The Renin-Aldosterone Axis in Postural Tachycardia Syndrome

Inclusion Criteria: * Diagnosis of postural tachycardia syndrome by the Vanderbilt Autonomic Dysfunction Center criteria * Age between 18-64 years * Male or females are eligible * Able and willing to provide informed consent * Healthy control subjects with no major medical problem (including postural tachycardia syndrome), free of medications during the study Exclusion Criteria: * Overt cause of postural tachycardia e.g., dehydration * Inability to give or withdraw informed consent * Pregnancy * Hypertension (BP \> 140/90) * Significant co-morbid condition * Other factors which in the investigator's opinion would prevent the subject from completing the protocol