Objectives: To investigate the long-term effect of radioactive iodine (131I) on thyroid function and size in patients with Graves' disease.
Setting: Out-patient clinic in Herlev Hospital.
Subjects: One hundred and seventeen consecutive patients (104 women) with Graves' disease selected for 131I treatment and followed for a minimum of 12 months (range 1-10 years, median 5 years).
Interventions: 131I dose was calculated based on thyroid volume and 24-h 131I uptake.
Main outcome measures: Standard thyroid function variables and ultrasonically determined thyroid volume before treatment as well as 0.75, 1.5, 3, 6 and 12 months after treatment, and then once a year were investigated.
Results: Seventy-eight patients were cured by one 131I dose and 30 by two doses, while the remaining nine patients received additional doses (range one to five doses, median one dose). Within one year, 25% developed hypothyroidism, and hereafter, hypothyroidism developed at a constant rate of 3% per year independent of antithyroid pretreatment. The cumulative 10-year risk of hypothyroidism was 60%. Initial median thyroid volume was 33 mL (range 9-106 mL). At 12 months after the last 131I dose, median thyroid volume was reduced to 14 mL (range 6-36 mL) (P < 0.00001). The median reduction being 58% (range 0-80%), hereafter no further reduction occurred. A significant reduction in thyroid volume was also noted in patients needing subsequent 131I doses and in those developing hypothyroidism within the first year.
Conclusions: 131I normalizes thyroid volume in patients with Graves' disease. Hypothyroidism seems an inevitable end result of this treatment. The present study suggests that it will be impossible to modify 131I therapy in a way to achieve both early control of hyperthyroidism and a low incidence of hypothyroidism.