In one of my lessons I found that increasing of dopamine production causes mental disorder such schizophrenia, and decreasing causes Parkinson disease.
Is it the only cause of these diseases, and how to measure dopamine concentrations?
You have been taught oversimplifications. Blood dopamine concentrations do not reflect those in the CNS. High dopamine concentrations are seen for example in feochromocytomas, and dopamine is used in shock to elevate blood pressure. Basic medschool stuff.