Abnormally low levels of the neurotransmitter serotonin are associated with

Depression

Explanation:

Serotonin is a neurotransmitter that helps in the relay of signals from one part of the brain to another. Although it is synthesized in the body but it is found mostly in the digestive tract and blood.

Imbalance in the level of serotonin is associated with depression, which is a mental health condition in which the person dose not feel happy and good.

This is because there is a low level of serotonin in the brain which makes the person inactive and dull due to late or no relay of signals.

answer: i would say the brain but it seem that you have a brain.