI, the TV was significantly lower, compared with non-responder group (25.1±8.8 vs. 52.8±28.2, P<0.001). When divided into low dose responder group (<15 mCi, n=46) and high dose responder group (≥15 mCi, n=39) based on total accumulated RAI doses, there were no differences in age, sex, disease duration and levels of TBII, but TV was significantly lower in patients treated with low dose RAI. The cut-off of thyroid volume for low dose responder group was 32.7 g (sensitivity 80.9% and specificity 76.7%).

Conclusions: Thyroid volume had a significant effect on the outcome of RAI in GD patients. The optimal fixed RAI dose for GD patients with larger goiter (≥33 g) should be at least 15 mCi to simplify the therapy and achieve the best outcome in iodine-replete Korea.