hormone
Meaning of hormone: Receptors, not the messenger, decide the effect, which is why a single hormone can quicken the heart and slow the gut at once.
Definition of hormone
A hormone is a chemical signal released into the bloodstream by one tissue to change the behavior of cells elsewhere. It reaches every part of the body but acts only where cells carry a receptor able to recognize it. That is what allows a message broadcast to the entire circulation to produce a specific response in a few organs and none at all in the rest.
Where do hormones come from?
The classic sources are the endocrine glands: pituitary, thyroid, parathyroid, adrenal, pineal, the islets of the pancreas, the ovaries and the testes. But describing a hormone as something a gland makes is now too narrow. Fat tissue releases leptin, which reports the size of the body's energy stores. The stomach releases ghrelin before meals and the small intestine releases hormones that prime the pancreas as food arrives. The heart releases a peptide when its chambers are stretched, the kidney releases the signal that drives red cell production, and bone releases hormones of its own. Tissue that no one would call a gland is doing endocrine work throughout the body.
How does one hormone produce several different effects?
Through the hormone receptor. The same molecule finds different receptor types on different tissues, and each receptor is wired to a different internal response. Adrenaline is the clearest example: it speeds the heart, widens the airways, narrows some blood vessels while widening others, and shuts down digestion, all from one signal. The chemistry of the hormone also sets its speed.
| Class | Examples | How it acts |
|---|---|---|
| Peptide and protein | Insulin, growth hormone | Bind receptors on the cell surface; effects within seconds to minutes |
| Steroid | Cortisol, estrogen, testosterone | Pass through the membrane and alter gene activity; slower, longer lasting |
| Amine | Thyroid hormones, adrenaline | Mixed, depending on the receptor involved |
What keeps levels steady?
Negative feedback. A control center releases a signal, the target gland responds, and the hormone produced feeds back to shut the original signal down, in the way a thermostat turns off a heater. Release is rarely constant: many hormones are secreted in pulses or on a daily rhythm, cortisol peaking in the early morning and falling overnight, so the time a blood sample is drawn can matter as much as the result.
Used in a sentence
Blood testing measured several hormone levels as part of the endocrine evaluation.
Good to know
A single hormone commonly affects several organs through receptors present in different tissues, which is why a change in one hormone level can produce a set of effects that appear unrelated to each other.
Word origin
Where “hormone” comes from
From Greek hormon, the present participle of horman, meaning to set in motion or urge on.