What is a hormone ?

Simple definition of a hormone:

A hormone is:

  • a chemical (compound - the smallest individual units of of a specific hormones being molecules of that substance, as opposed to atoms or ions)
  • that is produced and released by the endocrine system (see right for a diagram of the main human endocrine glands) of a living organism such as the human body*.

So, hormones are chemicals (sometimes described as 'chemical messengers') that are produced and released by cells and glands in the body that, together, form the endocrine system.

Strictly, endocrine glands are not the only structures in the body that secrete hormones. Certain hormones are secreted by endocrinocytes in some other organs & tissues.

There are many different hormones (see the list of hormones) that can be grouped together in different ways, e.g. according to which gland they are secreted by, which system(s) of the body they affect, and the chemical structure of the hormone molecules themselves - which determines how they interact with the body's cells in order to fulfill their function of conveying instructions to specific cells / tissues in the body.

What do hormones do ?

In simple terms, hormones act as 'messengers' by moving around the body then conveying instructions to the cells they interact with.

Hormones can affect many aspects of the body and its state of health, such as :

Specific hormones give specific instructions to specific types of cells.
Remember: Hormones can be thought of as the body's 'chemical messengers'.

Why Chemical ?

"Chemical" because hormones are chemicals and there is also another means by which messages (that is, instructions) are sent around the body. The other method is the nervous system, which sends electrical signals.

Types of Hormones

In general, hormones can take the form of protein-based molecules (water-soluble) or lipophillic (fat soluble). For more about how these differ see water-soluble hormones vs fat-soluble hormones.

There are groups or categories of hormones that share the same or similar characteristics, e.g. the 3 chemical classes of vertebrate hormones:

* International Nonproprietary Name and the names most commonly used in the United States.

Hormones compared with Nerves

Compare the effects of hormones with those of nerves:

Both hormones (chemical messengers) and nerves (which carry electrical signals) move information and instructions around the body. Hormones are produced by the glands of the endocrine system while nerves are part of the nervous system. Although both the endocrine system and the nervous system transmit information and instructions around the body and so help to maintain homeostasis, they operate in different ways. It is useful to be able to compare the endocrine system with the nervous system.

(Endocrine System)

(Nervous System)

Signals transmitted by:


Electrical impulses

Target cells:

Many different types of cell throughout the body can be affected by hormones. However, specific hormones only affect cells with receptors matching that particular hormone. Therefore hormones can be very accurate in affecting only the appropriate cells.

The nervous system sends signals to specific types of cells, such as nerves, muscles and certain gland cells. However, a single neuron (nerve cell) can affect many muscle fibres (muscle cells) e.g. causing them all to contract.

Speed of signalling (transmission):

Relatively slow (seconds - days)

Very fast (m sec)

Duration of response:
(How long does the effect last?)


Very short (can seem immediate / instant e.g. pain response)

The nervous system and the endocrine system co-operate with each other to achieve and maintain homeostatis efficiently.

The neuroendocrine system is co-ordinated by the hypothalamus. Neuroendocrine cells receive neurally transmitted information (i.e. via the nervous system) and respond by releasing hormones to be transported to their target cells via blood circulation.

See also hormone solubility and the three triggers for hormone release.

More about the Endocrine System

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