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Thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH), a neurohormone, is the simplest of the hypothalamic neuropeptides. It consists essentially of three amino acids in the sequence glutamic acid–histidine–proline. The simplicity of this structure is deceiving for TRH is involved in an extraordinary array of functions. Not only is it a neurohormone that stimulates the secretion of thyroid-stimulating hormone from the pituitary, and, quite independently, the secretion of another pituitary hormone called prolactin, but it also subserves other central nervous system activities because it is a widespread neurotransmitter or neuromodulator within the brain and spinal cord. There is evidence that TRH is involved in the control of body temperature and that it has psychological and behavioral effects, at least in animals. It may also have therapeutic value. There are studies suggesting that it mitigates the damage induced by spinal cord injury and that it leads to some improvement in the nervous disease known as amyotrophic lateral sclerosis (Lou Gehrig's disease).

These multiple effects are less surprising when it is considered that TRH appeared very early in the evolutionary scale of vertebrates and that, while the concentration of TRH is greatest in the hypothalamus, the total amount of TRH in the remainder of the brain far exceeds that of the hypothalamus. The TRH-secreting cells are subject to stimulatory and inhibitory influences from higher centres in the brain and they also are inhibited by circulating thyroid hormone. In this way TRH forms the topmost segment of the hypothalamic-pituitary-thyroid axis.
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