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Dear Dr. Roach: I’m set to have a CT scan of my lungs with iodine dye. I have hypothyroidism and am on 25 mg of levothyroxine. Should I take the medicine the day of the test? Or should I have the CT scan without the dye?

L.T.M.

Dear L.T.M..: Most of the intravenous contrast dyes used in CT scans contain iodine (oral contrast dye is usually made with barium; MRI scanners use gadolinium dyes). Iodine is necessary in the production of thyroid hormone, and dietary absence of iodine has been a traditional cause of goiter, an enlargement of the thyroid. Iodine can affect the thyroid gland in people with and without known thyroid conditions.

The first effect of iodine on the thyroid is unexpected: It shuts off thyroid hormone production in most people. This effect is transient, lasting only a day to a few days. A very few people have long-lasting low thyroid levels after iodine exposure, such as the IV contrast dye for CT scan.

However, there also are people whose thyroid glands become temporarily, even occasionally permanently, hyperactive after exposure to iodine. It’s thought people in whom this occurs have a predisposition for hyperthyroidism, but either were iodine-deficient or for other reasons the cells in the thyroid hadn’t activated until the iodine exposure.

Even though both low- and high-thyroid conditions are possible after IV contrast dye, the likelihood is small. Published rates of developing high thyroid levels (hyperthyroidism) after IV contrast range from 0 to 6 percent of people within two years. The high thyroid levels usually were temporary. There was no measurable increased risk of long-term hypothyroidism.

People who have had a CT scan with IV contrast should report any symptoms of high thyroid levels — tremor, weight loss, difficulty sleeping and sweating — to their doctor, who may wish to order a blood test for thyroid hormone levels. People who already have thyroid abnormalities should have routine testing, unless further symptoms develop.

This is yet another reminder to avoid medical procedures unless they are necessary — even for my physician readers, because some patients push hard to get procedures. Don’t get contrast if you don’t need it. Don’t get a CT scan at all if the benefits aren’t expected to outweigh the risks.

Dear Dr. Roach: Do people who have never had chickenpox get shingles?

K.C.H.

Dear K.C.H.: Ninety-nine percent of people have had chickenpox and are at risk for shingles, and this includes people who think they haven’t had chickenpox. (Some cases are so mild they go unrecognized.) But only people who have had chickenpox (or the vaccine, which is a weakened form of the live virus) can get shingles. People who had the vaccine seem to be at lower risk of shingles than those who had the infection.

If the shingles vaccine technology can work for a chickenpox vaccine, then it’s possible we could see the end of chickenpox and of shingles forever, the way we were able to eliminate smallpox.

Email questions to ToYourGoodHealth@med.cornell.edu.

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