Hyperthyroidism is a condition that causes your metabolism to speed up. (Your metabolism refers to the trillions of chemical reactions taking place throughout your body that convert the food you eat into the energy you need to live.) Hyperthyroidism happens when your thyroid gland produces too many hormones, which is why it's sometimes also called an "overactive thyroid."

The thyroid is a butterfly-shaped gland in the front of your neck that produces two hormones: triiodothyronine (T3) and thyroxine (T4). These two hormones control the speed of your body's metabolism.

When you're pregnant, you make around 50 percent more total T3 and total T4 than you did before pregnancy because they play a critical role in a baby's brain development. But having too much of these thyroid hormones can increase the risk of certain pregnancy complications, such as miscarriage, preeclampsia, and premature birth. Hyperthyroidism can also affect your baby's development.

Sometimes the body doesn't make enough thyroid hormone. This is a different condition known as hypothyroidism, also called an underactive thyroid.

The symptoms most often associated with hyperthyroidism include:

• Weight loss (or not gaining weight during pregnancy)

• Feeling nervous or irritable

• Mood swings

• Tiredness

• Muscle weakness

• Shaky hands

• Racing and irregular heartbeat

• High blood pressure

• Feeling too hot

• Trouble sleeping

• Frequent bowel movements or diarrhea

• A swelling in the front of your neck (goiter)

Most cases (90 to 95%) of hyperthyroidism in pregnancy are caused by the autoimmune disorder Graves' disease. If you have Graves' disease, your immune system produces an antibody called thyroid-stimulating immunoglobulin (TSI), which attaches to thyroid cells and causes the thyroid to make too much thyroid hormone. Graves' disease is rare, affecting only about 2 in 1,000 pregnancies.

Yes. Your thyroid can become overactive during pregnancy if your body produces too much of the hormone human chorionic gonadotropin (hCG).

Very early in pregnancy, your body starts producing hCG, and the level of this hormone in your blood increases rapidly in the first trimester. If your hCG level gets very high, it can stimulate your thyroid, leading to hyperthyroidism.

Very high levels of hCG are common in women who are expecting twins or multiples, or who experience severe nausea and vomiting in pregnancy (hyperemesis gravidarum). 

Not everyone with hyperthyroidism experiences problems during pregnancy. Mild hyperthyroidism doesn't usually cause problems for you or your baby, so your healthcare provider may just monitor your condition rather than treat it.

But severe, untreated hyperthyroidism can cause complications, such as:

• Miscarriage

• Premature birth

• Low birth weight

• Stillbirth

• Preeclampsia

• Maternal heart failure

• Thyroid storm

If you know you have hyperthyroidism, getting the condition under control before pregnancy is the best way to reduce the risks. That means having two sets of thyroid tests one month apart, with results that show your thyroid is functioning well and producing a stable level of hormones.

Once you're pregnant, frequent monitoring and taking medication as prescribed can prevent complications.

Doctors don't typically screen for thyroid disease during pregnancy. Usually, they only test women at high risk for the disease before they become pregnant. You may be tested if you:

• Have symptoms of hyperthyroidism, such as a racing heart or shaky hands

• Have had thyroid disease or any treatment on your thyroid in the past

• Have tested positive for thyroid antibodies (TSI antibodies)

• Have a goiter

• Have a family history of thyroid disease

• Are older than 30

• Had difficulty conceiving

• Had a preterm birth

• Had a miscarriage or stillbirth

• Have type 1 diabetes or another autoimmune disorder

To see how your thyroid is working, your provider will give you blood tests to check your levels of thyroid-stimulating hormone (TSH) and T4.

Your brain's pituitary gland makes TSH, one of the hormones that controls your thyroid. When T4 levels get too high, your body stops producing TSH. A low level of TSH is a sign that your thyroid is producing too many hormones.

To confirm a diagnosis, your provider also needs to check your level of T4. A high level of T4 combined with a low level of TSH means you have hyperthyroidism.

Your provider may also test your blood for the TSI antibodies that cause Graves' disease. (You might also hear these called TSH receptor antibodies.)

If you have a condition such as Graves' disease, you'll probably need medication to slow down your body's production of thyroid hormones. But if high levels of pregnancy-related hCG are causing your hyperthyroidism, you'll probably get better early in your second trimester without any treatment. By the second trimester, hCG levels become more stable, so thyroid hormones usually return to normal on their own.

When you need medication to treat hyperthyroidism in pregnancy, most healthcare providers prescribe methimazole (MMI) or propylthiouracil (PTU). These drugs cross the placenta in small amounts and can affect a baby's health, so doctors typically prescribe the lowest effective dose to minimize any risk to the developing baby.

Not taking thyroid medication when you need it is risky for you and your baby. So if you're worried about the possible effects of the medication on your baby, discuss your concerns with your provider. They can help you weigh the benefits and risks of taking thyroid medication during pregnancy.

Treatment for hyperthyroidism in pregnancy varies depending on your situation. Here are a few possible scenarios:

• You could stop taking medication. Some women who have mild hyperthyroidism from Graves' disease can stop taking thyroid medication when they become pregnant. Your provider may suggest this if your condition is well controlled, or if you've been on a low dose of medication for a while. (Thyroid medication gradually reduces the antibodies that cause Graves' disease.)

• You could change medication early in pregnancy. If you took MMI before you became pregnant, your provider will probably switch you to PTU, at least until you reach 16 weeks. Early pregnancy is critical in a baby's development, and PTU is the safer option. If you use PTU for a long time, there's a very small risk of damaging your liver. So once you're past this early part of your pregnancy, your provider may recommend switching back to MMI.

• You could change medication late in pregnancy. Stopping or reducing medication in the third trimester may also be an option for some women. Thyroid-stimulating immunoglobin antibodies often fall toward the end of pregnancy. Around one-quarter of women who need thyroid medication during their pregnancy can stop taking it in the third trimester.

• You could change medication after delivery. Antibody activity can increase again in the postpartum period, so you may find you need to go back on medication or increase your dose after you give birth.

It's possible. Research suggests both hyperthyroidism and hypothyroidism may have negative impacts on fertility for both men and women.

For women, an overactive thyroid can cause shortened phases in the menstrual cycle, including ovulation and menstruation. That means a fertilized egg may be expelled before it can implant.

If you know you have this condition, try to have a plan in place before you get pregnant, given the higher risks for pregnancy complications. Once you suspect you're pregnant (your period is late, you have pregnancy symptoms, or you get a positive result on a pregnancy test) contact your doctor. It's important to review your thyroid medication early in your pregnancy.

You'll need to check in with your provider about your condition frequently during pregnancy too. For example:

• If your provider stops prescribing medication, it's likely that you'll need to see them more frequently in the first trimester so they can monitor your thyroid. If your thyroid hormones stay healthy, you'll probably see your provider every four to six weeks in your second and third trimester.

• If your provider continues to prescribe medication, you'll likely see your provider every two to four weeks.

In addition to your ob-gyn, you may see an endocrinologist, a type of doctor who specializes in hormone-related conditions. Making time for extra tests and appointments can be difficult while you're pregnant, but maintaining healthy hormone levels is important to keep you and your baby healthy.

You'll have a blood test to measure your TSH, T4, and T3 every month. Your provider will be checking to see how your thyroid is functioning and whether your medication needs to be adjusted. (Too much thyroid medication can cause a baby to develop an underactive thyroid.)

If you have Graves' disease, you may also have a test to check TSI antibody levels, which indicates how active your condition is. If it's active toward the end of your pregnancy, you may have extra monitoring to check your baby's health.

Managing your thyroid condition is important, but there are plenty of other things you can do to have a healthy pregnancy. Eating a nutritious pregnancy diet, exercising regularly, and reducing stress are just some of the ways you can ensure you're giving your baby the best possible start in life.

Most babies born to moms who have hyperthyroidism don't have any health problems.

If you have Graves' disease, there's a small chance that TSI antibodies could cross the placenta and enter your baby's bloodstream. Your baby could be at risk if your hyperthyroidism isn't under control, or if you have high levels of TSI antibodies in your blood.

It's uncommon, but between 1 and 5 babies out of every 100 born to a mom with Graves' hyperthyroidism also have an overactive thyroid when they're born. Signs of hyperthyroidism in a baby include:

• Crankiness and being hard to settle

• Not gaining weight

• The soft spot on a baby's head (fontanel) closing early

• Heart problems

• Breathing problems

If your provider thinks your baby is at risk for hyperthyroidism, a team of specialists will care for you during your pregnancy. You'll have extra monitoring and frequent ultrasounds to check for signs of hyperthyroidism in your baby. After birth, your baby will be tested to confirm or rule out a diagnosis.

All babies have a thyroid test between two and four days after birth. Newborn hyperthyroidism due to the mom's Graves' disease isn't usually permanent, and often lasts between one and three months. During that time, your baby will need thyroid medication, but the dose will gradually be lowered as your baby clears the maternal TSI antibodies from their body.

Usually, yes. Women with hyperthyroidism sometimes produce too much milk, but not all do. Some breastfeeding moms have problems with their letdown reflex.

If you have trouble breastfeeding, don't try to go it alone. Talk to your healthcare provider or a lactation consultant about your concerns so you can get the help you need.

For most women, it's safe to breastfeed while taking thyroid medication as long as you're not taking a high dose. Very small amounts of medication will pass into your breastmilk but not enough to harm your baby. For breastfeeding moms, the American Thyroid Association recommends a maximum daily dose of 20 milligrams (mg) of MMI or 450 mg PTU.

Learn more:

Is it safe to take thyroid medication while I'm pregnant?

Thyroid issues and concerns