Endocrine Abstracts

Background: The use of radioactive iodine treatment (RAI) for the definitive treatment of benign hyperthyroid disorders has been well established. This study was conducted to determine the factors predicting the development of hypothyroidism following RAI therapy.

Methods: All patients (n=104, 25 men, 79 women) who had RAI for hyperthyroidism between January 2008 and December 2009 were included. In 82.6% of patients antithyroid medications were used prior to RAI treatment.

Results: All patients were administered fixed dose of RAI (mean±S.D.; 402±25.4) MBq. The median (IQR) age of patients was 58 years (54–62). Ninety patients had only one dose of RAI where as fourteen patients received the second RAI treatment after at least 6 months after the first dose which rendered them euthroid or hypothyroid. The success rate of RAI treatment (percentage of patients rendered either hypothyroid (64.4%) or euthyroid (22.1%) after 1 year was 86.5% which is comparable to other studies.

Patients became hypothyroid 138±132 (IQR 32–560) days post RAI. The average dose of thyroxine replacement was 116±39 (range 50–200) μg. The median values for T₃ were 8.3, 5 and 4.4 pmol/;; medians for T₄ were 20, 14 and 13 pmol/l; medians for TSH were 0.05, 0.5 and 0.23 mIU/l at diagnosis, before and after treatment respectively.

When Cox regression analysis was used younger people, lower BMI, higher levels of T₃ and T₄ at diagnosis and prior treatment of antithyroid medications increased the chance of developing hypothyroidism subsequently. When Kaplan–Meir curve was plotted the risk of development of hypothyroidism was lower after 18 months of RAI treatment.

Conclusions: Younger age, lower BMI, higher levels of T₃ and T₄ at diagnosis and prior treatment with antithyroid medications were associated with subsequent development of hypothyroidism. The risk of developing hypothyroidism diminishes 18 months after RAI.