Indoleamines are organic compounds made up of indole groups that have an amine group on them. Indoles are nitrogen-containing molecules that comprise part of many biologically active molecules. In the case of indoleamines, they are part of the structure of the essential amino acid L-tryptophan. The metabolism of this compound is thought to be the first step in the biosynthesis of indoleamines.

Many of the indoleamines have pronounced biological activity. The prototypical example is serotonin. This compound is a neurotransmitter — a chemical used by the brain to transmit signals. Other indoleamines include melatonin, a potent hormone, and the compound N,N-dimethyltryptamine, a hallucinogen.

Serotonin is one of the most highly studied indoleamines. It is found in animals, plants, and fungi. It is consumed in the human diet in fruits and vegetables. In animals, most of the serotonin is found in the gastrointestinal tract. The remainder is found in the central nervous system.

Many studies have been conducted on serotonin for its effects on human physiology. It is thought to directly or indirectly affect most brain cells, and has great effects on mood. The indoleamine hypothesis of depression postulates that depression results from an imbalance in the levels of serotonin. To remedy this, Selective Serotonin Reuptake Inhibitors (SSRIs) have been developed. These compounds are widely used as antidepressants.

These drugs are thought to work because they prevent the reuptake of the serotonin. The neurotransmitter remains active, thus, the increased serotonin has a greater effect. If the SSRIs are removed, the serotonin levels decrease. This brings about an increase in depression.

Serotonin is inactivated by the enzyme monoamine oxidase, also known as MAO. MAO inhibitors (MAOIs) are an older class of antidepressants. They can sometimes interact with compounds in the human diet. These inhibitors are used less frequently than the SSRIs. There is some controversy about the use of SSRIs in young adults and children, since there is a side effect of an increased risk of suicide.

Melatonin is another indoleamine and is synthesized from serotonin. This hormone is widespread throughout the body, particularly in the skin, and helps to regulate the circadian rhythm. This is the body's way of keeping time over 24 hour cycles. Melatonin is sometimes taken as a supplement to help regulate sleep/wake cycles for people with jet lag or circadian rhythm disorders. Some countries do not allow the sale of melatonin as a supplement for humans.

The amino acid L-tryptophan is metabolized by the enzyme indoleamine 2,3-dioxygenase. This enzyme is found in a number of tissues. It degrades L-tryptophan to the compounds kynurenines. These compounds have antimicrobial properties.

The breakdown products of indoleamine 2,3-dioxygenase are also involved in the suppression of the immune response. If over-expressed, this can result in the compromise of the immune system. Researchers are investigating the use of indoleamine 2,3-dioxygenase inhibitors to block this activity and provide a form of immunotherapy.