Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Postural orthostatic tachycardia syndrome ?
    ### Response:
    What causes postural orthostatic tachycardia syndrome? The underlying cause of postural orthostatic tachycardia syndrome (POTS) is poorly understood. However, episodes often begin after a pregnancy, major surgery, trauma, or a viral illness and may increase right before a menstrual period.
    