Hypothyroidism (low thyroid) is one of the most common conditions in healthy reproductive age women, so it frequently coincides with pregnancy. The fetus doesn't start making its own thyroid hormone until about 12 weeks; early on it is dependent on the maternal thyroid hormone. Low concentrations of maternal thyroid hormone may lead to delayed neurodevelopment, lower IQ scores, and poor fetal growth. Hypothyroidism is found in about 2.5 percent of pregnant women.
The thyroid gland is located in the front of the neck. It makes the thyroid hormone that circulates in the bloodstream. Under normal circumstances, the pituitary gland in the brain measures the amount of thyroid hormone in the blood and sends a message to the thyroid gland telling it how much hormone to make. The messenger is TSH, thyroid stimulating hormone. If the TSH level is elevated it is as if the pituitary is shouting at the thyroid gland: make more hormone! A TSH level is the best way to judge if the amount of thyroid replacement you are making or taking is appropriate to your needs. In pregnancy, TSH should be under 2.5 milliunits per liter.
Women who know they are hypothyroid and planning a pregnancy should take thyroid hormone replacement so that they have normal TSH levels before getting pregnant. This is one situation in which taking medicine is much better for the baby than avoiding it! Prenatal vitamins, calcium, soy products, and iron may all interfere with the absorption of thyroid replacement hormone, so don't take vitamins, iron, or other minerals within four hours of taking your thyroid hormone replacement.
During pregnancy, women need more thyroid hormone. When the thyroid gland works properly, a mother-to-be will make the extra hormone that she and the baby need. Hypothyroid women aren't able to increase their production of thyroid hormone, so they must take more thyroid hormone to keep up a good level in the blood. There are two schools of thought about how to deal with this: one is to measure the TSH level every three to four weeks, starting as soon as you know you are pregnant, and adjust how much medication you are taking based on the result. The other is to automatically increase the amount of thyroid replacement by about 30 percent, and then to adjust from there based on TSH levels. TSH levels under 2.5 assure that you are getting enough thyroid hormone for you and your developing fetus.
Some practices are currently screening all their pregnant patients for hypothyroidism. Others screen only if you have risk factors, like a history of thyroid disease, or a family history of thyroid or other auto-immune condition. Talk to your doctor or midwife if you have questions about hypothyroidism and pregnancy.
