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Yes, it's safe. In fact, it's unsafe not to take thyroid medication during pregnancy if you need it. Whether you have hypothyroidism (an underactive thyroid) or the much less common hyperthyroidism (an overactive thyroid), you'll probably need to take medication and be closely monitored.
The most common reason expecting moms take thyroid medicine is to treat hypothyroidism – when your thyroid gland doesn't produce enough thyroid hormone. Levothyroxine, a synthetic form of thyroid hormone that's safe for your baby, is the standard treatment. It poses no danger to your developing baby.
If you're pregnant, you must continue to take levothyroxine and stay in close contact with your doctor. (It's fine to take a generic brand of the medication, but this isn't the time to switch from a generic to a brand name or vice versa. You need to stay on the same medicine during pregnancy that you were taking before.)
During pregnancy, the thyroid gland needs to produce about 40 percent more thyroid hormone for both you and your developing baby. Women who don't receive enough thyroid hormone during pregnancy are at greater risk of complications, including miscarriage, preeclampsia, and preterm delivery. Some studies show that children whose mothers didn't have enough thyroid hormone during pregnancy may have lower IQs.
In 2011 the American Thyroid Association published clinical guidelines written by a committee of international experts on the care of women with thyroid disease before, during, and after pregnancy. The guidelines make the following points about caring for moms-to-be who have hypothyroidism:
- Most women on levothyroxine need to increase their dose as soon as they find out that they're pregnant. Of course, this should be done with your doctor's help.
- The exact dose of medication needed will be based on your TSH (thyroid-stimulating hormone) level. (The goal is a TSH of less than 2.5 to 3.0 mIU/L [milli-international units per liter].) Your TSH level is the best way to tell if you're getting enough thyroid hormone. It's measured by a simple blood test. During the first half of pregnancy, women on levothyroxine should have their TSH measured every four weeks. In the second half of pregnancy, it should be measured at least once, between 26 and 32 weeks.
After your baby arrives, your doctor will test your TSH level again, usually at six weeks. Your dose of levothyroxine will probably need to be reduced again to your pre-pregnancy level.
A rarer condition that occurs in only 0.5 percent of women is hyperthyroidism. If you have this condition, your thyroid is overactive (rather than underactive). The most common cause of hyperthyroidism is Graves' disease, an autoimmune condition in which the body produces an antibody that causes the thyroid gland to release too much hormone.
Untreated hyperthyroidism, when severe, may adversely affect the pregnancy. However, mild levels of maternal hyperthyroidism are often safe.
Women with moderate to severe Graves' disease may therefore need to receive appropriate treatment. Two antithyroid drugs – propylthiouracil and methimazole – are the mainstay of treatment of Graves' disease during pregnancy. These drugs work to reduce the amount of hormone that the thyroid gland releases.
Unfortunately, both propylthiouracil and methimazole have been associated with rare birth defects. The type and severity of birth defects may be worse with methimazole, but both medications should be avoided (especially in early pregnancy) unless required.
Nonetheless, there are times when such medications are necessary and can help reduce medical complications. Your thyroid physician can help make the best decision about the use of these medications.
In general, when they're needed, your doctor will give you the least amount of drug necessary to achieve control of your thyroid hormones.
The care of a pregnant woman with Graves' disease is complicated. For one thing, the dose of the medicine needed diminishes as the pregnancy progresses. In fact, many (but not all) women are taken off all antithyroid drugs before delivery.
In addition, both of these antithyroid drugs cross the placenta and can affect the developing baby. To complicate matters even further, the antibody that causes Graves' disease also (rarely) crosses the placenta and can affect the baby. Even women who have been successfully treated for Graves' disease may still have this antibody and require special monitoring during pregnancy.
After your baby arrives, your doctor will continue to monitor you to make sure your medication is adjusted correctly. It's not uncommon for Graves' disease to flare up in the postpartum period.
Your baby's doctor will also monitor your newborn for thyroid problems that may be present at birth. This is a rare occurrence, but your doctor has to watch out for it.
Learn how thyroid problems can affect you if you're trying to get pregnant.