When TRH was chemically sequenced and synthesised, it became possible to use specific and sensitive methods to study its tissue distribution. One of the most surprising findings to arise from this work was that TRH was distributed widely outside of the classical thyrotrophic area of the hypothalamus. TRH has been found in virtually all parts of the brain, including the cerebral cortex; the spinal cord; nerve endings abutting upon the ventral motor horn cells and upon the intermediolateral column of the spinal cord; the neurohypophysis; and the pineal gland. TRH has also been found in pancreatic islet cells and in various parts of the gastrointestinal tract. Although present in low concentrations in these areas, the aggregate in extrahypothalamic tissue far exceeds the total amount in the hypothalamus.
