Q. My sister recently was diagnosed as having an overactive thyroid gland (hyperthyroidism). Her doctor wants to treat her with radioactive iodine and says she should take birth control pills or have her tubes tied to avoid pregnancy. However, she is considering having a child and at age 36 doesn't feel she has many years left to become pregnant.

Can you tell me why getting pregnant would be dangerous during the radioactive iodine treatment? Are there any alternative treatments?

A. I assume your sister has Graves' disease, a common type of hyperthyroidism. Overactivity of this gland speeds up the body's metabolism, leading to symptoms such as weight loss, nervousness and rapid heartbeat. There are three treatments for this condition -- medications, surgery and radioactive iodine.

The concern some doctors have about radioactive iodine is the theoretical risk of harmful effects in developing fetuses or a woman's eggs. But after years of use, radioactive iodine has never been shown to cause cancer, birth defects or other toxic reactions in people taking it or in future pregnancies.

After radioactive iodine is swallowed, it travels to the thyroid gland, where its radiation effects destroy overactive tissue. The radiation decays gradually, decreasing by half every eight days. After one month, less than 10 percent remains; after two months, less than 1 percent of the initial dose remains -- a negligible amount. To be on the safe side, many physicians recommend postponing pregnancy for four to six months -- some say up to a year -- after radioactive iodine treatment.

The advantage of removing most of the thyroid gland by surgery is that the condition is generally cured immediately, without affecting one's ability to have children in the future. The main disadvantage, which is shared with radioactive iodine treatment, is the high chance of eventually developing an underactive thyroid gland (hypothyroidism).

Hypothyroidism is simple to treat by taking a thyroid pill once a day. But because its symptoms can be subtle, I recommend that people treated for hyperthyroidism have a thyroid blood test at least once a year for the rest of their life to detect this complication.

There are two similar medications doctors use to treat Graves' disease -- propylthiouracil (PTU) and Tapazole. In your sister's case, their advantage is that they can be taken during pregnancy. Their disadvantage is that only about half of those taking one of these medicines -- even up to two years -- are cured of their condition. And even though these medicines are sometimes used to treat hyperthyroidism during pregnancy, they carry a small risk of causing harmful hypothyroidism in the developing fetus.

Given these options, your sister should be able to get treatment for her hyperthyroidism and preserve her ability to have children. I suggest she discuss these alternatives with her doctor and get a second opinion if she thinks it would help her decide.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington. Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.