This test measures the amount of lithium in blood. It is used to monitor treatment with lithium, or evaluate for a toxic level of lithium in the body. Lithium is one of the most widely used and studied medications for treating bipolar disorder. Bipolar disorder is a mental condition that is characterized by alternating periods of depression and mania. Lithium helps reduce the severity and frequency of mania. It may also help relieve or prevent bipolar depression. Lithium is excreted primarily by the kidneys.