According to the 2011 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum," some of the changes that take place include the following:
- In women who are not iodine-deficient, the thyroid typically increases around 10% in size during pregnancy.
- In women who are iodine-deficient, the gland typically increases from 20% to 40% in size during pregnancy.
- Typically, production of thyroxine (T4) and triiodothyronine (T3) increases by 50% during pregnancy.
- A pregnant woman has a 50% increase in her daily iodine requirement.
- The normal reference range for thyroid stimulating hormone (TSH) has a lower high-end cutoff point during the first trimester, typically around 2.5 mIU/L.
- During the first trimester, approximately 10% to 20% of pregnant women are thyroid peroxidase (TPO) or thyroglobulin (Tg) antibody positive, with TSH in the normal reference range.
- An estimated 16% of women who are within the normal reference range but positive for TPO or Tg antibodies during the first trimester will go on to have a TSH above 4.0 mIU/L during the third trimester.
- Some 33% to 50% of pregnant women who are positive for TPO or Tg antibodies in the first trimester will develop postpartum thyroiditis.
The Guidelines authors write: "In essence, pregnancy is a stress test for the thyroid, resulting in hypothyroidism in women with limited thyroidal reserve or iodine deficiency, and postpartum thyroiditis in women with underlying Hashimoto's disease who were euthyroid prior to conception."
Changes to Thyroid Function During PregnancyBecause normal thyroid function is different during pregnancy, the Guidelines have established trimester-specific normal ranges for various thyroid tests, and specifically for the TSH test.
According to the Guidelines, if a laboratory has not established its own trimester-specific reference ranges for TSH, the following reference ranges should be used:
- First trimester: 0.1-2.5 mIU/L
- Second trimester: 0.2-3.0 mIU/L
- Third trimester: 0.3-3.0 mIU/L
Stagnaro-Green, Alex, et. al. "Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum." Thyroid. Volume 21, Number 10, 2011 (Online)