Two pregnancy-related hormones—human chorionic gonadotropin (hCG) and estrogen — cause increased thyroid hormone levels in the blood.
Two pregnancy-related hormones—human chorionic gonadotropin (hCG) and estrogen — cause increased thyroid hormone levels in the blood. Made by the placenta, hCG is similar to TSH and mildly stimulates the thyroid to produce more thyroid hormone. Increased estrogen produces higher levels of thyroid-binding globulin, a protein that transports thyroid hormone in the blood. These normal hormonal changes can sometimes make thyroid function tests during pregnancy difficult to interpret.
Thyroid hormone is critical to normal development of the baby’s brain and nervous system. During the first trimester, the fetus depends on the mother’s supply of thyroid hormone, which it gets through the placenta. At 10 to 12 weeks, the baby’s thyroid begins to function on its own. The baby gets its supply of iodine, which the thyroid gland uses to make thyroid hormone, through the mother’s diet.
Women need more iodine when they are pregnant—about 250 micrograms (μg) a day. In the United States, about 7 percent of pregnant women may not get enough iodine in their diet or through prenatal vitamins. Choosing iodized salt — salt supplemented with iodine — over plain salt is one way to ensure adequate intake.
The thyroid gland enlarges slightly in healthy women during pregnancy, but not enough to be detected by a physical exam. A noticeably enlarged gland can be a sign of thyroid disease and should be evaluated. Higher levels of thyroid hormone in the blood, increased thyroid size, and other symptoms common to both pregnancy and thyroid disorders—such as fatigue—can make thyroid problems hard to diagnose in pregnancy.
The thyroid is a small butterfly-shaped gland in the front of the neck below the larynx, or voice box. The thyroid gland makes two thyroid hormones, triiodothyronine (T3) and, in much greater quantities, thyroxine (T4). Thyroid hormones affect metabolism, brain development, breathing, heart and nervous system functions, blood cell formation, body temperature, muscle strength, bone health, skin dryness, menstrual cycles, weight, and cholesterol levels. Thyroid hormone production is regulated by another hormone called thyroid-stimulating hormone (TSH). TSH is made by the pituitary gland, a pea-sized gland located in the brain.
Thyroid disease is a disorder that results when the thyroid gland produces more or less thyroid hormone than the body needs. Too much thyroid hormone is called hyperthyroidism and can cause many of the body’s functions to speed up. Too little thyroid hormone is called hypothyroidism, in which many of the body’s functions slow down.
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