How Hormones Work: The Body's Chemical Messages
Hormones travel in the blood to control mood, growth, sleep and hunger. Here is how these tiny messengers run the body.
The Body's Chemical Messaging Network
Your nervous system communicates through electrical signals that travel in milliseconds. But the body has a second, slower communication system that is every bit as vital: the endocrine system, which sends chemical messages called hormones through the bloodstream.
Hormones coordinate processes that unfold over minutes, hours, days or even years — from the immediate surge of adrenaline in a stressful moment to the gradual changes of puberty, pregnancy and ageing. They regulate how fast your cells work, when you sleep, how hungry you feel, how you respond to stress, and much more.
For foundational background, see our hormones explained guide and the endocrine system guide.
What Exactly Is a Hormone?
A hormone is a signalling molecule produced by a specialised cell or gland and released directly into the blood. It travels through the circulation until it reaches a target organ or tissue — which may be on the other side of the body — where it triggers a specific response.
The key feature that makes hormonal signalling precise is the receptor. Each target cell carries specific receptor proteins on its surface (or inside the cell) that fit a particular hormone like a lock and key. Only cells with the matching receptor respond to a given hormone, even though the hormone washes past every cell in the body.
Hormones come in several chemical types. Steroid hormones (like cortisol, testosterone and oestrogen) are made from cholesterol and can cross cell membranes to act directly on genes inside the nucleus. Peptide hormones (like insulin and growth hormone) are made from proteins and bind to receptors on the cell surface, triggering internal signalling cascades.
The Main Hormone-Producing Glands
The endocrine system is made up of glands scattered around the body, each with its own specialised hormone output. The hypothalamus and pituitary gland — both in the brain — act as master controllers, regulating many of the other glands through their own hormones.
| Gland | Key hormones | Main roles |
|---|---|---|
| Hypothalamus | Various releasing/inhibiting hormones | Master regulator; controls the pituitary |
| Pituitary gland | Growth hormone, TSH, ACTH, FSH, LH | Controls other glands; growth; reproduction |
| Thyroid gland | Thyroxine (T4), T3 | Metabolic rate, heart rate, body temperature |
| Adrenal glands | Adrenaline, cortisol, aldosterone | Stress response, blood pressure, salt balance |
| Pancreas | Insulin, glucagon | Blood sugar regulation |
| Ovaries / testes | Oestrogen, progesterone / testosterone | Reproduction, puberty, secondary sex characteristics |
| Pineal gland | Melatonin | Sleep-wake cycle |
How Feedback Loops Keep Hormones Balanced
The body does not simply produce hormones at a constant rate and hope for the best. Most hormonal systems operate through elegant feedback loops that automatically keep levels within a healthy range.
Negative feedback is the most common type. When a hormone level rises, it signals the producing gland — or the hypothalamus and pituitary — to reduce production. As levels fall, the brake releases and production resumes. This keeps most hormones oscillating within a fairly tight band.
The thyroid system illustrates this well. The hypothalamus releases thyrotropin-releasing hormone (TRH), which prompts the pituitary to release thyroid-stimulating hormone (TSH), which prompts the thyroid gland to produce thyroxine. Rising thyroxine levels then feed back to suppress TRH and TSH, slowing their own production. If thyroxine levels fall, the feedback lifts and production ramps up again.
Blood sugar regulation uses a similar principle. After a meal, blood glucose rises; the pancreas releases insulin to drive glucose into cells and bring blood sugar back down. If blood sugar falls too low, the pancreas releases glucagon, which signals the liver to release stored glucose.
Adrenaline and the Stress Response
Few hormones produce effects as immediately felt as adrenaline (also called epinephrine). When the brain perceives a threat or intense stress, it signals the adrenal glands — two small glands sitting atop the kidneys — to release adrenaline into the bloodstream within seconds.
The effects are rapid and coordinated: heart rate rises, airways in the lungs widen, blood is redirected from digestion to muscles, and glucose is released from stores. Pupils dilate. The body is primed for rapid physical action — the classic "fight or flight" response.
Cortisol, another adrenal hormone, plays a complementary role over a longer timescale. It sustains the stress response, regulates immune function, maintains blood pressure and ensures a supply of energy. Chronically elevated cortisol — from prolonged stress — has measurable negative effects on immune function, sleep, memory and metabolism. This is one of the biological pathways through which chronic stress affects physical health.
Hormones of Growth and Development
Growth hormone (GH), produced by the pituitary gland, drives physical growth throughout childhood and adolescence. It acts on bone and muscle tissue to stimulate cell division and protein synthesis. GH is released in pulses, predominantly during sleep, which is one reason adequate rest is so important for growing children.
Puberty is orchestrated by a cascade of hormones. The hypothalamus and pituitary begin releasing gonadotropins (FSH and LH), which stimulate the gonads to produce sex hormones: testosterone in males, and oestrogen and progesterone in females.
These hormones drive the physical changes of puberty — growth spurts, development of reproductive organs, changes in body composition and the gradual maturation of the reproductive system. The process typically begins between ages 8 and 13 in girls and 9 and 14 in boys, with considerable individual variation.
Insulin: The Blood Sugar Manager
Insulin is one of the most studied hormones in medicine, not least because its dysfunction underlies diabetes. Produced by beta cells in the pancreas, insulin is released in response to rising blood glucose after a meal.
Insulin acts like a key: it binds to receptors on muscle, fat and liver cells, unlocking the door for glucose to enter. This lowers blood glucose back toward a normal range and allows cells to use glucose as fuel or convert it to storage forms (glycogen or fat).
In type 1 diabetes, the immune system destroys the insulin-producing beta cells, so no insulin is made. In type 2 diabetes, cells become less responsive to insulin (insulin resistance), and the pancreas eventually cannot compensate by producing ever more. Both conditions require medical management. This article is educational only — for any concerns about blood sugar, please consult a healthcare professional.
Melatonin and the Sleep Cycle
Melatonin, produced by the pineal gland in the brain, is often called the "sleep hormone" — though "darkness hormone" might be more accurate. Its production rises in the evening as light levels fall and drops again in the morning as light increases.
Melatonin does not force sleep; it signals to the body that it is nighttime, helping to synchronise the internal clock (circadian rhythm) with the environment. It is one part of a broader system that promotes sleep, including falling core body temperature and rising levels of a sleep-promoting substance called adenosine.
Exposure to bright light — particularly the short-wavelength blue light from screens — in the evening suppresses melatonin production, which is why using screens before bed can make it harder to fall asleep. This is a real biological effect, not merely speculation.
Explore more on the defence and hormones category page, or take the biology revision tool to test your knowledge. The human body flashcards are also a good way to memorise the key glands and their hormones.