Does Radiation Affect Thyroid Function?

QUESTION: Do you know of any studies about radiation affecting thyroid function? I have radiation therapy (30 treatments) and my thyroid function was normal before but when my doctor checked my serum levels of TSH {thyroid stimulating hormone} after radiation, it was low. Any comments or thoughts?

ANSWER: Radiation given to the thyroid gland can reduce its production of hormones. Radiation to the breast should have no effect on thyroid production. Even radiation to the lymph nodes at the base of the neck, the supraclavicular area, shouldn't affect your thyroid's activity. If the whole gland were in the field, then yes, thyroid function would be likely to decrease, but the decrease wouldn't come this fast (while you're still in the midst of treatment). Why was your thyroid function measured in the first place-did you have a thyroid condition before your treatment? It would make sense to repeat the TSH just to make sure that the result was accurate. If the lab result is in fact correct, then work with your doctors to identify another cause of low thyroid function, besides radiation.

—Marisa Weiss, M.D.