Foetal own production of thyroid hormones

 

Foetal own production of thyroid hormones -starts when ?? At what gestation?? We must remember that, though it seems paradoxical that maternal Free T3( which is bioactive) can’t pass through placenta but Free T4 can easily  pass (bioinactive)  : Al foetal tissues including foetal brain has receptors of Free T3 only. But as mentioned exogenous FT3 can’t pass from through placenta ( hostile  mother!!!!) . So whatever Free T4 foetus receives from maternally ingested L –Thyroxine (say Eltroxin, Thyronorm)  up to 12 weeks is first converted to Free T3 by all foetal tissue . This conversion, of  maternally derived Free T4 à to Free T3 occurs preferentially at foetal brain simply due to presence of  excess deiodinase enzyme at brain. Foetal thyroid initiates its  function at 11-12 weeks of gestation .Till then it depends on maternally derived Free T4 and subsequent  conversion. The fetal thyroid does not begin to concentrate iodine until 10–12 weeks of gestation, and the synthesis and secretion of thyroid hormone controlled by fetal pituitary thyroid stimulating hormone (TSH) ensues at approximately 20 weeks of gestation .As such, particularly during early pregnancy, the fetus is reliant on maternal thyroxine, which cross the placenta in small quantities to maintain normal fetal thyroid function. At birth, approximately 30% of the T4 in cord blood originates from the infant’s mother