[Physiology of the thyroid during pregnancy. Various exploratory tests]

Area of Science:

• Endocrinology
• Obstetrics
• Perinatology

Background:

• Thyroid function undergoes significant physiological changes during pregnancy.
• Maternal hyperestrogenism increases thyroxine-binding globulin (TBG) and total thyroid hormone levels.
• Free thyroid hormone levels (FT3, FT4) and TSH show dynamic changes throughout gestation.

Purpose of the Study:

• To elucidate the physiological adaptations of maternal thyroid function during pregnancy.
• To examine the role of the placenta in thyroid hormone transport and metabolism.
• To understand the implications for fetal thyroid development and management of maternal thyroid disorders.

Main Methods:

• Review of current literature on thyroid physiology in pregnancy.
• Analysis of hormonal changes in maternal and fetal compartments.
• Evaluation of placental transfer of thyroid hormones and related substances.

Main Results:

• Maternal thyroid gland often shows hypertrophy, yet function remains normal.
• Free T3 and T4 levels decrease in the second and third trimesters, while TSH moderately decreases.
• The placenta is a key regulator, allowing transfer of TRH, iodine, and immunoglobulins, but not TSH, T3, T4, or thyroglobulin.

Conclusions:

• Fetal thyroid function begins by 12 weeks, with T4 and TSH secretion by 18-20 weeks.
• Maternal medications like synthetic anti-thyroid drugs, lithium, and beta-blockers cross the placenta, impacting fetal thyroid status.
• Fetal thyroid hormone regulation is dependent on iodine availability and lacks auto-regulation, necessitating careful management of maternal thyroid conditions.