What happens to the thyroid gland and its function in pregnancy?

During pregnancy, the thyroid gland undergoes a series of physiological changes in order to meet the increased metabolic needs. In particular, a woman’s thyroid gland increases in size and becomes palpable in the first trimester.

What is the role of the mother’s thyroid in the developing fetus?

The developing fetus is initially fully dependent on its mother’s thyroid hormones (T4 and T3) for the development of its brain. The ability of the developing fetus to produce its own thyroid hormones takes place gradually and begins around mid-gestation (18-20 weeks). Thyroxine (T4) transport across the placenta is therefore vital for the developing embryo.

On the other hand, normally, placenta permeability to maternal thyroid hormones is minimal, deactivating through an enzyme most of the thyroid hormone presented from the maternal to fetal circulation, in order to protect the fetus. Consequently, T4 clearance is increased, and therefore T4 needs are also increased.

Overall, total T4 availability during pregnancy should increase by 40%-50%. In a healthy woman who becomes pregnant, human chorionic gonadotropin (hCG), which is produced by the placenta and has an intrinsic thyrotrophic activity takes over, mainly in the first half of pregnancy, and along with the thyroid stimulating hormone (TSH), which is produced by the pituitary gland, stimulate the increased production of maternal thyroid hormones, in order to maintain the euthyroid state (normal thyroid function).