What the endocrine system does
The endocrine system is a set of glands that release hormones, chemical messengers, into the blood. Hormones regulate long-term processes: growth, metabolism,stress responses, reproduction, and homeostasis (keeping conditions such as blood glucose, water and salt levels steady). It works alongside the nervous system: nerves send fast, targeted signals, while hormones act more slowly but often for longer and over the whole body.
The major glands
| Gland | Main hormones | Main effects |
|---|---|---|
| Hypothalamus | Releasing hormones, ADH | Controls the pituitary; links brain and hormones |
| Pituitary | Growth hormone, TSH, ACTH, FSH, LH, ADH | Growth; controls other glands; water balance |
| Thyroid | Thyroxine (T4), T3 | Metabolic rate, growth and development |
| Adrenal glands | Adrenaline, cortisol, aldosterone | Fight or flight, stress, salt balance |
| Pancreas | Insulin, glucagon | Blood glucose control |
| Ovaries / testes | Oestrogen, progesterone / testosterone | Puberty and reproduction |
How hormones work
Hormones travel everywhere in the blood, but only target cells with the matchingreceptor respond, like a key fitting a lock. Some hormones, such as adrenaline and insulin, bind to receptors on the cell surface and trigger rapid changes inside. Others, such as thyroid and steroid hormones, enter the cell and switch genes on or off, causing slower, longer-lasting effects.
Negative feedback: the thyroid
Most hormone levels are controlled by negative feedback. The hypothalamus releasesTRH, which tells the pituitary to release TSH, which tells the thyroid to makethyroxine (T4). T4 then reduces TRH and TSH. If T4 rises, the chain is turned down; if it falls, the chain is turned up. This explains thyroid problems: lack of iodine means the thyroid cannot make enough T4, so TSH stays high and the gland swells into a goitre; in an overactive thyroid, T4 is high and TSH almost disappears.
Controlling blood glucose
Cells need a steady supply of glucose, and the brain especially. After a meal, rising glucose triggers the pancreas to release insulin, which helps muscle, fat and liver cells take up glucose; the liver stores it asglycogen. Between meals, falling glucose triggers glucagon, which tells the liver to break glycogen down and release glucose. Together they keep glucose at about 4–7 mmol/L. In type 1 diabetes the insulin-making cells are destroyed, so insulin must be injected; in type 2 diabetes cells respond poorly to insulin.
Stress, growth and reproduction
In an emergency the adrenal glands release adrenaline within seconds: heart rate and breathing increase and glucose is released for the muscles. Longer-term stress raises cortisol, which keeps glucose high but can weaken the immune system. Growth hormone from the pituitary drives growth in childhood. Sex hormones from the ovaries and testes cause puberty and control the menstrual cycle and sperm production.
Hormones and health
Many conditions involve hormones, including diabetes, thyroid disorders and growth disorders. Blood tests measure hormones such as TSH, T4 and glucose. Treatments often replace a missing hormone, such as insulin injections or thyroxine tablets, which is why understanding feedback matters: doses are adjusted using feedback signals such as TSH levels.
Key terms
- Hormone
- A chemical messenger made by a gland and carried in the blood to target organs.
- Target organ
- An organ whose cells have receptors for a particular hormone.
- Negative feedback
- A control system where a change triggers a response that reverses it.
- Homeostasis
- Keeping internal conditions within narrow limits.
Common misconceptions
- “Hormones only matter during puberty.” Hormones control body processes every minute of life.
- “Each hormone affects every cell.” Only cells with the right receptors respond.
- “More hormone is always better.” Too much is as harmful as too little; feedback keeps levels balanced.
Educational model only. These are simplified models: real hormone systems involve many more hormones, feedback loops and differences between people. Values shown are illustrative, and this simulator cannot be used to diagnose or manage diabetes, thyroid conditions or any other condition, or to choose doses of medicine. Always follow the advice of a qualified healthcare professional. See our educational and medical disclaimer.
Key takeaways
- Endocrine glands release hormones into the blood; only target cells with receptors respond.
- The hypothalamus and pituitary control many other glands.
- Negative feedback keeps hormone levels steady, as in the thyroid axis.
- Insulin and glucagon keep blood glucose in a narrow range.
Model note: thyroid levels are relative to normal and use a simple three-hormone feedback loop; the glucose model uses simplified rates tuned to typical textbook responses.