Restoring Iron Deficiency in POTS

Inclusion Criteria * Age 18-45 years old. * Diagnosis of POTS, defined by a sustained heart rate increase of at least 30 bpm within 10 min of standing or head-up tilt (or absolute upright heart rate \>120 bpm), in the absence of orthostatic hypotension, and with at least 3 months of symptoms of orthostatic intolerance. * Absence of other diseases or medications that can explain orthostatic tachycardia or symptoms. * Serum ferritin \<50 ng/ml. * Patients on fludrocortisone will be allowed to participate as long as they have been on the same dose for at least one month before enrollment and continue on the same dose throughout the study. Exclusion Criteria * Unable or unwilling to provide consent. * Pregnancy. * Being chair- or bed-ridden. * Unable to hold POTS medications during study procedures (i.e., autonomic testing). * History of active bleeding (other than menstruation). * History of hypersensitivity to any iron preparation of recent (within 6 months) treatment with any IV iron preparation.