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.