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Monday, March 10, 2014
How does pregnancy affect the thyroid?
The increased estrogen during pregnancy stimulates the liver to make more of a protein called "thyroid binding globulin." This protein binds thyroid hormone in the blood stream, making it unavailable to the body's cells. Normally in pregnancy the thyroid gland simply revs up and makes more thyroid hormone, so that the TOTAL amount (protein-bound plus unbound) of thyroid hormone in the blood is higher than normal, but the amount of UNBOUND ("free") thyroid hormone in the blood is normal. (It is the "free" thyroid hormone in the blood that really determines whether or not the body's cells are being exposed to a healthy amount of thyroid hormone.)
If the thyroid is not completely normal, it may not be able make enough extra thyroid hormone to keep the levels of free hormone normal during pregnancy. Patients who are already on thyroid hormone pills prior to the pregnancy may need an increase in the dose during pregnancy. On the other hand, the placental hormone Human Chorionic Gonadotropin can stimulate the thyroid gland to make a little too much thyroid hormone. During the first part of the pregnancy it can be normal for there to be evidence of a slight increase in free thyroid hormone levels.
Thomas A Murphy, MD, FACP, FACE
Associate Professor of Medicine
School of Medicine
Case Western Reserve University