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

What's the thyroid?

Yes, it can make you bloat up and shrink down faster than a breakup. But your thyroid can also do so much more. It produces thyroid hormones, which control your body's use of vitamins, proteins, carbohydrates, fats, electrolytes and water, according to the American Thyroid Association. These hormones also regulate the body's immune response and how your body reacts to other hormones and drugs.

The thyroid gland is controlled by thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH).

What's thyroid hormone replacement therapy (THRT)?

If you suffer from a thyroid hormone deficiency, or hypothyroidism, you can encounter symptoms as varied as lack of energy, depression, constipation, weight gain, hair loss, dry skin, dry coarse hair, muscle cramps, decreased concentration, aches and pains, swelling of the legs and increased sensitivity to cold. Possible causes of hypothyroidism include a non-functioning thyroid gland, thyroid-gland damage due to surgery or radiation treatment, or a non-functioning pituitary gland.

If you suspect your thyroid isn't working as it should, your doctor can test your thyroid levels using a TSH test or T4 test, says Kent Holtorf, M.D., Medical Director of Holtorf Medical Group, which studies women's health and endocrinology. A TSH test measures your body's levels of thyroid-stimulating hormone, which helps control the thyroid gland. Your doctor may also order a T4 test, which measures the levels of the T4 hormone in the body. T4 is one of two types of thyroid hormones. The other, T3, comes from T4, so it's not commonly measured.

These tests will let your doctor know if your thyroid isn't working properly. If you do suffer from hypothyroidism, your doctor will likely put you on thyroid hormone replacement therapy (THRT), which is basically extra T4 hormones. You've got to bump those babies up.

Common synthetic THRT medications include Synthroid, Levoxyl, Levothyroid, Unithroid. The most common natural THRT medication is Armour Thyroid and is made from porcine (pig) thyroid glands.

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.

What Can I Do?

Get yourself a knowledgeable physician who understands the complex relationship between estrogen and thyroid hormones. Your physician should monitor your thyroid levels and how any medications, including oral contraceptives, are affecting them, and adjust your dosage accordingly.

All of your doctors must know all of the medications you are taking -- no "ifs, ands, or buts" about it, says Dr. Holtorf . Your doctors don't exchange all of their medical records about you each time there's an update, so unless you tell them your current list of medications, your physicians don't know what drug interactions to look out for.

Although changes in T4 hormones from estrogen-containing oral contraceptives are not dangerous, you may want to avoid them altogether by using a birth control pill that doesn't contain estrogen, Dr. Wilcox says. Keep in mind, however, that progesterone-only birth control pills are not as effective as those that contain estrogen and progesterone, and have to be taken at the same time every day. Alternatively, you can use a different form of birth control altogether.

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