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Radioactive Iodine Treatment of Hyperthyroidism

by David V. Becker, MD, James R. Hurley, MD, and Ronald Detres
Division of Nuclear Medicine, New York Hospital/Cornell Medical Centre, New York, NY

What is Radioactive Iodine?
Radioactive iodine (commonly called radioiodine) is a form of iodine chemically identical to nonradioactive iodine. Therefore, the thyroid gland, which takes up iodine to make thyroid hormone, cannot distinguish between the two. However, the nucleus of a radioactive iodine molecule has excess energy and gives off radiation that can have effects on the cells in which it is concentrated.

How is Treatment Given?
Radioactive iodine is given either in a capsule or in a tasteless solution in water.

How Does the Treatment Work?
Radioactive iodine treatment of hyperthyroidism makes use of the thyroid gland's natural need for iodine to make thyroid hormone. The thyroid is the only part of the body that collects and retains iodine. In hyperthyroidism, the thyroid cells are over-stimulated and make larger amounts of thyroid hormone. The excess amount of hormones are secreted into the blood, and produce the symptoms of hyperthyroidism. When radioiodine is given, the thyroid gland cannot tell if the iodine is radioactive or not, and collects it in the normal way in proportion to the activity of the thyroid. Radioiodine thus accumulates in the cells that make thyroid hormone and remains there long enough to radiate the gland and to slow thyroid production. Radioiodine that is not retained by the thyroid gland is secreted rapidly by the body (within two or three days), primarily through the kidneys into the urine.

How Long Does it Take to Cure Hyperthyroidism?
There is considerable individual variation in the timing and extent of response to radioiodine. However, by three months after it is given, the treatment has usually had its maximum effect. Most patients are no longer hyperthyroid by that time, although in some patients this may take as long as six months. A few patients may have too little effect and a second treatment may be required. Even a third dose of radioiodine can be easily given without side effects.

Will I Develop an Underactive Thyroid?
Hypothyroidism (underactivity of the thyroid) frequently occurs in hyperthyroid individuals. It may happen even in the absence of treatment or when treatment other than radioiodine (such as medication or surgery) is used. Of course, treatments which damage or remove thyroid tissue, such as radioiodine or surgery, increase the incidence of hypothyroidism and shorten the interval before it appears. This may occur as early as two months after treatment or as late as twenty or thirty years. The appearance of hypothyroidism after radioiodine is a very common complication of the treatment as it is a natural consequence of the disease which is accelerated by the treatment.

How is Hypothyroidism Diagnosed and Treated?
Hypothyroidism is easy to diagnose, primarily by a blood test which may show its presence even before its symptoms appear. As soon as thyroid hormone levels drop below the normal range, your pituitary gland begins to increase production of TSH. This, and increased level of TSH in the blood is the way your doctor will tell when your thyroid function drops below normal.

It is simple to treat hypothyroidism. A small and inexpensive thyroid pill taken once a day should bring thyroid hormone levels to normal and eliminate symptoms of hypothyroidism. It is likely that if you do develop hypothyroidism, it will be necessary for you to take such a thyroid hormone tablet for the rest of your life. An annual checkup will be necessary to make sure that your dose of thyroid hormone is correct, but such checkups are usually recommended for all hyperthyroid patients, no matter how they are treated.

Will I Gain Weight?
Radioiodine treatments cure hyperthyroidism, lower thyroid hormone levels, and thus cause symptoms to disappear. Weight gain is not an inevitable consequence of treatment.

All hyperthyroid patients have an increase rate of metabolism which causes their bodies to burn up food more rapidly than normal. In many, appetite and food consumption are also increased. Because of individual differences in metabolism and appetite, some hyperthyroid patients lose weight, some maintain their weight, and some actually gain weight. When they are cured by any method of treatment, body metabolism decreases and food is burned more slowly. Appetite usually decreases as well, and most patients return to their prehyperthyroid weight. However, the increased appetite may persist longer than the increased metabolism. If this happens to you, there may be a tendency for you to gain weight unless you voluntarily decrease your food intake. However, this can be readily controlled by decreasing food intake.

What if I am Pregnant?
Pregnancy is one important situation in which radioiodine must not be used. The reason is that after the third month of pregnancy the thyroid gland of the developing fetus begins to make thyroid hormone and will collect radioiodine. Iodine readily crosses the placenta from the mother to the baby, and radioiodine in this situation might act to decrease the function of the thyroid of the unborn child. Methods of treatment other than radioiodine should be used in hyperthyroid pregnant women. As a precaution, hyperthyroid women of childbearing age who are to receive radioiodine should be asked about the possibility of their being pregnant and the time of their last menstrual period. Some doctors may ask such women to have a routine pregnancy test before radioactive iodine testing and treatment is done.

Avoid Radioiodine if Breast-feeding
If you are breast-feeding a baby, radioiodine which you take in a thyroid treatment could go from your body to the baby through the your breast milk. The result would be that your baby would concentrate that radioactivity in its thyroid gland and might experience a change in thyroid function or even develop thyroid nodules or other tumors in later years. As a result, if it is important that you receive radioactive iodine treatment while you are breast-feeding, your doctor will require that you stop breast-feeding until tests show that there is no longer any radioiodine in your milk.

Radioiodine Side Effects
There are no immediate side effects from radioiodine treatment. Do not expect nausea, vomiting, hair loss, and allergic reactions to iodine; they do not occur. Rarely, the thyroid gland may develop a slight tenderness which may last for a day or two.

Are There Alternative Treatments?
There are two alternative treatments: antithyroid drugs and surgery.

Antithyroid drugs act chemically on the thyroid gland to slow the production of thyroid hormone and thus control hyperthyroid symptoms. Propylthiouracil and methimazole (Tapazole) are the two antithyroid drugs used in the United States [and Canada]. Carbimazole, related to Tapazole, is used in England. In almost all patients they will be effective in returning thyroid hormone levels (and therefore symptoms) to normal. They usually require four to eight weeks if the medication is taken regularly and in the appropriate dose.

In a small proportion of hyperthyroid patients so treated (about 20 to 30 percent) a permanent cure occurs, although it is not possible to predict in advance which patients will be cured and remain normal after medication is discontinued. Regular doses of two to three antithyroid tablets a day for a least six to twelve months is usually recommended in attempts at permanent cure.

Antithyroid drugs occasionally produce side effects, most commonly a skin rash or itching. These are allergic reactions and occur in about 5 percent of patients and disappear when the medication is discontinued.

Occasionally, however, more serious reactions can occur and may result in arthritis, hepatitis, or a dangerous decrease in the number of white blood cells know as agranulocytosis. since white blood cells help protect us from infection, serious and sometimes fatal infections may occur if agranulocytosis develops. Fortunately, the number of white blood cells usually returns to normal soon after the drug is discontinued.

Surgical removal of 80 to 90 percent of the thyroid gland is very effective in curing hyperthyroidism and has been used for years. Before an operation can be performed, however, hyperthyroidism must first be controlled with six to eight weeks of antithyroid drugs and iodine treatment to lower the levels of thyroid hormones.

Postoperative hospitalization for two to five days is usual. Surgery involves the risks of general anesthesia as well as possible damage to nearby neck structures such as the nerves to the vocal cords and the parathyroid glands that control calcium metabolism. Although surgical complications are uncommon when the operation is performed by an experienced and expert thyroid surgeon, they still occur in 2 to 5 percent of patients. As with radioiodine treatment, hypothyroidism is common after surgery, usually occurring within a few weeks of the operation. Even patients who remain well for several months are at risk for late hypothyroidism and require yearly blood tests to be sure that their thyroid levels remain normal.

Is the Treatment Safe?
This form of treatment of hyperthyroidism by radioiodine has been used routinely for more than 35 years on well over one million patients in the United States alone. Many studies of people treated and examined at a later time show no harmful side effects or complications to the patients or their offspring. Radioiodine is now recognized as the safest, least expensive, and most convenient and effective treatment for hyperthyroidism. In the United States approximately 90 percent of hyperthyroid patients are treated with radioiodine.

Radioactive Iodine Treatment of Hyperthyroidism
The treatment is safe because radioiodine can deliver sufficient radiation to the thyroid gland to slow its function while delivering only a small amount to the rest of the body. This occurs because radioiodine remains within the thyroid gland for a moderate period of time, while that not collected by the thyroid gland leaves the body rapidly in urine. The radiation exposure to the rest of the body is quite small, on the order of that of one or two routine diagnostic X-ray procedures such as a GI series.

Is There Any Danger to My Family?
No! Since the amount of radiation you receive from radioiodine treatment is not dangerous, it is reasonable that no one else in the vicinity receives a dangerous amount of radiation.

Nevertheless, physicians make an effort to minimize all radiation exposure to every individual. This is especially important for individuals likely to be more susceptible to the potential effects of radiation such as pregnant women and tiny children.

Exposure to others is most likely to occur if a patient who has received radioactive iodine is extremely close to another individual for a prolonged period of time. Since radioiodine accumulates in the thyroid, such contact would include dancing cheek to cheek for an evening, rocking a baby on your shoulder for a long period of time, or possibly sleeping together in the same bed. For this reason most physicians suggest that patients sleep alone for two days and avoid such close contact. In addition, since a small amount of radioiodine comes out into the saliva, kissing is not recommended for two to three days after treatment. There is no hazard for pets.

Who Should Receive Radioiodine?
In view of its safety, convenience, low cost, and effectiveness, most hyperthyroid patients and their physicians in the United States select radioiodine as the definitive treatment of choice.

Copyright © 1993 Thyroid Foundation of Canada/La Fondation canadienne de la Thyroïde.
Originally published in The Bridge Vol. 7, No. 4, 1992.
Reprinted from thyrobulletin, Vol. 14, No. 3, 1993.
To order a reprint ($2 P&H) contact the National Office or your local chapter.

Reviewed 2000