Thyroid hormones affect the growth, metabolism, and function of multiple organs in the body. If adaptive mechanisms were not in place during pregnancy, physiological increases in the quantity of circulating thyroid hormone–binding proteins would result in lower concentrations of unbound (free) thyroid hormones and subsequent adverse maternal and fetal consequences.1 Fortunately, the hypothalamic-pituitary-thyroid axis compensates, matching thyroidal output to maternal requirements, and, as long as the thyroid has adequate reserves, thyrotropin (often referred to as thyroid-stimulating hormone or TSH) and free thyroxine are maintained at normal concentrations.
Cappola AR, Casey BM. Thyroid Function Test Abnormalities During Pregnancy. JAMA. 2019;322(7):617–619. doi:10.1001/jama.2019.10159
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