The phrase “chemical imbalance” is thrown around a lot these days. What does it mean? Actually, very little. The human brain is a complicated organ with thousands of interconnecting neural pathways mediated by electrical discharges and chemical transmitters. Every time you move or think hundreds of neurons are firing and releasing neurochemicals that cause other neurons to fire. The way that your particular brain works is based on a combination of the way it is innately wired, which we can call innate traits, and the way your brain has learned to respond due to past experiences. Both traits and experiences can accentuate the reactions of certain pathways in the brain. For example: some people are wired to be able to multi-task well. They can jump from one task to another, keeping reasonable track of both. A person like this will do well in some jobs, though may become bored or distracted in tasks that require attention to a sole task. If this person is stuck repeatedly in that kind of single task environment, it is likely that this person will receive negative feedback leading to anxiety and low mood. This anxiety may further interfere with the ability to focus, resulting in performance that looks like a serious attention problem.
Actual “chemical imbalances” have not been shown to exist. In certain conditions, the ways certain pathways fire, may be altered. This is also true for many non-pathological conditions such as falling in love. There are some disorders where neurons may not be functioning properly, but this has not been proven for most emotional conditions. The way brain pathways come to fire that may cause symptoms can be seen as interplay between trait wiring, developmental issues and present stressors. Stress may also affect the ability of the brain to make or maintain neural connections.
Medication can affect the way many neural pathways function. This can be helpful, but it does not confirm that one has a "chemical imbalance"