Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream to regulate distant target organs. Unlike exocrine secretions that travel through ducts, endocrine signals reach nearly every cell via systemic circulation. Yet, only cells possessing specific receptors will respond to a given hormone. This chemical communication system coordinates long-term processes such as growth, metabolism, reproduction, and fluid balance.
Hormones are broadly categorized based on their chemical composition, which dictates their synthesis, storage, transport, and receptor interaction. The three primary structural classes include:
These hormones are synthesized from individual amino acids, primarily tyrosine and tryptophan. Catecholamines such as epinephrine and norepinephrine are derived from tyrosine and behave like water-soluble peptide hormones. Conversely, thyroid hormones (thyroxine and triiodothyronine) are also derived from tyrosine but are lipid-soluble, requiring carrier proteins in circulation.
Peptide and protein hormones consist of amino acid chains, synthesized initially as inactive large precursors called prohormones. These are packaged into vesicles and secreted via exocytosis when stimulated. Because they are water-soluble (hydrophilic), they dissolve easily in blood plasma but cannot cross cell membranes independently. Common examples include insulin, glucagon, and antidiuretic hormone.
Steroid hormones are hydrophobic molecules derived from cholesterol. They include sex hormones like estrogen, progesterone, and testosterone, as well as corticosteroids such as cortisol and aldosterone. Because they are lipid-soluble, they diffuse freely across cell membranes but require specific transport proteins to travel through the watery environment of the bloodstream.
The chemical nature of a hormone dictates its transport mechanism. Water-soluble hormones travel freely as free hormones in plasma and have a short half-life because enzymes degrade them quickly. Lipid-soluble hormones bind to plasma carrier proteins, which protect them from degradation and prolong their half-life in circulation. Only the small fraction of unbound lipid-soluble hormone remains biologically active and able to enter target cells.
The location of a hormone's receptor depends entirely on its chemical solubility. Water-soluble hormones bind to extracellular receptors on the cell membrane, activating second messenger systems like cyclic AMP to alter cell activity rapidly. Lipid-soluble hormones diffuse directly through the plasma membrane to bind intracellular receptors inside the cytoplasm or nucleus, directly influencing gene transcription and protein synthesis.