The Thyroid and Pregnancy: What to Know

Thyroid conditions, particularly hypothyroidism, are relatively common in pregnancy and important to treat. The thyroid is an organ located in the front of your neck that releases hormones that regulate your metabolism (the way your body uses energy), heart and nervous system, weight, body temperature, and many other processes in the body.

Thyroid hormones are particularly necessary to assure healthy fetal development of the brain and nervous system throughout your pregnancy. Hormones are delivered to your baby through the placenta. At around 12 weeks, the thyroid gland in the fetus will begin to produce its own thyroid hormones, but there is still dependence on maternal hormones.

There are two pregnancy-related hormones—estrogen and human chorionic gonadotropin (hCG)—that may cause your thyroid levels to rise. This may make it a bit harder to diagnose thyroid disorders that develop during pregnancy. However, your doctor will be on the lookout for symptoms that suggest the need for additional testing.

And if you have preexisting hypothyroidism or hyperthyroidism, you should expect more medical attention to keep these conditions in control while you are pregnant, especially for the first trimester. Occasionally, pregnancy may cause symptoms similar to hyperthyroidism (overactive thyroid); should you experience any uncomfortable or new symptoms, including palpitations, weight loss, or persistent vomiting, you should contact your physician. Complications of pregnancy such as hyperemesis gravidarum or molar pregnancy may also cause hyperthyroidism, which can range from temporary to serious in nature.

Untreated thyroid disorders during pregnancy may lead to premature birth, preeclampsia (a severe increase in blood pressure), miscarriage, developmental issues, and low birth weight among other problems. Therefore, it is important to talk to your doctor if you have had a history of hypothyroidism or hyperthyroidism so you can be monitored before and during your pregnancy, and to be sure that your medication is properly adjusted, if necessary.

Symptoms of Thyroid Problems in Pregnancy

Hypothyroidism (Underactive Thyroid)

Symptoms of hypothyroidism, such as extreme tiredness and weight gain, may be easily confused with normal symptoms of pregnancy. Other symptoms may include:

• Constipation

• Difficulty concentrating or memory problems

• Sensitivity to cold temperatures

• Muscle cramps

Hyperthyroidism (Overactive Thyroid)

Hyperthyroidism is less common during pregnancy than hypothyroidism, but it does occur, affecting an estimated 0.2% to 0.4% of pregnancies. Symptoms of hyperthyroidism may mimic those of normal pregnancy, such as an increased heart rate, sensitivity to hot temperatures, and fatigue. Other symptoms of hyperthyroidism include:

• Irregular heartbeat

• Heightened nervousness

• Severe nausea or vomiting

• Shaking hands (slight tremor)

• Trouble sleeping

• Weight loss or low weight gain beyond that expected of a typical pregnancy

Causes of Thyroid Problems in Pregnancy

Hypothyroidism

The most common cause of hypothyroidism is the autoimmune disorder known as Hashimoto’s thyroiditis. In this condition, the body mistakenly attacks the cells of the thyroid gland, leaving the thyroid without enough cells and enzymes to make enough thyroid hormone to meet the body's needs.

Hyperthyroidism

The most common cause of maternal hyperthyroidism during pregnancy is the autoimmune disorder Graves’ disease. In this disorder, the body makes an antibody (a protein produced by the body when it thinks a virus or bacteria is present) called thyroid-stimulating immunoglobulin (TSI) that causes the thyroid to overreact and make too much thyroid hormone.

Even if you’ve had radioactive iodine treatment or surgery to remove your thyroid, your body can still make the TSI antibody. If these levels rise too high, TSI will travel through your blood to the developing fetus, which may cause its thyroid to begin to produce more hormone than it needs. So as long as your doctor is checking your thyroid levels, both you and your baby will get the care needed to keep any problems in check.

Diagnosing Thyroid Problems in Pregnancy

Hypothyroidism and hyperthyroidism in pregnancy are diagnosed based on symptoms, physical exam, and blood tests to measure levels of thyroid-stimulating hormone (TSH) and the thyroid hormones T4 and (for hyperthyroidism) T3. The presence of thyroid peroxidase (TPO) antibodies indicates a likely autoimmune cause of thyroid conditions.

Treatment of Thyroid Problems in Pregnancy

Hypothyroidism is treated with a synthetic (man-made) hormone called levothyroxine, which is similar to the hormone T4 made by the thyroid. Your doctor will adjust the dose of your levothyroxine at diagnosis of pregnancy and will continue to monitor your thyroid function tests every 4-6 weeks during pregnancy. If you have hypothyroidism and are taking levothyroxine, it is important to notify your doctor as soon as you know you are pregnant, so that the dose of levothyroxine can be increased accordingly to accommodate the increase in thyroid hormone replacement required during pregnancy. Because the iron and calcium in prenatal vitamins may block the absorption of thyroid hormone in your body, you should not take your prenatal vitamin within 3-4 hours of taking levothyroxine.

For women who require treatment for hyperthyroidism, an antithyroid medication that blocks production of thyroid hormones is used. This medication—propylthiouracil (PTU)—is usually given during the first trimester, and—if necessary, methimazole can be used, after the first trimester. In rare cases in which women do not respond to these medications or have side effects from the therapies, surgery to remove part of the thyroid may be necessary. Hyperthyroidism may get worse in the first three months after you give birth, and your doctor may need to increase the dose of medication.