Abstract: Thyroid hormone （TH） is essential for the normal development and metabolism of different tissues,
and also is also vital for fetal development and pregnancy. There is evidence that maternal thyroid hormones can cross the
human placenta and act to modulate fetal endocrine system development before the onset of the fetus’s own thyroid hormone
production. The transport of thyroid hormones has now been shown to require specific transporter proteins, and as yet few data
are available to define the important transporting proteins in the human placenta. To date, members of the organic anion-
transporting polypeptide, L-type amino acid, and of the monocarboxylate transporter families, sodium iodide symporter, have
been identified as thyroid hormone transporters. However, further research is necessary to determine the role of these and
other proteins in placental transport of thyroid hormone, and to investigate how modulations of their function could affect
fetal pathologies such as intrauterine growth restriction, abortion, implantation failure and embryonal, fetal death.

Key words: Thyroid hormone, Placenta, Amino acid transporters, Organic anion transporting polypeptide, Monocarboxylate transporter, Sodium iodide symporter