Levothyroxine is a synthetic thyroid hormone approved by the Food and Drug Administration (FDA) for the treatment of hypothyroidism. Even though obstetricians try to minimize the number of medications a woman takes during pregnancy, levothyroxine is safe for the pregnant woman with hypothyroidism. If left untreated, hypothyroidism can lead to anemia, muscle pain, congestive heart failure and preeclampsia in pregnant women. It can also significantly affect the brain development of the fetus and cause low birth weight. During the first trimester, the fetus depends completely on the mother's thyroid hormones. It is not until the second trimester that the fetal thyroid gland develops and the fetus can synthesize its own hormone.
Hypothyroidism and levothyroxine
The thyroid is a butterfly-shaped gland in the front of the neck that produces a hormone called thyroxine or T4. T4 is necessary for many basic functions such as metabolism, digestion, normal growth of the skin, hair and the functioning of the brain. When the thyroid does not produce enough T4, hypothyroidism is generated. People with hypothyroidism manifest fatigue, weight gain, dry skin, hair loss, constipation, loss of memory, problems concentrating and depression. In some countries, iodine deficiency is a cause of hypothyroidism. In the United States, where table salt is added with iodine, the most common cause of hypothyroidism is autoimmune; that is, that people's own antibodies attack their thyroid gland. This condition is called Hashimoto's thyroiditis. Hypothyroidism can also be caused by radiation, certain medications, and thyroid surgery. Levothyroxine can cause adverse effects if the dose is very high. These side effects often mimic hyperthyroidism and include: palpitations, sweating, anxiety, diarrhea, heat intolerance, weight loss, abdominal and head pain.
Hypothyroidism and pregnancy
Women who have hypothyroidism should have thyroid tests as soon as they become pregnant and every six to eight weeks during pregnancy. It is recommended that a pregnant woman check her thyroid using two tests: measurement of thyroid stimulating hormone (TSH) and free T4 (FT4). TSH rises in a person with hypothyroidism without treatment because the pituitary gland releases TSH when the thyroid is not producing enough hormones. FT4 is the amount of "free" thyroxine in the blood. In the bloodstream, T4 is converted to T3, which is the active form of the hormone.Both T3 and T4 are linked to proteins in the blood and this makes them inactive. During pregnancy, the number of thyroid hormone-binding proteins increases so that the amount of free T3 and T4 in the blood decreases. To compensate for this, the dose of levothyroxine should be increased during pregnancy by 25 to 50 percent. The thyroid hormone tests should be repeated four weeks after adjusting the dose until the TSH is in normal ranges.
Controversy exists as to whether all women, not just those with pre-existing hypothyroidism or a higher risk of having it, should have TSH tests during pregnancy. Risk factors for developing hypothyroidism include family history, previous thyroid surgery or the presence of goiter. A pregnant woman should consult with her doctor about this test even if she does not have any of these risk factors.