Thyrotropin-releasing hormone (TRH), is a widely distributed tripeptide which acts as a hormone, a paracrine regulatory factor and a neurotransmitter/neuromodulator; it is expressed in many parts of the brain as well as in various peripheral tissues. nIn the hypothalamus, TRH is secreted from the paraventricular nucleus (VPN) by TRH neurons prior to a post-translational processing from its precursor, pro-TRH. A key player in the regulation of the hypothalamus-pituitary-thyroid (HPT) axis, TRH is released into the hypophyseal portal circulation of the median eminence and reaches the pituitary, where it binds to TRH receptors, thereby stimulates the secretion of tyrothropin (thyroid-stimulating hormone; TSH). TSH, in turn, affects the thyroid to produce T4 which is subsequently converted into its biologically-active analog, T3 - a negative mediator of TRH. In the tanycytes, ependymal cells of glial origin located in close proximity to the VPN, T4 is converted to T3 by the enzyme deiodinase II (dio2) - a counterpart to deiodinase III (dio3) which conversely inactivates the above mentioned thyroid hormones, thus so indirectly promotes the production of TRHs