Hypothyroidism is a condition that means your thyroid gland can't make enough hormones to keep your body running properly. This condition is sometimes referred to as an "underactive thyroid."

The thyroid is a butterfly-shaped gland at 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. 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.

When you're pregnant, you make about 50% more total T3 and total T4 than you did before pregnancy. Both T3 and T4 hormones play a critical role in a baby's brain development. A healthy thyroid can ramp up hormone production to meet this extra requirement during pregnancy, but if you have an underactive thyroid, you'll need to take a synthetic version of the hormones.

Hypothyroidism affects between two and three people out of 100 and is more common in women than men. You might not know you have hypothyroidism before you get pregnant. 

While the number of people diagnosed with hypothyroidism in the United States has increased in recent years, the percentage of reproductive-age women with the condition has remained stable. An estimated 2 to 10 out of every 100 pregnancies is affected by hypothyroidism.

Sometimes the body makes too much thyroid hormone. This is a different condition known as hyperthyroidism, or an overactive thyroid.

If your thyroid gland isn't making enough hormones, your body's functions will slow down. For example, you might feel like you can't think or move quickly. Other symptoms of hypothyroidism include:

• Fatigue

• Feeling cold

• Dry skin

• A puffy face

• Depression

• Muscle cramps

• Joint pain

• Weight gain

• Constipation

• Hair loss

For most people, the symptoms of thyroid disease develop slowly, over months or even years. And they're easy to overlook because they're so varied and common. That's especially the case during pregnancy, when it's normal to feel tired and gain weight. Other symptoms of hypothyroidism, such as constipation and muscle cramps, are also often chalked up to common pregnancy aches and pains.

The most common cause of an underactive thyroid during pregnancy is a condition called Hashimoto's disease, or Hashimoto's thyroiditis. It's an autoimmune disorder, which means that your immune system mistakenly attacks your body's healthy cells.

In the case of Hashimoto's disease, your immune system attacks your thyroid, causing swelling and damage to the thyroid gland. Over time, the damage prevents your thyroid from making enough hormones for your body to function properly. This process can accelerate during pregnancy, when your need for thyroid hormones goes up.

Other possible causes of hypothyroidism include:

• Surgery to remove part or all of your thyroid gland. Surgery is done to treat thyroid nodules (noncancerous tumors or lumps in the thyroid that can produce too much thyroid hormone), thyroid cancer, or the most common type of hyperthyroidism called Graves' disease.

• Radioactive iodine treatment for Graves' disease, which can permanently damage the thyroid and make it unable to produce enough hormones

• Radiation treatment on your neck for cancer treatment. This can damage your thyroid.

• Having too much or too little iodine in your body. Not having enough iodine is rare in the United States because table salt is supplemented with iodine.

• A problem with your thyroid from birth (congenital hypothyroidism).

• Certain medications, such as lithium and amiodarone, which can reduce thyroid hormones

If you take medication to help keep your thyroid hormones balanced, there's a good chance you'll have a healthy pregnancy without any complications. But without treatment, hypothyroidism may increase the risk for serious problems with your pregnancy, including:

• Miscarriage

• Premature birth

• Having a low birth weight baby

• Preeclampsia

• Placental problems, including placental abruption

• Anemia

• High blood pressure in pregnancy (gestational hypertension)

Thyroid hormones also play a critical role in your baby's brain development during pregnancy, so untreated hypothyroidism increases your child's risk of learning difficulties and developmental problems.

There are three main ways that hypothyroidism could impair fertility.

• Heavy or irregular periods could make it more difficult to conceive

• Low thyroid hormone levels can prevent ovulation

• Potential causes of the thyroid condition, such as pituitary disorders or autoimmune issues, could negatively affect fertility

If you're having trouble conceiving, your healthcare provider may run some thyroid tests. If you have hypothyroidism, treating the condition may help regulate menstruation and ovulation in a way that makes it easier to get pregnant.

It's possible. There's no universal screening for thyroid disease during pregnancy, but your healthcare provider will be looking for signs of hypothyroidism at your preconception appointment or first prenatal visit. If your provider thinks there's a chance you could have thyroid disease, your blood will be tested to see how your thyroid is working.

Your provider may give you a test if you:

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

• Have tested positive for thyroid antibodies, which could suggest an autoimmune disease

• Have a swollen neck from an enlarged thyroid gland (a goiter)

• Have a family history of thyroid disease

• Are age 30 or older

• Have a BMI of at least 40

• Have type 1 diabetes or another autoimmune disorder

• Had difficulty conceiving

• Had a preterm birth

• Had a miscarriage or stillbirth

If there's concern about how well your thyroid is working, your provider will check the level of thyroid-stimulating hormone (TSH) in your blood at your first prenatal visit. If your TSH levels are high, it could be a sign that your thyroid isn't making enough T3 and T4.

Your pituitary gland in your brain makes TSH, which controls your thyroid. Your body produces extra TSH to help boost your hormones when it can't produce enough T3 and T4.

Ask your provider what your target TSH level is. For most pregnant women it's below 2.5mIU/L, but this can vary depending on your circumstances. (Your target may also go up as your pregnancy progresses.)

If TSH levels are too high, a doctor will confirm a diagnosis by testing your blood for T4 levels and other substances in your blood.

If your T4 levels are low, it's likely you have hypothyroidism. If your T4 levels are normal, you may have a mild form of thyroid disease called subclinical hypothyroidism.

A hormone replacement medication called levothyroxine is used to treat hypothyroidism in pregnancy. Levothyroxine is a synthetic version of the T4 hormone your thyroid makes naturally. It's safe for you and your baby.

Taking levothyroxine minimizes the risk of complications during pregnancy and also helps to prevent the condition from affecting your baby's growth and development. But it's important to get the dose right: Too much levothyroxine can lead to hyperthyroidism (overactive thyroid), and too little won't treat your condition effectively.

A doctor will monitor your TSH levels regularly and can adjust the dose as needed. TSH is usually tested every four to six weeks until your hormone levels are stable. Your ob-gyn can provide this care during your pregnancy, but you may also see a doctor who specializes in hormone-related conditions, such as an endocrinologist or a maternal-fetal medicine specialist.

Your provider will give you instructions for how to take your medication. Generally, it's best to take it on an empty stomach at the same time each day. Thirty minutes before breakfast is ideal.

Avoid taking your prenatal vitamin, or calcium or iron supplements, within four hours of taking levothyroxine. Also, certain foods can interfere with the effectiveness of levothyroxine, making it harder for your body to absorb replacement hormones. Within several hours of taking your medication, avoid eating:

• Food made with soybean flour, such as meat alternatives, dairy alternatives, and some baked food

• Walnuts

• Foods that are high in dietary fiber, such as fruits and veggies, legumes (like black beans and lentils), and whole grain cereals and breads

• Calcium-fortified fruit juices

If you were taking levothyroxine before you became pregnant, contact your healthcare provider as soon as you think you may be pregnant. It's possible that your provider may have you increase your dose of levothyroxine before your next visit with them.

For your thyroid to work properly, it needs iodine, a mineral that is found in foods including dairy products, seafood, meat, and poultry. About half of all salt sold in the United States has had iodine added to it. Most people in this country get enough iodine through their diet, either by using iodized salt or by taking a multivitamin, so it's rare for hypothyroidism to be caused by a lack of iodine.

If you have hypothyroidism, check in with your provider frequently. They will likely check your thyroid hormone levels every four to six weeks and adjust your medication dosage based on the results.

Managing your thyroid condition is important, but there are plenty of other things you can do to have a healthy pregnancy. Eating nutritious food, 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.

Probably. Your dose of levothyroxine will likely need to be adjusted after you have your baby.

If you were taking medication for an underactive thyroid before you became pregnant, you'll probably return to your pre-pregnancy dose. If you just started taking levothyroxine during pregnancy, you may be able to stop after your baby is born. In either case, your TSH levels will be checked after about six weeks to ensure your level is appropriate.

Usually, yes. And continuing or starting treatment with levothyroxine while breastfeeding is safe for your baby.

But having hypothyroidism can make it hard to produce enough milk, or lead to a problem with your letdown reflex. If you have breastfeeding problems, don't try to go it alone. Talk to your healthcare provider or a lactation consultant about your concerns. 

Learn more:

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

Thyroid issues and concerns