Hormones are chemical messengers secreted by endocrine glands that travel through the bloodstream to regulate the activity of distant target tissues. They control metabolism, growth, reproduction, and homeostasis through highly specific interactions with cellular receptors.
Chemical Classification
Hormones are classified by their chemical structure, which determines their solubility, transport, receptor location, and mechanism of action. Peptide hormones are chains of amino acids ranging from small peptides such as thyrotropin-releasing hormone, a tripeptide, to large glycoproteins such as follicle-stimulating hormone. They are water-soluble, stored in secretory vesicles, and act through cell surface receptors. Synthesis involves transcription and translation, with most peptide hormones synthesized as larger precursor prohormones that are proteolytically processed to the active form.
Steroid hormones are derived from cholesterol and include the sex hormones, corticosteroids, and mineralocorticoids. They are lipid-soluble, synthesized on demand rather than stored, and bind to carrier proteins for transport in blood. Steroid hormones diffuse across the plasma membrane and act through intracellular nuclear receptors that directly regulate gene transcription. Amino acid-derived hormones include the catecholamines from tyrosine, thyroid hormones from tyrosine, and melatonin from tryptophan.
Endocrine, Paracrine, and Autocrine Signaling
Endocrine signaling involves hormones released into the bloodstream to act on distant target cells. This is the classical definition of hormone action. Paracrine signaling involves chemical messengers that act on neighboring cells without entering the bloodstream. Somatostatin in the pancreatic islets inhibits insulin and glucagon secretion from adjacent cells. Autocrine signaling acts on the same cell that produced the messenger. Prostaglandins and cytokines often act through autocrine and paracrine mechanisms.
Major Endocrine Glands
The hypothalamus produces releasing and inhibiting hormones that control the anterior pituitary. Thyrotropin-releasing hormone stimulates TSH release, corticotropin-releasing hormone stimulates ACTH release, and gonadotropin-releasing hormone stimulates LH and FSH release. Growth hormone-releasing hormone and somatostatin provide opposing control of growth hormone secretion.
The anterior pituitary produces growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, prolactin, luteinizing hormone, and follicle-stimulating hormone. The posterior pituitary stores and releases oxytocin and antidiuretic hormone synthesized in the hypothalamus. The thyroid gland produces triiodothyronine and thyroxine, which regulate metabolic rate, and calcitonin, which lowers blood calcium. The parathyroid glands produce parathyroid hormone, which raises blood calcium. The adrenal cortex produces cortisol, aldosterone, and androgens, while the adrenal medulla produces epinephrine and norepinephrine. The pancreatic islets produce insulin, glucagon, and somatostatin. The gonads produce sex hormones, and the kidneys produce erythropoietin and renin.
Hormone Transport
Peptide and catecholamine hormones are water-soluble and travel freely in plasma. Steroid and thyroid hormones are hydrophobic and require carrier proteins for transport. Sex hormone-binding globulin transports androgens and estrogens, corticosteroid-binding globulin transports cortisol, and thyroid-binding globulin transports thyroid hormones. Only the free, unbound fraction of these hormones is biologically active. Carrier proteins extend hormone half-life by protecting against metabolism and filtration, creating a reservoir that buffers rapid fluctuations.
Hormone Half-Life and Clearance
Hormone half-lives vary from seconds to days. Peptide hormones have short half-lives, typically minutes, and are cleared by proteolysis in the liver, kidneys, and blood. Catecholamines are rapidly inactivated by enzymatic degradation. Steroid hormones have intermediate half-lives of hours, and thyroid hormones have the longest half-lives, measured in days. Hormone clearance involves metabolic conversion in the liver, excretion in bile or urine, and receptor-mediated uptake.
Feedback Regulation
Hormone secretion is regulated primarily by negative feedback. In the hypothalamic-pituitary-thyroid axis, TRH stimulates TSH, which stimulates thyroid hormone production. Thyroid hormones then inhibit both TRH and TSH secretion, maintaining stable hormone levels. Positive feedback occurs in specific situations, such as the estrogen-induced LH surge that triggers ovulation.
