ates, 131-I is the initial modality of therapy selected by members of the American Thyroid Association for management of uncomplicated Graves’ disease in an adult woman . Two-thirds of these clinicians attempt to give 131-I in a dosage calculated to produce euthyroidism, and one-third plan for thyroid ablation.
131-I THERAPY FOR THYROTOXICOSIS OF GRAVES’ DISEASE
In many thyroid clinics 131-I therapy is now used for most patients with Graves’ disease who are beyond the adolescent years. It is used in most patients who have had prior thyroid surgery, because the incidence of complications, such as hypoparathyroidism and recurrent nerve palsy, is especially high in this group if a second thyroidectomy is performed. Likewise, it is the therapy of choice for any patient who is a poor risk for surgery because of complicating disease.
Treatment of children–
The question of an age limit below which RAI should not be used frequently arises. With lengthening experience these limits have been lowered. Several studies with average follow-up periods of 12 – 15 years attest to the safety of 131-I therapy in adults [ 39- 41]. In two excellent studies treated persons showed no tendency to develop thyroid cancer, leukemia, or reproductive abnormalities, and their children had no increase in congenital defects or evidence of thyroid damage [ 42- 44]. Franklyn and co workers recently reported on a population based study of 7417 patients treated with 131-I for thyrotoxicosis in England [44.1]. They found an overall decrease in incidence of cancer mortality, but a specific increase in mortality from cancer of the small bowel (7 fold) and of the thyroid (3.25) fold. The absolute risk remains very low, and it is not possible to determine whether the association is related to the basic disease, or to radioiodine treatment. Although there is much less data on long term results in children, there is a increased use of this treatment in teenagers over age 15-18, as discussed below.
The epidemic of thyroid cancer apparently induced by radioactive iodine isotopes in infants and children living around Chernobyl suggests caution in use of 131-I in younger children. Since the possibility of a general induction of cancer by 131-I is of central concern, it is interesting to calculate the risk in children using the data presented by Rivkees et al (44.2) who are proponents of use of RAI for therapy in young children..The risk of death from cancer due specifically to radiation exposure is noted by these authors to be 0.16%/rem for children, and the whole body radiation exposure from RAI treatment at age 10 to be 1.45 rem/mCi administered. Rivkees et al advise treatment with doses of RAI greater then 160 uCi/gram thyroid, to achieve a thyroidal radiation dose of at least 150Gy (about 15000 rads). Assuming a reasonable RAIU of 50% and gland size of 40 gm, the administered dose would thus be 40(gm) x 160uCi/gm x 2 (to account for 50% uptake) =12.8 mCi. Thus the long term cancer death risk would be 12.8 (mCi) x 1.45 rem (per mCi) x 0.16% (per rem) = 3%. For a dose of 15mCi the incremental risk a of cancer mortality would be 4% at age 5, 2% at age 10, and 1% at age 15 Whether or not accepting a specific 2-5% risk of death from cancer because of this treatment is of course a matter of judgment by the physician and family. However, this would seem to many persons to constitute a significant risk that should be avoided
Low 131-I uptake–
Certain other findings may dictate the choice of therapy. Occasionally, the 131-I uptake is significantly blocked by prior iodine administration. The effect of iodide dissipates in a few days after stopping exposure, but it may take 3-12 weeks for the effect of amiodarone or IV contrast dyes to be lost. One may either wait for a few days to weeks until another 131-I tracer indicates that the uptake is in the toxic range or use an alternative therapeutic approach such as antithyroid drugs.
Sometimes a patient with thyrotoxicosis harbors a thyroid gland with a configuration suggesting the presence of a malignant neoplasm. These patien