Maternal thyroid during pregnancy
During pregnancy, several important physiological changesoccur,with substantial repercussions for women’s thyroid gland. At the same time, maternal thyroid hormones (TH) play a vital role in the development and function of both the fetus and the placenta. Thyroid gland volume usually enlarges during pregnancy, and TH synthesis increases about 50% above the preconception level. These changes are in response to several factors. The normal pattern of human chorionic gonadotropin (hCG) secretion during pregnancy demonstrates a major increase during the first trimester and a plateau during midgestation, where it persists until shortly after delivery.4 hCG has a much researched thyroid-stimulating hormone (TSH)like activity secondary to specificity crossover at the TSH re
ceptor (TSHR). As a result, serum thyroxine (T4) and triiodothyronine (T3) levels are elevated, whereas serum TSH levels are reduced. Pregnancy-related hyperestrogenism induces a 100% rise in serum thyroxine-binding globulin (TBG) as a result of changes in TBG half-life secondary to altered glycosylation.Asaconsequence,byweek10ofgestation,total T4 and T3 serum concentrations are increased and plateau at this level until delivery.4,5 Other physiological adjustments also increase TH synthesis, such as elevation in the maternal glomerular filtration rate (GFR) and transplacental passage of T4.6 These changes mean that adjusted normal reference ranges for thyroid function tests, unique to pregnancy, must be consulted