ABSTRACT
The thyroid hormones thyroxine (T4) and triiodothyronine (T3) play an essential role in mammalian brain development, maturation, and function. Thyroid hormone effects are regional, temporal, and of a defined duration. The cellular targets, specific receptors, timing, and duration of thyroid hormone effects are still not fully understood. Whereas the thyroid synthesizes both hormones, the main hormone, T4, acts as a prohormone to the active hormone, T3. Therefore, collectively these hormones are known as “thyroid hormone.” Thyroid gland activity is predominantly regulated by the pituitary glycoprotein hormone, thyroid-stimulating hormone (TSH). Thyroid hormone exerts a negative feedback effect on pituitary secretion of TSH, such that when T4 concentrations drop, TSH concentrations increase, thus maintaining stable T3 levels. In the brain, T3 is derived in large part from 5′ deiodination of T4 in a pathway closely regulated by developmental and physiologic factors.1 This chapter deals with the relationship between thyroid hormone regulation and glial function. Before addressing the details of this relationship, it is important to note some general points about thyroid hormone transport, the effect of thyroid hormones on brain development, and the role of glia in neurodevelopment.
