Answer to Question #13751 Submitted to "Ask the Experts"
Category: Nuclear Medicine Patient Issues — Diagnostic Nuclear Medicine
The following question was answered by an expert in the appropriate field:
Does a nuclear stress test affect the thyroid? If so, how can I protect my thyroid before or during the procedure? I have heard potassium iodide protects the thyroid. I have put this procedure off for over a year due to being concerned about this issue.
I can understand your concern about the effects that the nuclear stress test may have on your thyroid gland. First, I can assure you that the amount of radioactive material used in nuclear cardiac stress tests is very small and will have no effect on your thyroid gland, so there is no need to use any drugs to protect it. Second, the risk to your life and health from undiscovered heart disease far exceeds the negligible risk to your thyroid gland from radiation exposure, so you should proceed with the nuclear stress test if your doctor has recommended it.
Millions of nuclear cardiac stress tests are performed every year in the United States. These tests are effective in assessing the presence of hidden blockages in the coronary arteries, which may lead to a heart attack. Three types of radioactive drugs are used for these exams which may be done while you are under physical stress (for example, running on a treadmill) and again while you are at rest. Your doctor can compare the stress and rest images to find any areas of your heart that are not getting enough blood flow. These radioactive drugs are given to you by injection into a vein, and "home in" on the heart to produce a picture of the heart's blood flow pattern. They also are concentrated in other body organs, including the thyroid gland. I checked the prescribing information for the three drugs, and the radiation dose to your thyroid gland is about the same as that from a year's worth of radiation from natural sources (the so-called "natural background"). This amount of radiation will not affect the functioning of your thyroid gland and poses virtually no risk of radiation-related thyroid cancer. This is true for other body organs that get even more radiation dose than the thyroid gland, including the gallbladder, colon, and urinary bladder.
Your concern with your thyroid gland in particular leads me to believe that you are thinking about radioactive iodine. It is true that the thyroid gland can be protected from the effects of radioactive iodine by taking nonradioactive ("cold") potassium iodide. In fact, taking "cold" potassium iodide is the recommended procedure for protecting people in the event of an accident at a nuclear power plant where radioactive iodine may be released into the environment. "Cold" potassium iodide does not protect the thyroid gland from the drugs commonly used in nuclear cardiac stress tests.
In summary, the very low radiation exposure from any of the radioactive drugs used in nuclear stress rests should not discourage you from having one. Either a negative or positive tests result can help guide your doctor in providing the best medical care to you.
I hope this response helps.
Robert E. Reiman, MSPH, MD