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Thyroid Disorders in Pregnant Women: An Overview

Women at risk should be screened early in pregnancy

An estimated 20 million Americans have some form of thyroid disease, and one woman in eight will develop a thyroid disorder during her lifetime, according to the American Thyroid Association (ATA).

For women, thyroid disorders can also have an impact on their pregnancy. Undiagnosed thyroid disorders can lead to preeclampsia, tachycardia and post-partum hemorrhage for the mother and severe neurologic abnormalities, mental disability, low birth weight and even stillbirth for the fetus, said Dr. John Barton of the Central Baptist Hospital in Lexington.

Thyroid disease is the second most common endocrine disorder in pregnancy after diabetes, said Barton. “One in 50 pregnancies probably has some mild sub-clinical hypothyroidism,” Barton said. “About one in 500 also have overt hyperthyroidism.”

For newly diagnosed women with hypothyroidism in pregnancy, an initial dose of 100 mcg of L-thyroxine is reasonable, said Barton. Subsequent adjustments can be made based on thyroid function tests and symptoms with an ultimate maintenance dose between 50 and 250 mcg of L-thyroxine per day. For newly diagnosed women with hyperthyroidism, the initial recommended dose of PTU is 100-450 mg/day with dosing three times per day and for methimazole 10-20 mg/day with dosing of once to twice per day. Once clinical improvement of hyperthyroidism occurs, the dose of antithyroid medication can often be reduced by one-half. 

The issue of universal screening and L-thyroxine treatment in women with subclinical hypothyroidism remains debated among professional societies including the Endocrine Society, the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine. “Congenital hypothyroidism as the result of severe iodine deficiency clearly carries long-term risk for mental retardation and growth retardation,” Barton said. Iodine deficiency induced cretinism is, however, very uncommon in developed countries. All 50 states in the United States currently utilize newborn metabolic screening for hypothyroidism. If diagnosed and treated in the first month of life, these newborns usually have normal intelligence. 

Although there is currently no research to support a universal thyroid stimulating hormone (TSH) screening for every pregnant woman, Barton said women who are at risk for thyroid disease should be screened prior to or in early pregnancy. Those include women who have another endocrine disease; those who have been previously diagnosed with thyroid disease; women with a family history of thyroid disease; and women who are morbidly obese or women older than 30 years.

Barton said a variety of tests can be used to check for thyroid disorders, but the most important thing is to realize how laboratory values in pregnancy differ from those in the non-pregnant state. “Just getting the labs and looking at the lab reference may not be appropriate,” he said. “You need to think about, one, that she’s pregnant, and, two, what trimester is she in to determine what’s normal or abnormal.”

According to Barton, hypothyroidism is more prevalent than hyperthyroidismin pregnancy and can be the result of prior thyroid surgery that interferes with thyroid function, previous hyperthyroidism that was over-treated with radioactive iodine or Hashimoto’s thyroiditis.

Pregnant patients with hypothyroidism will present with fatigue, lethargy, weight gain, cold intolerance and constipation. A physical exam may show dry coarse skin, brittle and coarse hair and bradycardia.

Barton said pregnant hypothyroid patients have a significant increase in their need for thyroid replacement in the first trimester. “It’s really important to increase their thyroid supplement in that first trimester. Most thyroid experts say you have to increase your dose about 30 percent,” said Barton. “One way of accomplishing this would be for the patient to take two extra doses of their medication [L-thyroxine] per week.”

Barton said women at risk of hypothyroidism should take a prenatal vitamin with at least 150 micrograms of potassium iodine each day.

For hyperthyroidism, patient symptoms may include anxiety, irritability, heat intolerance, increased appetite and weight loss. Patients with Graves disease may present with exophthalmos and vision change including double vision, photophobia and blurring of vision.

Barton said treatment for hyperthyroidism in pregnant women has been recently updated due to guidelines issued by an ATA task force in 2011 for the diagnosis and management of thyroid disease in pregnancy and postpartum. “Classically, we had given propylthiouracil [PTU] throughout pregnancy,” Barton said. “Now there’s concern that PTU may be associated with high risk for liver damage. The thought now is to use PTU in the first trimester and then switch to methimazole in the second and third trimesters.”

Physicians seeking additional guidance for treating pregnant women with thyroid disorders can check the ATA’s guidelines, which are available online at http://thyroidguidelines.net/pregnancy.

By Corrie Pelc, Staff Writer

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