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Thyroid Disease in Pregnancy

By Donald Aulds, M.D.

Hormonal changes during pregnancy also cause the thyroid gland to change. The size of the gland increases by approximately 10 to 15 percent. Most of the thyroid levels remain stable except for an increase in the total T3 and T4 and a decrease in the T3 resin uptake test. Therefore, pregnancy does not cause abnormal thyroid function, but low or high thyroid function may adversely affect pregnancy, the fetus or the ability to get pregnant. In regions of the world where inadequate levels of iodine in the diet are noted, thyroid disease occurs at a high rate. The World Health Organization recommends that the average pregnant woman consume 200 micrograms of iodine daily. The diets of American women have usually provided adequate amounts of iodine, so supplementation is rarely needed.


Because many of the symptoms of pregnancy are the same symptoms of hypothyroidism such as weight gain, fatigue, and swelling, it is often difficult to diagnose thyroid problems in pregnancy. Recently several medical organizations have recommended that all women should be tested in early pregnancy with thyroid function test including TSH (thyroid stimulation hormone), free T3, and free T4 and thyroid antibodies. Low thyroid function (hypothyroidism) is most commonly caused by an autoimmune disease called Hashimoto’s disease in which an antibody attacks the thyroid gland and decreases the production of thyroid hormones. Side effects in the pregnant woman of severe hypothyroidism may include anemia, muscle pain and weakness, congestive heart failure, pre-eclampsia, placental abnormalities, low birth weight infants, and postpartal hemorrhage. In mild cases of hypothyroidism, there may be no signs of side effects.

What are the risks to babies born to mothers with hypothyroidism?
For the first 10 to 12 weeks of pregnancy, the baby is dependent on the production of thyroid hormones by the mother. After 12 weeks the baby begins to make its own thyroid hormone but is still dependent on the mother to provide enough iodine to produce the hormones. Untreated hypothyroidism in the mother can lead to inadequate iodine being available to the baby, thus leading to difficulty for the baby to make its own hormones. Inadequate levels of thyroid hormone in the infant can lead to severe neurological and development disorders. Recent studies have suggested that mild brain development disorders may be present in children born to women who are untreated with mild hypothyroidism.

Treatment of hypothyroidism in a pregnant woman is usually done by thyroid hormone replacement. Synthetic thyroid such as Synthroid and Levothyroxin requirement may increase during the pregnancy and therefore levels may need to be checked every 6 to 8 weeks through the pregnancy to adjust for the changes in levels. Thyroid medications should not be taken at the same time as a prenatal vitamin as the vitamin contains iron which can interfere with absorption of the hormone. It is now recommended that the two should be taken 2 to 3 hours apart. Natural thyroid hormones therapy does not fluctuate as much as the synthetic but it is still best to monitor with blood tests through the pregnancy. It is vital that a pregnant woman does not stop her thyroid hormones during the pregnancy as she will be putting her infant at risk for problems during the vital months of development.

Congenital Hypothyroidism

This is an inability of a newborn to produce its own thyroid hormones. The thyroid hormones are critical for the brain development in the baby. Infants with unrecognized congenital hypothyroidism can have severe cognitive, neurological and development abnormalities. These can largely be prevented by recognizing and treating with thyroid hormones as soon as possible. All newborns in the US are required to be screened for congenital hypothyroidism so therapy can be started as soon as possible. Problems seen in babies with congenital hypothyroidism can include hoarse cry, constipation, poor feeding, mental retardation, poor bone maturation, short stature, difficulty walking, spastic movements, and sometimes autistic-like behavior.

More about Dr. Aulds

Donald G. Aulds, MD is an Obstetrician and Gynecologist and currently serves as the Medical Director for both the Women's Center and the Best Start Program of North Alabama. He is a Diplomat of the American Board of Obstetrics and Gynecology and Fellow of the American College of Obstetricians and Gynecologists.

Dr. Aulds completed his medical education at Louisiana State University School of Medicine, New Orleans, LA and his Internship and Residency in Obstetrics and Gynecology at Ochsner Medical Foundation, New Orleans, LA.

Dr. Aulds has been an active member of the Huntsville Hospital Medical Staff since 1980.