Radioactive iodine is a widely used treatment for hyperthyroidism caused by solitary autonomously functioning thyroid nodule (toxic adenoma). The aim of this retrospective analysis is to report the long term effects of this therapy on the thyroid function of patients with toxic adenoma treated in our department. Between 1968 and 1996, 160 patients received a single dose of (131)I (range 25-40 mCi) for hyperthyroidism caused by toxic adenoma. In 126 of these (110 females, 26 males), follow-up was feasible either in our Endocrine Outpatient Clinic or through correspondence. The mean observation period was 5.3 years (range 1-21 years, median 4.0). Post treatment evaluation revealed that: a) 57 patients became euthyroid and remained free of disease up to the last visit (mean observation period 5.76+/-0.52 years, range 1-21 years, median 5 years), b) 69 patients developed hypothyroidism, all within 1 to 12 months (5.9+/-0.49 months), c) persistence or recurrence of the disease (ie. thyrotoxicosis) was not observed, d) the (131)I dose, or the (131)I pretreatment TSH levels were not different between patients who developed hypothyroidism and those who became and remained euthyroid.
Conclusion: 131I administration in the above-mentioned dose to patients with toxic adenoma: a) was a safe and very effective therapy, and b) led to hypothyroidism which developed within the first year after (131)I administration in 55% of the patients.