Five years' experience with iodine-125 therapy of Graves' disease

Recurrent Hyperthyroidism After Radioiodine-Induced Hypothyroidism: Report of Two Cases and Literature Review

The practice at the Mayo Clinic in treating Graves' disease with radioiodine (131 I) is to achieve a hypothyroid state. Less than 10% of the patients need more than one dose. Although cases of transient hypothyroidism have been reported after treatment with 131 I, the recurrence of hyperthyroidism is unusual after hypothyroidism has been induced with 131 I. We studied two cases seen at our institution in the past year in which the circumstances behind the recurrence have never been reported previously. The first case illustrates a recurrence of hyperthyroidism after the patient had received 38.5 mCi of 131 I. The second patient had recurrence of hyperthyroidism after 22 years of replacement therapy with levothyroxine. Both patients had increased levels of thyroid-stimulating immunoglobulin. We believe that the recurrence of the hyperthyroid state in these two patients was due to incomplete ablation of the thyroid. Apparently 131 I caused hypothyroidism, but residual cells remained viable and under continuous stimulation of thyroid-stimulating immunoglobulin produced recurrent thyrotoxicosis.

Radio-iodine treatment of hyperthyroidism--a more liberal policy?

The main and most valid evidence we have is that on direct follow-up of treated patients. This has been well done in thousands of patients in two countries with very good medical care and over long periods--10 to 15 years or more. On the other hand there is no doubt about radiation carcinogenesis, and thyroid cancer has been caused in children by low-dose x-ray therapy. The possibility of other cancer and of leukaemogenesis is also real. The evidence for this is reviewed and risk estimates given. Experimental work and radiation biology is reviewed and it is suggested that the risk of thyroid cancer from the usual radio-iodine doses given for treatment is low because of cell killing or sterilization, and animal evidence is discussed that supports this. The final position appears to be that no risk has yet been demonstrated in man and evidence in general suggests that if there is indeed a cancer risk it will be less than about 0.1%. It is suggested that this is not dissimilar to surgical risks, except that these are more immediate as compared with the long latent period for development of radiation-induced cancer. Genetic risks are also discussed and again have not been demonstrated in man from this treatment and should in any case be low. The risk to children is also considered; there is no direct evidence of risk but only small numbers have been treated--nevertheless the hypothetical risk may be counterbalanced by the benefits. Administration of radio-iodine therapy to pregnant or lactating women must be scrupulously avoided; both cause significant risks. Finally, if a more liberal policy be accepted it is worth while giving optimum treatment, and evidence is given suggesting low-dose treatment similar in effect to surgical thyroidectomy and followed temporarily if need be by anti-thyroid drug treatment.

Potential future application with therapeutic agents

Several new approaches to radiation therapy with radionuclides have been discussed. Iron 55 is selectively utilized in the red cell developmental cycle and in therapeutic doses, can lower marrow and circulating erythrocyte levels with much smaller degrees of effect on other cell lines. A serious complication, noted in animal studies, is the induction of neoplasma, especially osteosarcoma. Selective irradiation of the cell nucleus is possible with 125IUdR. This results in highly efficient cell killing due to the highly concentrated region of ionization. High concentrations of densely ionizing radiation in the malignant cell may also be accomplished with 211At. The use of labeled liposomes is an additional approach to the delivery of intracellular irradiation. None of these approaches is applicable for the practical treatment of human malignancy at the present time. The importance of these approaches is their value as models for future development of methods that can provide highly selective radiation to target sites.

Adverse reactions to radiopharmaceuticals

This review covers the side effects and adverse reactions to radiopharmaceuticals that were reported in the literature over the past 25 years. The information published prior to 1970 is sporadic, but due to the increased utilization of nuclear medicine procedures and the recognition that radiopharmaceuticals may have pharmacologic side effects, a registry has existed since 1971 to tabulate information on such effects. This survey is medical, rather than pharmaceutical in emphasis and so the adverse reactions are classified according to the target-organ systems involved rather than according to the specific radionuclides or to pharmaceuticals. If any of the radiopharmaceuticals of present or past use are not mentioned in this review, it is because no reports on their side effects were retrived by us. Hopefully, the organized registry system suggested by the Society of Nuclear Medicine (SNM) will enable a more complete recording of side effects from radiopharmaceuticals in the future.

The assessment of 125I treatment of thyrotoxicosis

257 patients have been reviewed 1-5 years (mean 3 years 2 months) after receiving one of five dose regimes of 125I for thyrotoxicosis. The cumulative incidence of hypothyroidism was 34% and of persistent thyrotoxicosis 17%. The group receiving doses between 351 and 500 muCi/g had the highest proportion of euthyroid patients (65%) with the lowest requirement for repeat therapy (46%). In the euthyroid patients, increasing dose of 125I was associated with progressive decline in mean thyroxine (T4) level and free thyroxine index (FTI) within the respective normal ranges, and increase in mean thyroid stimulating hormone (TSH) level to above the normal range. Euthyroid patients with elevated TSH levels had significantly lower T4 and FTI values compared with those with normal TSH, and showed a 3-4-fold increased rate of development of hypothyroidism over 1 year. Euthyroid patients with elevated T3 levels remained euthyroid during the subsequent year and mean T3 levels declined significantly, suggesting that abnormally elevated T3 levels after 125I do not generally indicate impending relapse of thyrotoxicosis. It is concluded that the potential admantages of 125I therapy for thyrotoxicosis in reducing the incidence of hypothyroidism have not been realized in practice.

Intentional radioiodine ablation in Graves' disease

In a series of 50 cases of Graves' disease qualifying for therapy with radioiodine, "ablative" doses of radioiodine resulted in hypothyroidism in 92% of cases at six months. Hypothyroidism was treated by thyroxine or tri-iodothyronine replacement therapy. Rapid responses and predictable outcome were regarded as important advantages. Intentional ablation is particularly convenient in situations where clinical and biochemical follow-up may be difficult. Further follow-up studies are needed to determine whether ablative therapy is the treatment of choice in all cases of Graves' disease.