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Women's Health

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Estrogen Therapy Warning for Women With Thyroid Disease

WebMD Health News
Reviewed by Gary D. Vogin, MD

June 8, 2001 -- Women who are taking thyroid hormone either to supplement a gland that doesn't make enough or to suppress thyroid cancer will need their blood tested regularly if they decide to take estrogen either to combat menopause-related symptoms or as birth control.

A new study reported in the June 7 issue of The New England Journal of Medicine finds that while the situation isn't necessarily dangerous, to the surprise of some doctors, some of these women could end up with low levels of thyroid hormone in their blood, which can trigger several unwanted side effects.

According to Baha M. Arafah, MD, the study's author, the problem can arise when estrogen therapy interferes with thyroid medication and reduces the amount of available thyroid hormone, or thyroxine, in the blood.

"People who have been on thyroid medication for years would assume that whatever problems they are having [when starting estrogen therapy] have nothing to do with the thyroid medication," says Arafah, associate professor of medicine and endocrinology at University Hospitals of Cleveland. "But that's not necessarily going to be true."

Arafah studied 25 menopausal women who were using thyroid hormone either for hypothyroidism, a condition in which the thyroid doesn't make enough thyroxine, or to suppress the regrowth of a thyroid cancer that was removed. He compared these women with 11 healthy menopausal women.

All the women began taking daily estrogen replacement therapy for relief of menopause-related symptoms such as hot flashes and night sweats or to prevent the brittle-bone disease osteoporosis.

While healthy women experienced no significant changes in their thyroid function while on the estrogen therapy, about 40% of women taking the thyroid hormone had decreases in their blood levels of thyroxine, levels low enough to trigger hypothyroid symptoms, such as low energy and feeling tired, sluggish, and cold, or to put them at risk for regrowth of thyroid cancer.

Fortunately, the women in the study didn't experience any of these symptoms because they were being monitored closely, so their thyroxine dose was increased to compensate for the effects of the estrogen. This dose adjustment in thyroxine allowed them to continue taking estrogen to combat menopausal symptoms while keeping their thyroid disease in check.

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