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Hormones
Last reviewed: 05.07.2025
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Hormones are a group of compounds of various chemical structures, characterized by the ability, after being released from the cells in which they are formed, to reach target cells (most often with blood) and, by binding to specific protein molecules of target cells (receptors), cause more or less specific changes in metabolism in the latter. Approximately 100 hormones have been described in humans. After inactivation, hormones are excreted from the body in an inactive form. The rate of formation and destruction of hormones depends on the needs of the body.
The main sites of hormone synthesis are the hypothalamus, the anterior and posterior lobes of the pituitary gland, the thyroid and parathyroid glands, the islets of the pancreas, the cortex and medulla of the adrenal glands, the sex glands, the placenta, certain cells of the gastrointestinal tract, the brain, the myocardium, and adipose tissue. Hormones can also form tumors of non-endocrine tissues (the so-called ectopic production of hormones).
Hormones are transported by blood. Most hormones (especially those of protein and peptide nature) are highly soluble in water, and therefore in blood plasma. The exceptions are T4 and steroid hormones. They are transported by blood with the help of special carrier proteins. Solubility and interaction with the carrier affect the half-life of hormones in the blood. Most peptide hormones have a very short half-life - 20 minutes or less. Hydrophobic steroid hormones have a significantly longer half-life (cortisol approximately 1 hour, T4 - 7 days).
Hormones circulate in the blood in very low concentrations (usually approximately 10 -6 -10 -9 mol/l), but the number of molecules corresponding to this concentration is enormous (10 -1014 molecules/l) - practically trillions of molecules in 1 liter of blood. This enormous number of hormone molecules makes it possible for them to influence every single cell of the body and regulate its specific metabolic processes. Circulating hormones do not act on all cells equally. The selectivity of hormone action is ensured by specific receptor proteins localized on the cell membrane or in the cytoplasm of target cells. The number of receptors on cell membranes can be thousands or even tens of thousands. The number of receptors on a target cell is not constant and is usually regulated by the action of the corresponding hormones. Usually, with a constantly elevated concentration of a hormone in the blood, the number of its receptors decreases. The specificity of receptors is often low, so they can bind not only hormones, but also compounds similar to them in structure. The latter circumstance can cause a disruption in hormonal regulation, manifested by tissue resistance to the action of hormones.
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