Inclusion Criteria:  Patients (age 12 to years and older) with chronic (>3 months) symptoms of orthostatic intolerance, including but not limited to lightheadedness, syncope, headache, fatigue, weakness, sweating, nausea and heart palpitations. Symptomatic orthostatic heart rate increment ≥30 bpm if >19 years old or ≥40 bpm if <19 years old during a 10 minute 70 degree head up tilt study Presence of non-anemic iron deficiency, defined as serum ferritin levels <20 ug/L with hemoglobin no less than 1 gm/dL below the normal reference range as defined for age and gender Consent obtained from responsible guardian AND from subjects, 12-17 years of age Consent obtained for subjects 18 years of age and older  Exclusion Criteria:  Orthostatic hypotension (decrease of systolic BP>30 mmHg and/or diastolic BP>15mmHg within 3 minutes of 70 degree head up tilt study) Pregnant or lactating females The presence of failure of other organ systems or systemic illness that can affect autonomic function Concomitant therapy with anticholinergic, alpha-adrenergic antagonists, beta-adrenergic antagonists or other medications which could interfere with autonomic testing. Patients may participate if the potentially interfering medication is held for five half-lives prior to the study Laboratory evidence of anemia or iron overload Personal history of hematochromatosis or first degree relative with hematochromatosis Known sensitivity to Venofer® (iron sucrose injection, USP) or other intravenous iron preparation