Radiation and the Thyroid

Medha Joshi, MD, and David Lieb, MD, FACE, FACP
patient getting mri

The thyroid gland is very sensitive to radiation. Exposure to radiation is an established risk factor for thyroid cancer (most commonly papillary thyroid cancer) and other thyroid diseases, including benign thyroid nodules (lumps), hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). The risk of developing thyroid disease due to radiation depends on the patient age at exposure, radiation dose to the thyroid and time since radiation exposure.

Most radiation exposure occurs as part of treatment for different cancers, such as lymphoma, brain tumors, etc, or at the time of stem cell transplants. Additionally, the radiation used in x-rays and CT scans represents a significant source, with radiation doses from CT scans being much larger than those for x-rays. In the 1950s, radiation was used to treat benign diseases such as acne, but this practice is no longer employed. Accidental exposure to radiation can also occur, such as that seen after nuclear power plant accidents, for example after Chernobyl (1986 in Ukraine), Three Mile Island (1979 in Pennsylvania) and Fukushima Daiichi (2011 in Japan).

Finally, we are also exposed to small amounts of radiation from the environment, including space.

"The risk of developing thyroid disease due to radiation depends on the patient age at exposure, radiation dose to the thyroid and time since radiation exposure."

The most common thyroid disease seen after radiation exposure is hypothyroidism, or an underactive thyroid gland. It affects up to 30% of patients given radiation therapy to the neck region, often within the first five years after treatment. This is because the radiation destroys thyroid tissue. How severe the hypothyroidism is depends on the radiation dose. It’s important that people wear protective shields over their thyroid gland during any radiation treatment involving the head and neck.

Hyperthyroidism (overactive thyroid gland) can also occur due to high doses of radiation exposure, but is often temporary. Rarely, this can develop many years after radiation exposure, particularly in Hodgkin disease survivors.

Both thyroid nodules and thyroid cancer can occur after radiation exposure. Thyroid cancer is the most common endocrine cancer, and thyroid cancer cases have increased at a rate faster than any other cancer in the United States over the past few years. The most commonly seen cancer is papillary thyroid carcinoma. Thyroid cancers that occur following radiation exposure don’t seem to be more aggressive compared to those in patients who don’t have a history of radiation exposure. However, radiation-induced thyroid cancers are more likely to have lymph node spread.

It should be noted that children are especially sensitive to the effects of radiation because their tissues are growing, and cells are dividing more rapidly. Therefore, children exposed to radiation therapy (childhood cancer survivors) may be at an increased risk for thyroid cancer for up to four decades and probably for their lifetime after exposure. Thus, lifelong monitoring is recommended as thyroid diseases can develop decades after radiation therapy in these patients.

Protecting the thyroid gland from the effects of radiation is very important. Tests involving radiation should be used only when absolutely necessary, and people that receive radiation to the neck or that work in places with radiation exposure (such as radiology technicians) should wear thyroid shielding. Tests that do not involve radiation, like ultrasounds, should be used when possible.