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Radioactive Iodine Treatment of Hyperthyroidism

Thomas Higgins, M.D.
4/2/00

The thyroid gland concentrates iodine which it then uses to make thyroid hormone. T4, the major thyroid hormone, is called T4 because it contains 4 iodine atoms. The thyroid normally concentrates 8 to 20% of the dietary intake of iodine. The dietary iodine which is not used is removed from the body in the urine, sweat & saliva. Diets high in iodine lower the fractional dietary uptake of iodine where as diets low in iodine raise the fractional uptake. The fractional dietary uptake is also frequently increased in persons with hyperthyroidism. The dietary fractional uptake of iodine can be determined by a thyroid uptake test.

There are 2 forms of radioactive iodine which are frequently used in medicine: I-131 & I-123. Both release gamma rays which penetrate long distances & can be detected from outside the body. Since both have the gamma ray, both can be used to image the thyroid (thyroid scan) & perform a thyroid uptake test. In addition to the gamma ray, I-131 also release a beta particle which deposits all of its energy in a short distance & can be used for therapy. I-123 does not have the beta particle and is not used for therapy.

Currently in the United States, most people with hyperthyroidism are treated with radioactive iodine. This form of treatment has been available since 1942 & has been extensively used since the 1950’s. In a 1990 survey, 69 percent of North American thyroid specialists chose radioactive iodine for a hypothetical patient with Graves' hyperthyroidism. It is less popular elsewhere, being chosen as first-line therapy for a similar patient by only 22 and 11 percent of European and Japanese thyroid specialists, respectively. Radioactive iodine has become popular because it is effective, has minimal side effects & is comparatively inexpensive

Radioactive iodine is administered orally as sodium I-131 in solution or a capsule. The radioactive iodine is rapidly incorporated into the thyroid, and its radiation results in ablation of thyroid function over a period of 6 to 18 weeks in most patients.

Radioactive iodine appears to be quite safe aside from causing hypothyroidism which is usually considered a goal of treatment rather than a complication. Hypothyroidism is easy to treat. The only other definite complication is a one percent (or lower) incidence of radiation thyroiditis. Nonsteroidal antiinflammatory drugs are usually sufficient for pain relief, but prednisone may be required in severe cases.

A study of 35,593 patients followed for many years after radioactive iodine therapy revealed no increase in overall cancer mortality but did show a very small increase in thyroid cancer risk. The increase was seen mainly in patients treated for toxic nodular goiters. Since the risk of thyroid cancer is known to be slightly increased among patients with nodular goiters, it is likely that at least some of the excess thyroid cancer risk is related to underlying thyroid disease.

. Other Treatments for Hyperthyroidism

Antithyroid medication (PTU or Tapazole) can lower blood levels of thyroid hormone to normal but hyperthyroidism frequently returns after the medication is discontinued. Blood tests need to be monitored while the medication is being taken and side effects occur in some patients.

Surgical removal of the thyroid is also effective in controlling hyperthyroidism and the side effects are rare in the hands of an experienced surgeon. This procedure is rarely done for hyperthyroidism in this country & it is relatively expensive.

Specific Types of Hyperthyroidism

GRAVES' DISEASE: The primary goal of radioactive iodine therapy in Graves' disease is to cure the hyperthyroidism. In my experience, the dose of radioactive iodine which is given should be high enough to cause the development of hypothyroidism in about 3 months in 90% of patients.

Although attempting to lower thyroid function to normal with "low dose" radioactive iodine may appear desirable, this approach has several disadvantages:

  • less than one-third of patients have normal thyroid function 10 years after therapy.
  • "Low-dose" radioactive iodine therapy is more likely to result in treatment failure, necessitating another treatment in 6 to 24 months.
  • Many patients treated with "low-dose" radioactive iodine continue to have sub-clinical hyperthyroidism with its associated risks of heart rhythm disturbances and reduced bone density.
  • Patients who develop normal thyroid function shortly after radioactive iodine therapy subsequently develop hypothyroidism at a rate of 2 to 3 percent per year. Hypothyroidism may occur at a time when the presence of thyroid disease is no suspected resulting in delayed diagnosis. The rate of developing late hypothyroidism in this group of patients is the same in the "low-dose" & "high-dose" group.
  • Recurrent hyperthyroidism is more frequent after "low-dose" radioactive iodine. Recurrences generally occur soon after treatment & require retreatment.

SOLITARY HOT NODULES & TOXIC MULTINODULAR GOITERS: These are a much less frequent cause of hyperthyroidism than is Graves’ disease. Toxic nodules can occur at any age but toxic multinodular goiters generally occur in older persons. In general, these conditions are more resistant to radioactive iodine therapy and require higher doses of radioactive iodine than does Graves’ disease. The dose of radioactive iodine used for hot nodules & toxic multinodular goiters is frequently twice that used for a similar case of Graves Disease.

Since only a portion of the thyroid is producing thyroid hormone (and is taking up iodine), only a portion of the thyroid is exposed to the full radioactive iodine treatment. The result is post treatment hypothyroidism is less common than in cases of Graves’ disease but the lower radiation exposure may be associated with thyroid dysfunction or possibly cancer as noted above later in life.

My Recommendations

I recommend radioactive iodine therapy in most persons with Graves' disease. There are some situations where the antithyroid drugs or surgery are still recommended.

I calculate the dose of radioactive iodine by combining my estimate of the size of the thyroid gland requiring treatment and the percentage of iodine being taken up by the as indicated by the thyroid uptake test. Patients who have been treated with PTU or Tapazole prior to radioactive iodine are more resistant to the iodine and generally require higher doses.

My experience has been that approximately 90% of patients become hypothyroid and begin thyroid hormone therapy 3 months after radioactive iodine therapy, 5% take longer & 5% require a second treatment.

Children with Graves Disease are a special case but are still often best treated with radioactive iodine.

I generally recommend surgery for younger patients with solitary hot nodules & toxic multinodular goiters. I feel radioactive iodine is a good choice in older patients with these conditions.

There are 2 situations where I do not recommend radioactive iodine; pregnancy &

patients with active eye disease associated with Graves Disease: It has been my experience that the eye disease associated with Graves Disease can flair after radioactive iodine & I like to see it having been stable for at least 6 months prior to treatment.

Pregnancy: The fetus begins taking up iodine into its’ thyroid at about 12 weeks gestation & exposure to radioactive iodine at this time can cause the thyroid to fail to develop. Pregnancy should be delayed four to six months after radioactive iodine therapy. However, unintended pregnancies during this interval should be allowed to proceed to term. Birth defects do not appear to be more common in women treated with radioactive iodine.