Health effects of therapeutic use of 131I in hyperthyroidism

Since 1942, therapy with radioiodine (Na131I) has gained a major role in the treatment of benign thyroid disorders, notably hyperthyroidism caused by Graves' disease or toxic multinodular goiter. The very large series of patients treated so far offer the opportunity for an assessment of both benign and malignant side effects. Hyperthyroidism is sometimes observed after radioiodine therapy due to radiation induced thyroid hormone or by an immunological mechanism. Despite the numerous attempts to design dosage schedules aiming at euthyroidism, hypothyroidism occurs in the majority of patients throughout life. Transient hypothyroidism may be observed within the first year after therapy and is caused by an immunological mechanism. Radioiodine therapy in Graves' disease may induce or worsen ophthalmopathy, which can be prevented by steroids effectively. Hypoparathyroidism and hyperparathyroidism have been reported after radioiodine therapy but probably do not exceed the normal incidence. Sialitis is commonly observed but mostly in patients treated with radioiodine for thyroid cancer. There are no indications for induction of genetic abnormalities after radioiodine therapy although no definite conclusion can be reached. Much attention has been paid to malignant disease. In very large series, no effects of radioiodine therapy on survival have been observed. Some studies report an increased relative risk for certain types of cancer (notably thyroid cancer, stomach cancer, bladder and kidney cancer or hematological malignancies). However, these observations were not confirmed by other large studies, so that no definite conclusion with respect to risk for certain types of malignant disease can be drawn. However, radioiodine therapy for benign thyroid disorders has generally been considered safe and without major side effects, hypothyroidism being the most frequent one.