Radioiodine treatment of hyperthyroidism: prognostic factors affecting outcome

To assess the effectiveness of radioactive iodine (RAI) treatment in patients with hyperthyroidism and to evaluate prognostic factors affecting outcome.

Our cohort comprised 115 consecutive patients with hyperthyroidism treated with RAI at the Endocrinology Clinic at the Farabi Hospital, Trabzon, between 1994 and 2002. Data were retrieved from the endocrinology clinic database. Patients were categorized into three diagnostic groups: Graves' disease (GD), toxic multinodular (TMN) hyperthyroidism, and toxic adenoma. Our policy, over the period of the study, was to offer a single fixed first dose (10 mCi) 131I to all patients with toxic nodular goiter (TNG) for the first time and to all patients with relapsed GD.

There was no significant difference in the cure rate between GD and TNG, but Graves' patients had a significantly higher incidence of hypothyroidism (p < 0.001). In contrast, incidence of euthyroidism was significantly increased in TNG than those of the patients with GD (p < 0.05). The incidences of hyperthyroidism, euthyroidism, cure rate, and persistent hyperthyroidism did not vary significantly between females and males. Age at onset of hyperthyroidism at diagnosis was not associated with outcome of RAI therapy. The incidence of hypothyroidism in patients who had nonpalpable goiter was higher than those in patients who had medium or large goiter (p < 0.05). The means of serum FT3 and TT4 at presentation were correlated with the development of hypothyroidism after RAI therapy. Logistic regression analysis showed serum FT3 concentration at presentation to be significant contributing factor to failure to respond to a single dose of RAI. Patients who had higher FT3 concentrations at diagnosis were more likely to fail to respond to RAI therapy.

The results of the present study of a cohort of patients with hyperthyroidism demonstrate that a single fixed dose of 10 mCi of RAI is highly effective in curing GD as well as toxic nodular hyperthyroidism. Therefore, treatment protocols for these groups should be identical. The most important factors that determine efficacy of RAI treatment are serum FT3 concentrations at diagnosis before the initiation of treatment and goiter size. Therefore, these factors should be taken into consideration when planning treatment. If such factors are present, the initial dose of RAI should be increased.