Fetal thyroid function

TSH (thyroid stimulating hormone) is a glycoprotein.
TSH stimulates the release of thyroxine T4 and tri-iodothyronine T3.
The major action of thyroid hormones is to stimulate oxygen consumption.
The developing thyroid is first recognized as a thickening on the floor of the pharynx at around four weeks gestation. However, it does not begin to function until the end of the first trimester. Fetus uses maternal thyroid hormone T4 which crosses placenta in small amounts only during first trimester.
Fetal pituitary starts to secrete TSH in the second trimester and level increases steadily between 20 abd 30 weeks of gestation.
Fetal thyroid gland starts to secrete T3 and T4 from 10 weeks gestation onward.
From this point onwards, the fetal thyroid axis is independent and little relationship between maternal and fetal levels.