Thyroid Disease in Pregnancy: Reducing Risk

The thyroid, a small, butterfly-shaped gland in the front of the neck, releases hormones that control how the body uses energy. These hormones affect almost every organ in the body and, not surprisingly, play a crucial role in pregnancy. “Thyroid hormones are vital for normal development of the baby’s brain and nervous system,” says Maternal Fetal Medicine Specialist Dr. Kecia Gaither. “During the first trimester, the baby is dependent on the mother’s hormones, which pass through the placenta.

The baby’s thyroid starts working on its own around week 12 but does not produce enough hormones until 18-20 weeks. So untreated thyroid disorders that cause the mother to produce too little or too much thyroid hormone can have serious consequences for the pregnancy and the baby’s development.”

When the thyroid produces too much thyroid hormone, it is said to be overactive and the resulting condition, called hyperthyroidism, causes many of the body’s functions to speed up. When the gland produces too little hormone, it is underactive and the resulting condition, called hypothyroidism, causes the body’s functions to slow down. Both conditions respond well to medication. Women who have either one before becoming pregnant will be carefully monitored and their medication adjusted as needed. And since pregnancy-related hormones affect the thyroid gland, every woman must be alert to symptoms of hyper- and hypothyroidism and report them to her doctor. 

Hyperthyroidism in pregnancy

The most common cause of hyperthyroidism in pregnancy is Graves disease, an autoimmune disorder in which the thyroid makes too much hormone. Hyperthyroidism can also be caused by very severe morning sickness in the first trimester. Symptoms include fast and irregular heartbeat, shaky hands, and unexplained weight loss or failure to gain. Untreated, hyperthyroidism can cause preterm labor and pre-eclampsia, a serious condition that can cause high blood pressure and other damage. Effects on the baby might include fast heart rate, small size, and congenital malformation. Mild hyperthyroidism is generally monitored and not treated as long as the mother and baby are doing well. When more severe, hyperthyroidism is treated with an anti-thyroid drug at the lowest possible dose to control over-activity of the mother’s thyroid and ensure safety for the baby.  

Hypothyroidism in pregnancy

Hypothyroidism in pregnancy is usually caused by Hashimoto’s disease, an autoimmune disorder in which the immune system produces antibodies that attack the thyroid, causing inflammation and impairing its ability to produce enough thyroid hormone. Some of the symptoms of hypothyroidism, such as fatigue and weight gain, are common in pregnancy.

Further symptoms include unusual sensitivity to cold, constipation, muscle cramps, and problems with concentration or memory. Untreated hypothyroidism can lead to complications in the pregnancy, such as pre-eclampsia, anemia, and miscarriage. The baby might be of low birth weight, or, especially if the condition occurs in the first trimester, might be developmentally impaired. If lab tests indicate mild hypothyroidism and no symptoms are present, treatment may not be necessary. More severe deficits are treated with levothyroxine, a synthetic hormone that is identical to the one produced by the thyroid and is safe for mother and baby. Women who were taking levothyroxine before becoming pregnant may need an increased dose. “The thyroid gland has a profound effect on pregnancy and vice versa,” says Dr. Gaither. “The relationship is complex but we have highly sensitive laboratory tests that tell us exactly what is going on and we have highly effective treatments for any imbalance in hormone levels. Women who have a thyroid disorder before becoming pregnant and those who develop one during pregnancy can have every expectation that they will have a healthy pregnancy and a healthy baby.” 

Kecia Gaither, MD, MPH, FACOG, a perinatal consultant and women’s health expert, is a double board-certified physician in OB/GYN and Maternal-Fetal Medicine in New York City. Dr. Gaither is Director of Perinatal Services at Lincoln Medical and Mental Health Center, a member of NYC Health + Hospitals System in Bronx, New York.

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