Aldosterone & Sodium Regulation in Postural Tachycardia Syndrome - Screening

Inclusion Criteria: * Age between 18-50 years old * Patients with suspected POTS and healthy volunteers * Sedentary healthy subjects of similar age and gender than patients will also be recruited. * Only female participants are eligible. * Able and willing to provide informed consent Exclusion Criteria: * Smokers * Overt cause for postural tachycardia, i.e., acute dehydration * Significant cardiovascular, pulmonary, hepatic, or hematological disease by history or prior testing * Highly trained athletes * Subjects with somatization or severe anxiety symptoms will be excluded * Pregnant (positive pregnancy test) or breastfeeding * Hypertension defined as supine resting BP\>145/95 mmHg off medications or needing antihypertensive medication * Other factors which in the investigator's opinion would prevent the participant from completing the protocol, including poor compliance during previous studies or an unpredictable schedule * Unable to give informed consent