The movement of thyroid hormones in the central nervous system.

Thyroid hormones in the central nervous system (CNS) present a number of complexities which include a marked difference in hormone effects on the developing brain compared to the mature brain, extensive regional heterogeneity within the brain, and alternate ways by which the hormones gain entry to the CNS and its cells. Furthermore, most of the T3 found within brain cells is derived from the T4 that they have accumulated. This brief review concerns the movement of the thyroid hormones as they leave the bloodstream and make their way into and through the CNS. There are many sites for potential control of thyroid hormone transport into the central nervous system. The blood brain barrier and the choroid plexus may work in tandem as two parallel resistors subjected to different controls to maintain brain interstitial levels of thyroid hormone at near constant levels. Thyroid hormone may enter neurons directly from the interstitial fluid or indirectly from the glial cells. Nonetheless, both neurons, perhaps at the synaptosome, and glial cells are capable of transporting and concentrating thyroid hormones at physiological levels.