**8. Future challenges**

Further studies are still needed to improve our understanding of the mechanisms mediating the transplacental transport of THs in both human and animals, particularly the role of the different THTs, and the mechanisms that ensure that sufficient amounts of THs are protected from D3 inactivation during their transit across the placenta. Such knowledge would facilitate the development of interventions to increase TH passage in pathological situations, in order to ensure normal fetal development. A better understanding of these mechanisms would also permit us to refine the timing and dosage of the increase in levothyroxine therapy in hypothyroid pregnant women and to establish whether thyroxine on its own is indeed the best form of TH replacement in pregnancy.

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1284.

Elucidation of tissue-, cell-, and sex-specific expression of individual Ds and THTs during the development of both human and animals, in the adult, during aging and when sick.

I hope that new insights into the complex actions by which the THs and their receptors control cell proliferation and differentiation will be provided in the near future.
