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How does blood carry hormones from endocrine glands to target tissues?

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Blood plasma — blood Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).

Blood carries hormones from endocrine glands to target tissues primarily through the plasma, the liquid component of blood. Most hormones are secreted directly into this general circulation to reach distant target organs.12

In the serum, hormones exist in two states: as free, unbound molecules or bound to specific transport proteins produced by the liver. These proteins, such as sex hormone-binding globulin or thyroxine-binding globulin, act as reservoirs that maintain a stable concentration of biologically active free hormones. As free hormones leave the circulation to act on tissues, bound hormones are released from their transport proteins to maintain equilibrium.2

Additionally, the body utilizes specialized portal circulations for more direct transport. For example, the hypothalamic-hypophyseal portal system delivers neurohormones directly from the hypothalamus to the pituitary gland, while the hepatic portal circulation routes hormones from the pancreas to the liver before they enter general circulation.2