It is unknown precisely what causes depression. It is thought to be likely that an imbalance of neurotransmitters or hormones in the body can lead to the disorder.

An association has been made between depression and serotonin, although scientists are unsure whether decreased levels of serotonin contribute to depression or depression causes a decrease in serotonin levels.

Although it is possible to measure the level of serotonin in the bloodstream, via a serum serotonin level test, it is currently not possible to measure serotonin levels within the brain. Researchers do not know whether serotonin levels in the bloodstream reflect the serotonin levels in the brain.

It is believed that medication such as selective serotonin reuptake inhibitors (SSRIs) that can affect the levels of serotonin in the body work as antidepressants and are able to relieve the symptoms of depression. It is unknown precisely how they work, however.

A recent study may offer evidence contrary to these widely-held theories. Scientists ran mice without the capacity to create serotonin through a series of behavioral tests.The mice did not show any signs of depression, suggesting that factors other than serotonin could be involved with the development of depression.¹² A similar conclusion was drawn by a professor of psychiatry writing an editorial piece for The BMJ in April 2015. He suggested that low serotonin is a mythical cause of depression