INFLUENCE OF COMPENSATED RADIOIODINE THERAPY ON THYROID VOLUME AND INCIDENCE OF HYPOTHYROIDISM IN GRAVES-DISEASE

Objectives. To investigate the long-term effect of radioactive iodine (I-131) 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 s elected for I-131 treatment and followed for a minimum of 12 months (r ange 1-10 years, median 5 years). Interventions. I-131 dose was calcul ated based on thyroid volume and 24-h I-131 uptake. Main outcome measu res. Standard thyroid function variables and ultrasonically determined thyroid volume before treatment as well as 0.75, 1.5, 3, 6 and 12 mon ths after treatment, and then once a year were investigated. Results. Seventy-eight patients were cured by one (131) dose and 30 by two dose s, while the remaining nine patients received additional doses (range one to five doses, median one dose). Within one year, 25% developed hy pothyroidism, and hereafter, hypothyroidism developed at a constant ra te of 3% per year independent of antithyroid pretreatment. The cumulat ive 10-year risk of hypothyroidism was 60 %. Initial median thyroid vo lume was 33 mL (range 9-106 mL). At 12 months after the last I-131 dos e, median thyroid volume was reduced to 14 mL (range 6-36 mi,) (P < 0. 00001). The median reduction being 58% (range 0-80%), hereafter no fur ther reduction occurred. A significant reduction in thyroid volume was also noted in patients needing subsequent I-131 doses and in those de veloping hypothyroidism within the first year. Conclusions. I-131 norm alizes thyroid volume in patients with Graves' disease. Hypothyroidism seems an inevitable end result of this treatment. The present study s uggests that it will be impossible to modify I-131 therapy in a way to achieve both early control of hyperthyroidism and a low incidence of hypothyroidism.