Long-term follow-up study of radioiodine treatment of hyperthyroidism

OBJECTIVE To determine the cumulative incidence of hypothyroidism during long-term follow-up in patients treated for hyperthyroidism by radioactive iodine I-131 (RAI) therapy, the significance of clinical factors in predicting the development of hypothyroidism, and the outcome after a fixed 7 mCi (259 MBq) dose of RAI. DESIGN Prospective cohort study of patients treated for hyperthyroidism by RAI. PATIENTS AND MEASUREMENTS Since 1965, details on 2043 patients treated by RAI therapy in Tampere University Hospital were entered into a computerized register. Following RAI treatment, thyroid status was monitored every 1-3 months during the first year, and subsequently at 1-3-year intervals until June 2002 or until the patient died or moved out of the Tampere University Hospital district. RESULTS The cumulative incidence of hypothyroidism in patients with Graves' disease and toxic multinodular goitre at 1, 10 and 25 years was 24%vs. 4%, 59%vs. 15% and 82%vs. 32%, respectively. In a Cox regression model, previous partial thyroidectomy [risk ratio (RR) = 1.63 in patients with Graves' disease and RR = 1.59 in those with toxic multinodular goitre] and age at the first RAI treatment (RR = 0.998 and RR = 0.996 per year) were statistically significantly associated with the development of hypothyroidism both in patients with Graves' disease and in those with toxic multinodular goitre. Antithyroid medication preceding RAI therapy (RR = 0.47) decreased and female gender (RR = 1.53) increased the risk of hypothyroidism only in patients with Graves' disease. Administration of a single dose of RAI resulted in the control of hyperthyroidism in 75% of patients, while two to six RAI treatments were needed in 25% of patients to achieve either a hypothyroid or a euthyroid state in both groups. None of the clinical factors studied was associated with the remission rate either in patients with Graves' disease or in those with toxic multinodular goitre. The remission rate did not differ between the patients who received a dose of RAI calculated according to the uptake of RAI and thyroid size and those who received an empirical dose of RAI. The fixed 7 mCi (259 MBq) dose of RAI cured 80% of patients. CONCLUSION RAI treatment is effective in treating hyperthyroidism in patients with Graves' disease, but hypothyroidism will develop in 82% of patients in 25 years. Because the development of hypothyroidism seems to be inevitable and unpredictable by any clinical factors, the objective of RAI treatment should be to minimize the persistence of hyperthyroidism with the simplest possible form of treatment. We recommend a fixed 7 mCi dose of RAI to be used as the first empirical dose in the treatment of hyperthyroidism, at least in Graves' disease.