What Is a Hormone? A Message Sent to Everywhere and Read by a Few

The mechanism has three parts and the specificity lives entirely in the third. A gland synthesises and releases the hormone, circulation distributes it indiscriminately, and a target cell responds only if it carries a receptor for that molecule. The chemistry determines where the receptor sits. Water-soluble hormones, including peptides such as insulin and the catecholamines, cannot cross a cell membrane, so their receptors are on the cell surface and binding triggers a cascade of internal messengers that amplifies the signal, producing effects within seconds to minutes. Fat-soluble hormones, meaning the steroids and thyroid hormones, pass straight through the membrane and bind receptors inside the cell, which then act directly on DNA to change which genes are transcribed, producing effects over hours to days that persist. That is why adrenaline works instantly and testosterone works over months, and why steroid effects are so hard to reverse quickly.

The endocrine system is a distributed set of organs, several of which do other jobs as well:

• •The hypothalamus, which links the nervous and hormonal systems and controls the pituitary directly
• •The pituitary, historically called the master gland, which releases growth hormone and a set of hormones whose job is to instruct other glands
• •The thyroid, setting metabolic rate throughout the body, with underactivity producing fatigue and weight gain and overactivity the reverse
• •The adrenals, producing cortisol for the stress response and metabolism, aldosterone for salt and water balance, and adrenaline for immediate emergencies
• •The pancreas, whose islet cells release insulin to lower blood glucose and glucagon to raise it, the failure of which is diabetes
• •The ovaries and testes, producing the sex hormones that drive development, reproduction and a great deal else
• •Less obvious sources, since the gut releases hormones controlling appetite and digestion, fat tissue releases leptin signalling energy stores, bone releases hormones affecting metabolism, and the heart releases a hormone regulating blood pressure

Almost every hormonal system is a negative feedback loop, which is what keeps levels within a range without any central controller. The thyroid axis is the standard illustration: the hypothalamus releases a hormone that tells the pituitary to release another, which tells the thyroid to produce thyroid hormone, and circulating thyroid hormone then suppresses both the hypothalamus and the pituitary, so that rising levels reduce the signal to produce more. That structure is why a blood test measuring the pituitary hormone is the standard screen for thyroid disease, since it responds sensitively to whether the thyroid is delivering. It also explains a clinical hazard: administering a hormone from outside suppresses the body's own production through the same loop, which is why long courses of steroids must be tapered rather than stopped, since the adrenal glands have been idle and need time to resume. Positive feedback loops exist and are rare because they are unstable by nature, which is exactly why they are used for events meant to run to completion, including childbirth and ovulation.

Endocrine disorders divide into too much, too little and resistance. Too little insulin action produces diabetes, in the first type because the insulin-producing cells are destroyed by the immune system and in the second because tissues stop responding to normal levels, which is resistance rather than deficiency. Too little thyroid hormone is among the commonest disorders anywhere and is straightforward to treat; too much produces weight loss, tremor and heart rhythm problems. Excess cortisol, from a tumour or from prescribed steroids, produces a characteristic syndrome, and insufficient cortisol is life-threatening. Growth hormone excess before the growth plates close produces gigantism and afterwards acromegaly. Because hormones are potent at extremely low concentrations, measured in nanograms per millilitre, small changes matter, and because the effects are systemic, the symptoms are frequently vague and attributed to something else for years, which is why endocrine conditions are commonly diagnosed late.

A substantial research area concerns compounds in the environment that bind hormone receptors or block them, called endocrine disruptors. The concern arises because the systems operate at very low concentrations and because timing matters enormously during development, so an exposure that is harmless to an adult may not be to a foetus. The clearest historical case is diethylstilbestrol, prescribed to pregnant women from the 1940s and causing a rare cancer and reproductive abnormalities in their daughters, which established that prenatal hormonal exposure can produce effects appearing decades later. Bisphenol A, certain phthalates and some pesticides have receptor activity and are regulated in several jurisdictions, with continuing argument about doses and about whether standard toxicology, which assumes effects rise with dose, applies to compounds whose natural counterparts work in loops with thresholds. Separately, the deliberate use of hormones is enormous and routine, covering contraception, fertility treatment, thyroid replacement, insulin, gender-affirming care and the hormone therapies used in several cancers.

A hormone is broadcast through the bloodstream to every tissue and acts only where a matching receptor exists, which is what makes hormonal signalling slower and broader than nervous signalling. Water-soluble hormones bind receptors on the cell surface and act in seconds, while fat-soluble ones enter the cell and change gene transcription over hours to days. Almost every axis is a negative feedback loop, which is why administered hormones suppress the body's own production and why steroids must be tapered, and disorders divide into excess, deficiency and receptor resistance.