Lithium is a naturally occurring alkali metal present in trace amounts in the human body. It is used as a mood stabilizer to treat bipolar disorder and prevent manic-depressive episodes. While lithium has antidepressant effects, high concentrations can damage the nervous system, kidneys, and endocrine system by causing issues like hypothyroidism. Research on lithium's mechanism of action has yielded inconsistent results, but it may work by influencing neurotransmitter levels and second messenger systems in the brain.