Thyroid hormones are vital for fetal development and can act directly on placental tissues to modify their metabolism, differentiation and development. There is evidence that maternal thyroid hormones can cross the human placenta and act to modulate fetal development before the onset of the fetus's own thyroid hormone production. Plasma membrane transport of thyroid hormones has now been shown to require specific transporter proteins. Several proteins have recently been identified as specific thyroid hormone transporters. However, as yet few data are available to define the functionally important transporter proteins in the human placenta. To date, members of the organic anion-transporting polypeptide, L-type amino acid, and of the monocarboxylate transporter families, have been identified as thyroid hormone transporters that are active in a variety of placental cell types. 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.