The thyroid is a small gland located below the Adam's apple that regulates the body's metabolism. A low thyroid level, which is called hypothyroidism, means the thyroid gland does not make enough thyroid hormone to keep the body running normally. Although there are many possible causes for hypothyroidism in pregnancy, most often, hormonal fluctuations of pregnancy or autoimmune disease are the culprits, according to "Thyroid Disease and Pregnancy," a 2005 brochure from the American Thyroid Association.
Hashimoto's Thyroiditis
The most common cause of hypothyroidism during pregnancy is Hashimoto's thyroiditis, a disorder in which the body's own immune system attacks the thyroid gland, according to "Thyroid Disease and Pregnancy." Hashimoto's thyroiditis decreases thyroid levels in the blood, resulting in fatigue, muscle aches and higher cholesterol levels, notes the MayoClinic.com. A key sign of Hashimoto's thyroiditis is unexplained weight gain.
Pregnancy Hormones
Normal hormonal changes during the first weeks in pregnancy cause a temporary drop in thyroid levels, according to "Thyroid Disease and Pregnancy." This is due to high blood levels of two hormones: the human chorionic gonadotropin, which is the hormone detected by pregnancy tests, and estrogen, which is the main female hormone. After the first trimester, the thyroid gland returns to normal.
Abnormal Iodine Levels
A constant lack of iodine in the diet causes low thyroid levels. This frequently happens in developing countries, but is rare in the United States because iodine is added to table salt, explains the Merck Manual. Paradoxically, too much iodine can worsen or even bring on hypothyroidism, as well.
Thyroid Surgery
Pregnant women whose thyroid gland has been surgically removed typically have low thyroid levels, explains "Hypothyroidism," a 2005 brochure from the American Thyroid Association. If some of the gland remains, however, it may be able to keep blood levels of thyroid hormone normal.
