| THE HYPOTHALAMO-PITUITARY-THYROID AXIS
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| Thyrotropin-releasing hormone (TRH)
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| TRH is manufactured in the hypothalamus and transported via the portal circulation to the pituitary where it ultimately leads to secretion of TSH
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| TRH is a modified tripeptide synthesized as a 26 kDa prohormone by peptidergic hypothalamic nuclei and transported, after processing, to the anterior pituitary by the portal circulation. It is secreted in pulsatile fashion. TRH stimulates TSH synthesis and secretion by binding to receptors on the pituitary thyrotroph cell membrane that are linked to phospholipase C. The resulting phosphoinositides stimulate the release of calcium from intracellular storage sites and so lead to secretion of TSH. More chronic actions of TRH include stimulation of TSH subunit biosynthesis and TSH glycosylation. The number of TRH receptors on the thyrotrophs is down-regulated both by the concentration of TRH itself and by thyroid hormones.
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