What is a hormone ?

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 :

• Growth and development (including puberty)
• Metabolism incl. metabolic rate, Basal Metabolic Rate (BMR) and the risk of obesity - see also health risks of obesity.
• Physical appearance (including body shape)
• Reproduction
• Emotions and emotional responses

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

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:

• Peptide hormones consist of chains of amino acids and vary in size and complexity.
  • Small peptide hormones
    e.g. thyrotrophin-releasing hormone and anti-diuretic hormone (also known as vasopressin).
  • Proteins - Peptides that are formed from 20+ up to 100s of amino acids are proteins
    e.g. insulin and human growth hormone (HGH)
  • Glycoprotein hormones are even more complicated molecules that have carbohydrate side-chains
    e.g. luteinizing hormone (LH), follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH)
• Nonpeptide hormones - another category of water-soluble hormones.
• Lipid and phospholipid-derived hormones include steroid hormones derived from cholesterol and the eicosanoids (also known as icosanoids*). Examples of steroid hormones include testosterone and cortisol.
• Monoamines are derived from aromatic amino acids such as phenylalanine, tyrosine and tryptophan. Note that in this context the term "aromatic" refers primarily to the structure of the molecules - not to their scent or odour as in aromatherapy.

* 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.

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.