Abstract
A euthyroid pregnant woman will normally have a fetus that displays normal fetal development. However, studies have long demonstrated the role of T3 (Triiodothyronine), T4 (Thyroxine), and TSH (Thyroid Stimulating Hormone) and their degree of penetrability into the fetal circulation. Maternal thyrotropin-releasing hormone (TRH) crosses the placental site and, from mid-gestation onward, is able to promote fetal TSH secretion. Its origin is not only hypothalamic, as was believed until recently. The maternal pancreas, and other extraneural and extrahypothalamic organs, can produce TRH variants, which are transported through the placenta affecting, to a degree, fetal thyroid function. Antithyroid drugs (ATDs) also cross the placenta and, because of their therapeutic actions, can affect fetal thyroid development, leading in some cases to adverse outcomes. Furthermore, there are a number of TRH analogues that share the same properties as the endogenous hormone. Thus, in this narrative review, we highlight the interaction of all the above with fetal growth in uncomplicated pregnancies.
Highlights
Over the years, a large number of studies have been conducted focusing on maternal and fetal thyroid function and their interaction [1,2,3,4]
Articles focusing on the effect of thyrotropin-releasing hormone (TRH) and Antithyroid drugs (ATDs) on fetal thyroid function were included
This review does not focus on placental hormones that potentially affect fetal thyroid function except in reference to those reviewed, such as placental TRH
Summary
A large number of studies have been conducted focusing on maternal and fetal thyroid function and their interaction [1,2,3,4]. A fetal thyroid is unable to secrete its own thyroid hormones until the 18th week of gestation and is dependent on the maternal component to achieve normal thyroid function and development [8,9,10]. Following its binding to the TSHR, TSH mediates a number of effects on thyroid hormone metabolism, some being iodotyrosine synthesis, Tg synthesis, iodine trapping, and hormone release. In this narrative review, we analyze recent data on and new insights into the effect of thyrotropin-releasing hormone and ATDs on fetal thyroid function in uncomplicated pregnancies
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