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What is hyperthyroidism?
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 is 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 T3 and 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.
What are the symptoms of hyperthyroidism?
Symptoms of hyperthyroidism can be different for everyone but may include:
- Weight loss (or not gaining weight during pregnancy)
- Feeling nervous or irritable
- Mood swings
- Muscle weakness
- Shaky hands
- Racing and irregular heartbeat
- Feeling too hot
- Trouble sleeping
- Frequent bowel movements or diarrhea
- A swelling in the front of your neck (goiter)
What causes hyperthyroidism in pregnancy?
Most cases of hyperthyroidism in pregnancy are caused by the autoimmune disorder Graves' disease. When 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.
Your thyroid can also become overactive during pregnancy if your body produces too much of the hormone human chorionic gonadotropin (hCG). This condition is called gestational transient thyrotoxicosis. 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 hCG level goes very high, it can stimulate your thyroid, leading to hyperthyroidism.
Very high levels of hCG are also common in women who are expecting more than one baby, or who experience severe nausea and vomiting in pregnancy (hyperemesis gravidarum). You may have your thyroid checked if you have very bad pregnancy sickness, or if you're expecting twins or more. This condition usually resolves on its own after the first trimester.
What are possible pregnancy complications of hyperthyroidism?
Not everyone with hyperthyroidism experiences problems during pregnancy. Mild hyperthyroidism doesn't usually cause problems – for you or your baby – so in this case, your provider may just monitor your condition rather than treat it.
But severe, untreated hyperthyroidism can cause complications, such as:
- Premature birth
- Low birth weight
- Maternal heart failure
If you know you have hyperthyroidism, getting the condition under control before pregnancy is the best way to reduce the risks of hyperthyroidism during pregnancy. 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.
Will I be tested for hyperthyroidism during pregnancy?
Doctors don't typically screen for thyroid disease during pregnancy because most women who have the condition know before they start trying to conceive. It's more likely that your healthcare provider will ask you questions about your health and medical history at a preconception or first prenatal visit. 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
- Have been pregnant more than once
- Had difficulty conceiving
- Had a preterm birth
- Had a miscarriage or stillbirth
- Have a BMI of 40 or higher
- 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.)
What's the treatment for hyperthyroidism during pregnancy?
If you have a condition like 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.
It's important for you to know that not taking thyroid medication when you need it is also 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 healthcare provider. She can help you weigh the benefits and risks of taking thyroid drugs during pregnancy.
The bottom line is that treatment varies depending on your situation. There 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.
How do I have a healthy pregnancy when I have an overactive thyroid?
If you know you have this condition, ideally you'll have a plan in place before you get pregnant. In any case, contact your provider right away as soon as your period is late, you have any pregnancy symptoms, or you get a positive result on a pregnancy test. It's important to review your thyroid medication early on 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 her once a week in the first trimester to 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. Coping with these extra tests and appointments can be difficult while you're pregnant, but maintaining healthy hormone levels is important for keeping you and your baby healthy.
You'll have a blood test to measure your TSH, T4, and T3 at each visit. 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. Following a healthy 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.
How does hyperthyroidism affect my baby?
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 isn't usually permanent, usually lasting between one and three months. During that time, your baby will need thyroid medication, but the dose will gradually be lowered as your baby recovers.
Can I breastfeed my baby if I have hyperthyroidism?
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.
It's unlikely that your baby will need special monitoring because you're taking medication, but your baby's doctor will monitor your baby's growth and development at routine checkups just to be sure.