Thyroid disease is the second most common disorder of the endocrine system in women of childbearing age.  The thyroid gland, located in the neck, produces the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine). These thyroid hormones are then released into the bloodstream when they are needed by the body.  The pituitary gland, found at the base of the brain, produces a hormone called thyroid stimulating hormone (TSH), which regulates the release of thyroid hormones. Both TSH and T4 levels are analyzed by endocrinologists to determine the presence of thyroid disorders.
Hypothyroidism is defined as an underactive thyroid gland, and is diagnosed by the presence of low free T4 levels and high TSH levels. It occurs in 3-5 out of every 1,000 pregnancies.  Hypothyroidism can develop during pregnancy in women with no known history of the disease.  Additionally, in women who were already hypothyroid prior to conception, the disease can worsen and become uncontrolled. During pregnancy, the elevation of estrogen and human chorionic gonadotropin (hCG) causes women’s bodies to require up to 20-40% more thyroid hormone. As a result, the thyroid increases production of T3 and T4, which can lead to thyroid dysfunction in some women. 
It is necessary to treat hypothyroidism during pregnancy in order to prevent adverse events in both the mother and baby. Untreated hypothyroidism during pregnancy can increase the risk for fetal growth abnormalities, deficiencies in fetal cognitive development, gestational hypertension (high blood pressure), anemia, preeclampsia, preterm birth, placental abruption, and miscarriage.  Children whose mothers had uncontrolled hypothyroidism during pregnancy have been shown to have lower IQs and impaired learning abilities in school. To prevent these detrimental effects, it is imperative for pregnant women with hypothyroidism to receive proper treatment with levothyroxine.
Levothyroxine is an oral medication used to treat hypothyroidism. It is essentially a synthetic form of thyroxine, or T4, which is one of the thyroid hormones produced by the thyroid gland.  It is marketed under different brand names, including Levoxyl® or Synthroid®. Levothyroxine is pregnancy category A and studies have shown that there is no increased risk of congenital abnormalities when used in pregnant women. 
Prior to conception, hypothyroidism can reduce fertility and increase the risk for miscarriage.  Therefore, hypothyroid women who are planning to conceive should strive to achieve controlled hypothyroidism through their endocrinologist. Early after conception, many women who had controlled hypothyroidism prior to pregnancy will develop increased levels of TSH, indicative of uncontrolled hypothyroidism. For this reason, hypothyroid women should contact their doctor immediately if pregnancy is suspected. The Guidelines of the American Thyroid Association recommend that after a positive home pregnancy test or missed period, hypothyroid women taking levothyroxine should increase their levothyroxine regimen by 2 extra doses every week in order to prevent the large increases in TSH levels that can occur after conception.
Currently, it is not recommended to screen pregnant women who are asymptomatic and have no prior history of thyroid disease for hypothyroidism. This is because treatment of subclinical hypothyroidism, defined as an elevated TSH with a normal T4, has not been shown to improve cognitive functioning of the child.  Throughout pregnancy, it is important for women to monitor for symptoms of hypothyroidism so that they can be diagnosed and treated if necessary.  Symptoms may include weight gain, fatigue, constipation, difficulty focusing, intolerance to the cold, dry skin, or hair loss. Women that are diagnosed with hypothyroidism will be started on a levothyroxine regimen by their endocrinologist, who will adjust the dose based on TSH levels until the disease becomes controlled. TSH will need to be tested every 4-6 weeks until the woman’s levothyroxine dose is stable or until 20 weeks of gestation. Thereafter, the endocrinologist will measure TSH levels at 24-28 weeks of gestation and again at 32-34 weeks of gestation.
After delivery, it is recommended that the levothyroxine dose be gradually decreased to the preconception dose, as postpartum TSH levels should return to pre-pregnancy levels.  TSH levels will need to be tested again 6 weeks after delivery to ensure that the dose of levothyroxine is correct. Some women that started levothyroxine treatment during pregnancy may no longer need treatment postpartum and may be able to discontinue therapy. This decision will be dependent on the woman and her endocrinologist.
Hypothyroidism can also cause problems postpartum with lactation. Lactating women with hypothyroidism have been shown to have difficulty producing enough milk. Levothyroxine treatment has been shown to improve lactation and is recommended in lactating women with hypothyroidism who want to breastfeed their child. Levothyroxine does not readily pass into milk, and it has not been associated with any side effects in the breastfeeding infant. [7,8]
It is also necessary for women to be aware of the proper instructions for taking levothyroxine for the medication to work most effectively. Levothyroxine should be taken with water in the morning, about one-half to one hour prior to any meals.  It is important to take levothyroxine at least 4 hours apart from any vitamins containing calcium or iron. However, for women that already take levothyroxine, they should continue to take it the same way that they have been or discuss their dosing schedule with their endocrinologist before making any changes. Alterations in how and when levothyroxine is taken can change levels of the drug that are absorbed and can alter thyroid tests.
- Carney L, Quinlan J, West J. Thyroid Disease in Pregnancy.
- National Institute of Diabetes and Digestive and Kidney Diseases. Pregnancy & Thyroid Disease.
- Alexander E, Pearce E, Brent G, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum.
- The American Association of Endocrine Surgeons. Diseases of Thyroid Function: Hypothyroidism.
- The American College of Obstetricians and Gynecologists. Thyroid Disease in Pregnancy.
- Levothyroxine Use While Breastfeeding.
- American Academy of Pediatrics. The Transfer of Drugs and Other Chemicals into Human Milk.