Inm-7: Hypothyroidism and Pregnancy

A normal pregnancy results in a number of important physiological and hormonal changes that alter thyroid function. These changes mean that laboratory tests of thyroid function must be interpreted with caution during pregnancy. Levels of thyroxine binding globulin (TBG) and production of T3 and T4 hormones and the daily requirement of iodine in pregnancy are increased to 50%, TSH receptor stimulated by HCG and TSH levels reduced. The thyroid gland can increase in size during pregnancy especially in iodine-deficient areas. This is usually only a 10-15% increase in size and is not typically apparent on physical examination by the physician. In fact, pregnancy is a stressful situation for the thyroid gland that is the cause of hypothyroidism in women with limited reserve of thyroid hormone (due to autoimmune destruction or iodine deficiency). Hypothyroidism is a common disorder of the thyroid in pregnancy. The most common cause is chronic autoimmune thyroiditis (Hashimoto's thyroiditis), but in iodine deficient areas, deficiency of this element can be associated with goiter or hypothyroidism. For the first 10-12 weeks of pregnancy, the fetus is completely dependent on the mother for the production of thyroid hormone. By the end of the first trimester, the fetus thyroid begins to produce thyroid hormone on its own. However, remains dependent on the mother for ingestion of adequate amounts of iodine, which is essential to make the thyroid hormones. A small amount of thyroid hormones reaches from the mother to the fetus is essential for brain development. Untreated maternal hypothyroidism can lead to reduction in IQ and mental retardation in the fetus. In addition to abortion, preterm labor, preeclampsia, pregnancy-induced hypertension are complications of hypothyroidism in pregnancy. These complications can be prevented by timely and appropriate treatment of hypothyroidism during pregnancy. It is important to note that levothyroxine (LT4) requirements frequently increase during pregnancy, often times by 25 to 30 percent. Occasionally, the LT4 dose may double. Ideally, hypothyroid women should have their LT4 dose optimized prior to becoming pregnant. Women with known hypothyroidism should have their thyroid function tested as soon as pregnancy is detected and their dose adjusted by their physician as needed to maintain a TSH in the normal range. Thyroid function tests should be checked approximately every 6-8 weeks during pregnancy to ensure that the woman has normal thyroid function throughout pregnancy. It is also important to recognize that prenatal vitamins contain iron and calcium that can impair the absorption of thyroid hormone from the gastrointestinal tract. Consequently, LT4 and prenatal vitamins should not be taken at the same time and should be separated by at least 3-4 hours. As soon as delivery of the child occurs, the woman may go back to her usual prepregnancy dose of LT4 and 6-8 weeks postpartum, thyroid function tests should be rechecked.