Thyroid hormones (TH; T₄ and T₃) are important for the normal physiological and morphological development of the central nervous system. The T₄-binding protein, transthyretin (TTR) transports T₄ from the maternal blood to the fetal brain. T₄ transported to the brain is taken up by astrocytes via the Oatp1c1 cell membrane transporter. T₄ is deionated via D2 within the astrocyte and produced T₃. Then, it is secreted out of the cell via MCT8 which is a cell surface transporter protein in astrocytes. T₃ is taken up via MCT8 by neurons. TH receptors are T₃-dependent nuclear transcription activators and mediate the biological activities of T₃.

As a result, the studies showed that the effects of TH on embryonal brain development is affected byTH level, presence of selenoenzymes and TH transporters, THR functions and these effects showed variations according to different parts of brain and at different level of development.