Between the eighth and twelfth weeks of foetal development, the thyroid and the pituitary differentiate histologically and functionally. By the twelfth week, the thyroid begins to produce hormone. Serum T4 and thyroxine-binding globulin (TBG) levels rise rapidly, in parallel, to plateau at term levels by the end of the second trimester. Serum T3 levels are low, but detectable, in the third trimester and remain lower than maternal levels even at term (Fig. 15.4), resulting from a low foetal peripheral conversion of T4 to T3. Tissue levels, especially brain T3, are close to adult levels. TSH is detectable at the same gestational age as circulating T4. Levels of TSH approach term values by sixteen weeks and are higher in the foetus than in normal childhood or in maternal serum. Foetal thyroid hormone receptors are present and occupied by T3 during the second and third trimester.
