How do you measure serotonin deficiancey?

Serotonin--a chemical in the brain that regulates mood, sleep, appetite and aggression--is difficult to measure. It is not possible to directly measure levels of serotonin in the brain of a living person. Because the cells that produce serotonin either reabsorb it or release it into the bloodstream, where it breaks down into different compounds, researchers have developed a test that can measure the amount of these compounds in the blood to estimate serotonin levels. Only a doctor can perform these tests. Interestingly, doctors test for these compounds because they are leading indicators of a specific type of cancer in the digestive system--not for depression or mental health.