The physiology of thyroid function in pregnancy.

Pregnancy has a variety of effects on maternal thyroid function. Thyroid gland enlargement is common particularly in areas of relative iodine deficiency. The renal clearance of iodine is increased in pregnancy and together with an increased volume of iodine distribution, leads to a low plasma inorganic iodine and thus increases the thyroidal iodine clearance. However, the absolute iodine uptake and hormone production rate remain unchanged. There is an increase in the serum thyroxine (T4)and triiodothyronine (T3) concentration largely due to an increase in thyroid hormone-binding proteins. Free thyroxine and free T3 remain unchanged in pregnancy as does the Free Thyroxine Index, which gives the single most accurate measure of thyroid function. The placenta secretes a number of thyroid stimulators including human chorionic gonadotrophin and possibly chorionic thyrotrophin and molar thyrotrophin whose physiological role is to date poorly understood. The fetal thyroid develops independently, and although fetal T4 CONCENTRATION RISES PROGRESSIVELY To maternal by term, the T3 concentration is markedly reduced owing to preferential formation of inactive reverse T3.