Thyroid Hormone Replacement Drugs and Birth Control Pills: A Safe Combo?

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Can birth control pills and THRT medications affect each other?

If you're taking an estrogen medication (like birth control pills or hormone replacements) during THRT, you may have to adjust your dosage of T4 hormones, according to the American Thyroid Association.

Why? Ninety-nine percent of T4 (one of the two thyroid hormones) should bind to little proteins called thyroxine-binding globulins (TBG) in the bloodstream, making it pretty much useless. Estrogen in oral contraceptives causes TBG to hang out in the body longer, increasing your body's level of TBG -- and a percentage of all T4 stuck to it, says Wendy C. Wilcox, M.D., Vice Chairperson of the North Bronx Healthcare Network Department of Obstetrics and Gynecology.

Since the remaining 1 percent of "free" T4 is what affects your body, your thyroid gland should work to produce more T4 to keep enough of the hormone gallivanting around your body free from TBG, according to the American Thyroid Association. But sometimes (especially in patients who already have a thyroid condition) the thyroid can slack off, meaning you have to up your dosage of T4.

Whether or not you have to change your dose, the estrogen will throw off your levels of T4, since an absurd amount of T4 will be hanging out with TBG, Dr. Wilcox says. For that reason, when having your thyroid levels checked, your doctor should opt for the THS test, not the T4 test. Your high T4 levels will give the false impression your thyroid is putting in double time.

The good news is that THRT should not affect the effectiveness of birth control medication, Dr. Holtorf says. But thyroid disease can decrease fertility by affecting ovulation.

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