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Pregnancy & Prenatal Testing

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First Trimester: Thyroid Stimulating Hormone (TSH)

Pregnancy causes normal changes in the function of many endocrine glands, but it has a marked effect on the thyroid gland, which produces hormones such as thyroxine (T4) and triiodothyronine (T3) that are critical to the healthy development of a fetus as well as the health of the mother.

Women with known thyroid conditions will usually require careful monitoring if they become pregnant. A healthcare practitioner may use tests for thyroid stimulating hormone (TSH) to monitor a woman's thyroid function throughout her pregnancy. TSH is produced by the pituitary, a small gland in the brain, in response to low T4 or T3 levels. Increased TSH levels in women who are taking thyroid hormone replacement may mean that their dose of hormone replacement needs to be increased.

Some have advocated screening pregnant women during the first trimester (or even before pregnancy) for elevated TSH even if they do not have a history of thyroid disease. This is because a significant percentage of women may have an underlying thyroid disorder that is unsuspected and that will cause problems during pregnancy. However, most guidelines do not recommend this as being necessary.

National Institute of Diabetes and Digestive and Kidney Disorders: Pregnancy and Thyroid Disease