The reason for early termination is difficulties recruiting that caused a premature ending of financial resources ensuring protected time for research.

In this study the investigators aim at addressing potential relationships between iron stores and glucose homeostasis. Iron (i.e. Ferric Carboxymaltose) will be perfused to pre-menopausal, iron-deficient non-anaemic women suffering from a chronic fatigue syndrome and parameters related to glucose homeostasis, parameters related to metabolic syndrome and inflammation will be measured before and after the intervention.

Inclusion Criteria:

• Premenopausal women.
• Negative pregnancy test.
• Adequate contraception during the study period and for 1 month following study completion.
• Overt or relative iron deficiency at screening defined as follows:

Serum ferritin <50 ng/mL AND transferrin saturation <20%, OR Serum ferritin <30 ng/mL.

- Serum C-reactive protein: <5 mg/L if not on oral contraception, OR <20 mg/L if use of oral contraception

• Intolerance to oral iron formulations, or lack of efficacy of oral iron formulations.
• Minimum total score of 5 on the Visual analogic scale of fatigue.
• Normal levels of vitamin B12 and folic acid at screening.
• Availability and willingness to complete all study visits and procedures per protocol.
• Ability to sign an informed consent.

Exclusion Criteria:

• Age <18 years.
• Menopause (defined as an amenorrhea of at least 12 months).
• Irregularly menstruating women (menstrual cycle outside a range of 24-38 days in duration) or experiencing overt metrorrhagia (simple spotting not being an exclusion criteria).
• Body mass index <18.5 kg/m2 or >30 kg/m2.
• Diabetes, defined as subjects with HbA1c ≥ 6.5 % and/or with fasting blood glucose levels ≥ 7 mmol/l and/or with a history of diabetes and/or by the use of anti-diabetic drugs.
• Hb level <117 g/L or known haemoglobinopathy or haemochromatosis.
• Blood transfusion within the last 12 weeks.
• Intake of iron preparations 4 weeks prior to screening.
• Known hypersensitivity to FCM or to any other iron preparation.
• Suspicion of major depressive disorder based on Patient Health Questionnaire.
• Known chronic inflammatory disease, including human immunodeficiency virus, hepatitis B or hepatitis C virus infection.
• Active malignancy.
• Decreased renal function (estimated glomerular filtration rate using the CKD-EPI equation<60 ml/min/1.73m2).
• Liver dysfunction (aspartate aminotransferase and alanine aminotransferase > 3-fold upper limit).
• Angina (Class IV).
• Asthma.
• Documented sleep apnoea.
• Important recent weight loss (>10% within the past month).
• Thyroid dysfunction (thyroid stimulating hormone >4 µU/mL).
• Reported weekly alcohol consumption > 14 standard drinks.
• Drug abuse (any drug consumption reported in the past 12 months).

To ensure that patients are unaware of the study drug they are receiving, the infusion pouch will be prepared in a separate room by members of the pharmacy unit and opaque bags will cover the infusion kits and infusions will be done via dark coloured infusion sets. Finally, a curtain will be used to shield the injection site from the patient's view.

To ensure that Outcomes Assessors are unaware of the study drug the patient is receiving, a seperate team of care providers will supervise the infusion of the investigation product and collect and/or manage adverse events (AEs) and serious adverse events (SAEs). The Outcome Assessor will not have access to participant related data during the randomised part of the study.