Thyroid disease is a group of disorders that affects the thyroid gland. The thyroid is a small, butterfly-shaped gland in the front of your neck that makes thyroid hormones. Thyroid hormones control how your body uses energy, so they affect the way nearly every organ in your body works—even the way your heart beats.

Sometimes the thyroid makes too much or too little of these hormones. Too much thyroid hormone is called hyperthyroidism and can cause many of your body’s functions to speed up. “Hyper” means the thyroid is overactive. Too little thyroid hormone is called hypothyroidism and can cause many of your body’s functions to slow down. “Hypo” means the thyroid is underactive.

If you have thyroid problems, you can still have a healthy pregnancy and protect your baby’s health by having regular thyroid function tests and taking any medicines that your doctor prescribes.

You’ve probably heard of the thyroid gland, but do you know what it does? You might not give it a second thought unless something goes wrong. Thyroid trouble can cause a range of seemingly unrelated problems, including drastic changes to your weight, energy, digestion, or mood. Learn to recognize signs of thyroid disorder, so you can get treatment if needed.

The thyroid is a small but powerful butterfly-shaped gland located at the front of your neck. It controls many of your body’s most important functions. The thyroid gland makes hormones that affect your breathing, heart rate, digestion, and body temperature. These systems speed up as thyroid hormone levels rise. But problems occur if the thyroid makes too much hormone or not enough.

Nearly 1 in 20 Americans ages 12 and older has an under-active thyroid, or hypothyroidism. When thyroid glands don’t produce enough hormones, many body functions slow down. A smaller number of people—about 1 in 100—has an over-active thyroid, called hyperthyroidism. Their thyroids release too much hormone.

Thyroid problems are most likely to occur in women or in people over age 60. Having a family history of thyroid disorders also increases the risk.

Thyroid problems are often caused by autoimmune disorders, in which the immune system mistakenly attacks and destroys the body’s own cells. For example, an autoimmune disorder called Graves’ disease can cause the thyroid to be over-active, while one called Hashimoto’s disease can make the thyroid under-active.

Thyroid disorders can be hard to diagnose, because the symptoms are similar to other conditions. “Hypothyroidism can be very subtle,” says NIH’s Dr. Monica Skarulis, an expert on the thyroid. If a thyroid disorder is suspected—maybe because of a weight change or fatigue—blood tests can help to confirm the diagnosis and find its cause.

Patients with under-active thyroids can be treated with artificial thyroid hormones. Over-active thyroids are often treated with medications that reduce hormone levels.

During pregnancy, thyroid hormones can affect the health of both the mother and the developing baby. Thyroid hormone levels sometimes need to be carefully monitored and adjusted, even if the expectant mother never had thyroid problems before. After pregnancy, some women have abnormal levels of thyroid hormone for a year or more.

The thyroid gland also can be affected by cancer. Thyroid cancer usually has no symptoms. It’s sometimes first noticed as a lump in the neck—although such bumps are more likely to be harmless nodules.

“Thyroid nodules are extremely common, whereas thyroid cancer is pretty rare,” Skarulis says. A doctor can determine if a nodule is cancerous by removing and examining a tiny piece of it. If it shows signs of cancer, the nodule or even the entire thyroid will be removed.

If you notice signs of thyroid disease, talk with a health professional. Based on your family history, symptoms, and medical exam, your provider can help you decide if further testing or treatment is needed.

Symptoms of Thyroid Disorder

Hypothyroidism:

• Fatigue
• Depression
• Joint and muscle pain
• Cold intolerance
• Slowed heart rate
• Constipation
• Weight gain

Hyperthyroidism:

• Fatigue
• Nervousness or irritability
• Trouble sleeping
• Muscle weakness
• Heat intolerance or increased sweating
• Rapid and irregular heartbeat
• Frequent bowel movements or diarrhea
• Weight loss

Thyroid hormones are crucial for normal development of your baby’s brain and nervous system. During the first trimester—the first 3 months of pregnancy—your baby depends on your supply of thyroid hormone, which comes through the placenta. At around 12 weeks, your baby’s thyroid starts to work on its own, but it doesn’t make enough thyroid hormone until 18 to 20 weeks of pregnancy.

Two pregnancy-related hormones—human chorionic gonadotropin (hCG) and estrogen—cause higher measured thyroid hormone levels in your blood. The thyroid enlarges slightly in healthy women during pregnancy, but usually not enough for a health care professional to feel during a physical exam.

Thyroid problems can be hard to diagnose in pregnancy due to higher levels of thyroid hormones and other symptoms that occur in both pregnancy and thyroid disorders. Some symptoms of hyperthyroidism or hypothyroidism are easier to spot and may prompt your doctor to test you for these thyroid diseases.

Another type of thyroid disease, postpartum thyroiditis, can occur after your baby is born.

Both hyperthyroidism and hypothyroidism can make it harder for you to get pregnant. This is because problems with the thyroid hormone can upset the balance of the hormones that cause ovulation. Hypothyroidism can also cause your body to make more prolactin, the hormone that tells your body to make breastmilk. Too much prolactin can prevent ovulation.

Thyroid problems can also affect the menstrual cycle. Your periods may be heavier or irregular, or you may not have any periods at all for several months or longer (called amenorrhea).

Pregnancy-related hormones raise the level of thyroid hormones in the blood. Thyroid hormones are necessary for the baby's brain development while in the womb.

It can be harder to diagnose thyroid problems during pregnancy because of the change in hormone levels that normally happen during pregnancy. But it is especially important to check for problems before getting pregnant and during pregnancy. Uncontrolled hyperthyroidism and hypothyroidism can cause problems for both mother and baby.

Hyperthyroidism that is not treated with medicine during pregnancy can cause:

• Premature birth (birth of the baby before 39 to 40 weeks, or full-term)
• Preeclampsia, a serious condition starting after 20 weeks of pregnancy. Preeclampsia causes high blood pressure and problems with the kidneys and other organs. The only cure for preeclampsia is childbirth.
• Thyroid storm (sudden, severe worsening of symptoms)
• Fast heart rate in the newborn, which can lead to heart failure, poor weight gain, or an enlarged thyroid that can make it hard to breathe
• Low birth weight (smaller than 5 pounds)
• Miscarriage

Hypothyroidism that is not treated with medicine during pregnancy can cause:

• Anemia (lower than normal number of healthy red blood cells)
• Preeclampsia
• Low birth weight (smaller than 5 pounds)
• Miscarriage
• Stillbirth
• Problems with the baby's growth and brain development

Untreated hyperthyroidism during pregnancy can lead to

• miscarriage
• premature birth
• low birthweight
• preeclampsia—a dangerous rise in blood pressure in late pregnancy
• thyroid storm—a sudden, severe worsening of symptoms
• congestive heart failure

Rarely, Graves’ disease may also affect a baby’s thyroid, causing it to make too much thyroid hormone. Even if your hyperthyroidism was cured by radioactive iodine treatment to destroy thyroid cells or surgery to remove your thyroid, your body still makes the TSI antibody. When levels of this antibody are high, TSI may travel to your baby’s bloodstream. Just as TSI caused your own thyroid to make too much thyroid hormone, it can also cause your baby’s thyroid to make too much.

Tell your doctor if you’ve had surgery or radioactive iodine treatment for Graves’ disease so he or she can check your TSI levels. If they are very high, your doctor will monitor your baby for thyroid-related problems later in your pregnancy.

An overactive thyroid in a newborn can lead to

• a fast heart rate, which can lead to heart failure
• early closing of the soft spot in the baby’s skull
• poor weight gain
• irritability

Sometimes an enlarged thyroid can press against your baby’s windpipe and make it hard for your baby to breathe. If you have Graves’ disease, your health care team should closely monitor you and your newborn.

Untreated hypothyroidism during pregnancy can lead to

• preeclampsia—a dangerous rise in blood pressure in late pregnancy
• anemia
• miscarriage
• low birthweight
• stillbirth
• congestive heart failure, rarely

These problems occur most often with severe hypothyroidism.

Because thyroid hormones are so important to your baby’s brain and nervous system development, untreated hypothyroidism—especially during the first trimester—can cause low IQ and problems with normal development.

Because the thyroid uses iodine to make thyroid hormone, iodine is an important mineral for you while you’re pregnant. During pregnancy, your baby gets iodine from your diet. You’ll need more iodine when you’re pregnant—about 250 micrograms a day. Good sources of iodine are dairy foods, seafood, eggs, meat, poultry, and iodized salt—salt with added iodine. Experts recommend taking a prenatal vitamin with 150 micrograms of iodine to make sure you’re getting enough, especially if you don’t use iodized salt. You also need more iodine while you’re breastfeeding since your baby gets iodine from breast milk. However, too much iodine from supplements such as seaweed can cause thyroid problems. Talk with your doctor about an eating plan that’s right for you and what supplements you should take. Learn more about a healthy diet and nutrition during pregnancy.