Thyrotropin Releasing Hormone

Synonym: TRH

	Table of contents
	1. Definition 2. Composition 3. Physiology 3.1. Hypothalamo-pituitary system 3.2. Central nervous system 3.3. Gastrointestinal tract 3.4. Contextual isolation 4. Clinical significane

Definition

Thyrotropin releasing hormone (TRH) is a signalling substance that is produced in several organ systems. There it plays an important role in their respective systeomics.

Composition

The Tripeptide TRH (pyroGlu-His-ProNH₂) has a molecular weight of 362. Its structure has been first described in 1962.

Physiology

Hypothalamo-pituitary system

TRH acts as a releasing hormone that is formed by hypothalamic neurons. It is conveyed via the portal vessel system that connects hypothalamus and pituitary gland. In the pituitary it stimulates secretion of TSH after being bound to TRHR1-receptors in the context of the thyrotropic feedback control.

Via G-proteins it delivers a setpoint for the feedback control. Additionaly, formation of TRH is inhibited in the form of degenerative feedback by thyroid hormones.

Hormones and Transmitters that are apt to influence TRH secretion are:

• Stimulating factors
  • Catecholamines
  • Alpha-MSH
  • CART
  • VIP (presumably)
  • Histamin (presumably)
• Inhibiting factors
  • T4 und T3 (in thyrotropic zone only)
  • AGRP
  • Dopamin
  • GABA
  • NPY
  • Estrogenes
  • Glucocorticoids
  • Cytokines (presumably)
  • Serotonin (presumably)
  • Somatostatin (presumably)
  • TRH (presumably)

Central nervous system

TRH can be found in several areas of central nervous system. More than 2/3 of this central fraction are to be found outside the zone that governs thyrotropic feedback control. Being bound to TRHR1- and TRHR2-recetpors TRH acts here as neurotransmitter or neuromodulator. It plays a central role in body temperature control, in the limbic system, in learning, in sleep, in processing of pain and in sensoric perception.

Gastrointestinal tract

In the gastrointestinal tract TRH has multiple effects, particularly on motility of stomach and intestine and on secretion of Insulin and pancreatic enzymes.

Contextual isolation

In peripheral blood plasma TRH is found in negligibly low concentration. Its several sites of action are contextually isolated by blood-brain-barrier or the concentration gradient at the end of the portal vessel systems that are found both between hypothalamus and pituitary and in gastrointestinal tract, respectively. Thus, interactions among the different functions of TRH are prevented.

Clinical significane

As TRH is found in blood plasma in low concentration only and as this concentration is unrelated to the respective function state of the organism TRH is usually not determined in plasma. In contrast, a TRH-test is useful to identify certain disorders of thyrotropic feedback control. It is also used in combined pituitary tests.