What ferritin level should be targeted and how should iron be supplemented in a patient with restless‑legs syndrome?

Ferritin Targets and Iron Supplementation in Restless Legs Syndrome

In patients with RLS, you should target a serum ferritin >100 ng/mL or transferrin saturation ≥20%, and use IV ferric carboxymaltose 1000 mg as first-line iron therapy when ferritin is ≤100 ng/mL, reserving oral ferrous sulfate 325-650 mg daily (or every other day) for patients who cannot access IV iron. 1

Iron Assessment Protocol

Check serum ferritin and transferrin saturation in all patients with clinically significant RLS before initiating any treatment. 1 The blood draw must be performed:

• In the morning 1
• After 24-hour avoidance of all iron-containing supplements and foods 1
• Fasting is implied by the timing requirement

Critical caveat: Ferritin assays vary significantly between manufacturers. The Roche method reads approximately 58% higher than the Beckman method at the 100 ng/mL threshold (158 μg/L vs 100 μg/L). 2 If your laboratory uses the Roche assay, adjust the cutoff upward proportionally to avoid withholding beneficial treatment. 2

Iron Supplementation Thresholds (Adults)

The 2025 AASM guideline establishes RLS-specific thresholds that are higher than general population standards: 1

When ferritin ≤75 ng/mL OR transferrin saturation <20%:

• Use either oral iron (ferrous sulfate 325-650 mg daily or every other day) OR IV iron 1
• IV ferric carboxymaltose 1000 mg is strongly recommended (strong recommendation, moderate certainty) 1
• Oral ferrous sulfate receives only a conditional recommendation 1

When ferritin is 76-100 ng/mL:

• Use IV iron only (not oral iron) 1
• IV ferric carboxymaltose 1000 mg remains the preferred formulation 1

When ferritin is 101-300 ng/mL:

• The 2025 AASM guideline does not recommend routine iron supplementation in this range 1
• However, emerging evidence challenges this cutoff: A 2025 retrospective study demonstrated that IV ferric carboxymaltose 1000 mg produces equivalent improvements in IRLS scores and substantia nigra iron deposits in patients with ferritin 100-300 ng/mL compared to those with ferritin <100 ng/mL, with minimal risk of iron overload (only 2/58 patients developed transferrin saturation >45%, neither with hepatic dysfunction). 3

Iron Formulation Selection

Intravenous Iron (Preferred)

IV ferric carboxymaltose 1000 mg is the only IV formulation with a strong recommendation (moderate certainty of evidence). 1 It achieves:

• Significant IRLS improvement by week 12 (mean difference -4.66 points vs placebo, P=0.021) 4
• Response rates of approximately 70% vs 40% with placebo 5
• Slow-release pharmacology enabling CNS iron penetration via H-ferritin binding and macrophage uptake 1

Alternative IV formulations (conditional recommendations, very low certainty):

• IV low molecular weight iron dextran 1
• IV ferumoxytol 1

Do not use IV iron sucrose in non-dialysis RLS patients. High-quality RCTs failed to demonstrate clinically significant benefit, likely because fast-release, low-dose formulations lack the CNS penetrance of slow-release preparations. 1

Oral Iron

Ferrous sulfate 325-650 mg daily or every other day receives a conditional recommendation. 1 Key limitations:

• Poorly absorbed when ferritin >50-75 ng/mL (unlike IV iron which bypasses GI absorption) 1
• Constipation and GI side effects limit tolerability 1
• Should be taken with vitamin C to enhance absorption 6

Special Population: End-Stage Renal Disease

ESRD patients require different thresholds and formulations: 1, 7

• Check iron when ferritin <200 ng/mL AND transferrin saturation <20% (both criteria must be met) 1, 7
• Use IV iron sucrose (not ferric carboxymaltose) in dialysis patients 1, 7
• Add vitamin C supplementation as an adjunct to enhance iron utilization 1, 7
• These higher thresholds reflect the inflammatory state and altered iron metabolism in ESRD 7

Special Population: Pediatric RLS

In children with RLS and ferritin <50 ng/mL, use ferrous sulfate with pediatric dose adjustment (conditional recommendation, very low certainty). 1, 7 The lower threshold reflects different developmental iron requirements. 1

Special Population: Pregnancy

RLS affects 22% of pregnant individuals, especially in the third trimester. 7 Iron supplementation is particularly important in this group, but prescribers must consider pregnancy-specific safety profiles of all treatments. 1

Clinical Pitfalls and Caveats

Iron supplementation alone may not suffice. Low serum ferritin is associated with increased risk of augmentation (iatrogenic worsening) when dopamine agonists are used. 8 This underscores why the 2025 AASM guideline shifted away from dopamine agonists as first-line therapy. 1

Timing matters for IV iron response. Ferric carboxymaltose may require 12 weeks to achieve maximal benefit (week 4 showed nonsignificant improvement, P=0.163; week 12 reached significance, P=0.021). 4 Counsel patients about this delayed response to prevent premature discontinuation.

Recheck iron studies after supplementation to guide repeat dosing, though the 2025 guideline does not specify exact intervals. 1 The 2018 IRLSSG consensus (superseded by the 2025 AASM guideline) suggested ferric carboxymaltose could be used as first-line treatment for moderate-to-severe RLS when ferritin <300 ng/mL, 9 but the current AASM standard is more conservative at ≤100 ng/mL. 1

Address exacerbating factors concurrently: Discontinue serotonergic antidepressants, dopamine antagonists (metoclopramide), and centrally acting antihistamines (diphenhydramine); minimize alcohol and caffeine; treat obstructive sleep apnea. 1, 5 These steps are mandatory before attributing treatment failure to inadequate iron repletion.