Biological Factors Associated with Suicide

Studies have linked suicidal behavior to decreased activity of brain 5- HT pathways. Markers of 5- HT function, such as cerebrospinal fluid (CSF) 5- HIAA and the density of 5- HT transporter sites, are lowered in suicide victims. Abnormalities in brain 5- hydroxytryptamine (5-HT) neurotransmission have been reported in patients with impulsive and aggressive behavior, and low levels of the 5- HT metabolite 5- hydroxy indole acetic acid (5- HIAA) have been found in the cerebrospinal fluid of subjects who have committed acts of unpremeditated violence. 

References

1. Harrison PJ, Cowen P, Burns T, Fazel M. Shorter Oxford Textbook of Psychiatry. Oxford: Oxford University Press; 2018., 7th edition, Chapter 21, page no 613). 
2. Linnoila VM, Virkkunen M. Aggression, suicidality, and serotonin. The Journal of clinical psychiatry. 1992 Oct).