What influences early hypothyroidism after radioiodine treatment for Graves' hyperthyroidism?

The objective of this study was to evaluate the factors influencing the occurrence of early hypothyroidism after radioiodine treatment of Graves' hyperthyroidism.

Of 147 patients with Graves' disease (GD) treated with radioactive I-131 (RAI) in our thyroid clinic between July 2003 and December 2004, 84 were followed at 2 and 4 to 5 months after treatment. The age range was 12 to 75 years and the dosage range in these patients was 7.4 to 29.9 mCi. Twenty-four were males and 60 were females. Factors possibly contributing to post-RAI hypothyroidism are: dosage of I-131, age, gender, size of the gland, initial serum free T4, free T3, thyroid-stimulating hormone (TSH) levels, pretreatment with antithyroid drugs, radioactive iodine uptake, and duration of disease.

All patients had low TSH, elevated FT4, and elevated radioactive iodine uptake (RAIU) at 4 and/or 24 hours. Of the 84 patients followed, 46% of the males and 62% of the females became hypothyroid at 4 to 5 months (57% of the total). Twenty-one patients remained hyperthyroid and 14 patients became euthyroid. Multivariate analysis of these 84 patients showed no statistically significant single contributing factor for the development of early hypothyroidism.

The early onset of hypothyroidism after RAI in GD is very common (57%) and unpredictable. Thus, after RAI treatment, all patients must be closely monitored for the development of this disorder.