Hormones are messenger substances that are produced in glands or specialized cells of the body. Hormones are used to transmit information to control metabolism and organ functions, whereby each type of hormone is assigned a suitable receptor on a target organ. To reach this target organ, hormones are usually released into the blood (endocrine). Alternatively, the hormones act on neighbouring cells (paracrine) or on the hormone producing cell itself (autocrine).


Depending on their structure, hormones are divided into three groups: Peptide hormones consist of protein (peptide = egg white), glycoprotein hormones also have a sugar residue (protein = egg white, glykys = sweet, “sugar residue”). As a rule, these hormones are first stored in the hormone-producing cell after their formation and only released (secreted) when required. Steroid hormones and calcitriol, on the other hand, are derivatives of cholesterol.

These hormones are not stored, but released directly after their production. Tyrosine derivatives (“tyrosine derivatives”), the last group of hormones, include catecholamines (adrenaline, noradrenaline, dopamine) and thyroid hormones. The basic structure of these hormones consists of tyrosine, an amino acid.

  • Peptide hormones and glycoprotein hormones
  • Steroid hormones and Calcitriol
  • Tyrosine derivatives

Hormones control a variety of physical processes. These include nutrition, metabolism, growth, maturation and development. Hormones also influence reproduction, performance adjustment and the body’s internal environment.

Hormones are initially formed either in so-called endocrine glands, in endocrine cells or in nerve cells (neurons). Endocrine means that the hormones are released “inwards”, i.e. directly into the bloodstream and thus reach their destination. The transport of the hormones in the blood takes place bound to proteins, whereby each hormone has a special transport protein.

Once they have reached their target organ, hormones unfold their effect in different ways. First and foremost, a so-called receptor is required, which is a molecule with a structure that matches the hormone. This can be compared with the “key-lock principle”: the hormone fits exactly into the lock, the receptor, like a key.

There are two different types of receptors: Depending on the type of hormone, the receptor is located on the cell surface of the target organ or inside the cells (intracellular). Peptide hormones and catecholamines have cell surface receptors, whereas steroid hormones and thyroid hormones bind to intracellular receptors. Cell surface receptors change their structure after hormone binding and thus initiate a signaling cascade inside the cell (intracellular).

Via intermediate molecules – so-called “second messengers” – reactions with signal amplification take place, so that the actual effect of the hormone finally occurs. Intracellular receptors are located inside the cell, so that the hormones first have to overcome the cell membrane (“cell wall”) that borders the cell in order to bind to the receptor. Once the hormone has bound, the gene reading and the resulting protein production are modified by the receptor-hormone complex.

The effect of the hormones is regulated via activation or deactivation by changing the original structure with the help of enzymes (catalysts of biochemical processes). If hormones are released at their place of formation, this either occurs in an already active form or alternatively, enzymes are activated peripherally. The deactivation of the hormones usually takes place in the liver and kidney.

  • Cell surface receptors
  • Intracellular receptors