In a 61-year-old man with restless‑leg syndrome, daytime sleepiness (Epworth Sleepiness Scale score 12), BMI 28, baseline serum ferritin 30 ng/mL, periodic limb movement index 45/hr, who has been started on iron therapy, which of the following findings indicates that iron supplementation should be discontinued: serum ferritin 50 ng/mL, total iron‑binding capacity 300 µg/dL, hemoglobin 13.5 g/dL, or transferrin saturation 50%?

Discontinue Iron Therapy at Transferrin Saturation of 50%

The correct answer is (d) transferrin saturation of 50%, as this indicates iron overload and is a clear indication to stop iron supplementation to prevent hemochromatosis and iron toxicity.

Rationale Based on Current Guidelines

The 2025 AASM guidelines for RLS treatment provide specific parameters for iron supplementation 1. According to these consensus guidelines, iron therapy should be initiated when:

• Serum ferritin ≤ 75 ng/mL OR
• Transferrin saturation < 20%

Why Each Answer is Correct or Incorrect:

Option (a) - Serum ferritin 50 ng/mL: INCORRECT

• This ferritin level is still below the target range
• The AASM guidelines recommend continuing iron supplementation until ferritin reaches at least 75-100 ng/mL 1
• This patient started at 30 ng/mL, so 50 ng/mL represents improvement but not completion of therapy
• Historical data shows RLS patients have fewer symptoms when ferritin exceeds 50 mcg/L 2

Option (b) - TIBC 300 µg/dL: INCORRECT

• This is a normal TIBC value (normal range: 250-450 µg/dL)
• TIBC alone does not indicate iron overload
• This value provides context for calculating transferrin saturation but is not itself a stopping criterion

Option (c) - Hemoglobin 13.5 g/dL: INCORRECT

• This is a normal hemoglobin level for males (normal: 13.5-17.5 g/dL)
• The patient did not have anemia to begin with (baseline ferritin 30 ng/mL indicates iron deficiency without anemia)
• Normal hemoglobin does not indicate iron overload
• RLS iron therapy targets brain iron deficiency, not just correction of anemia 1

Option (d) - Transferrin saturation 50%: CORRECT

• Transferrin saturation ≥45-50% indicates iron overload 3, 4
• This level suggests excessive iron accumulation and risk of hemochromatosis
• Case reports document hemochromatosis development in RLS patients receiving iron therapy with transferrin saturation reaching 88% 4
• Continuing iron therapy beyond this point risks organ damage from iron deposition

Critical Safety Considerations

Monitoring Requirements During Iron Therapy:

The evidence emphasizes several monitoring pitfalls 4:

1. Pre-treatment screening: Measure transferrin saturation and ferritin before initiating iron therapy to exclude hemochromatosis
2. During treatment: Re-measure iron parameters once or twice yearly
3. Post-infusion timing: Wait 4-8 weeks after IV iron before checking levels, as circulating iron interferes with assays 3

Iron Overload Warning Signs:

A transferrin saturation of 50% represents a critical threshold because:

• Normal transferrin saturation is 20-45%
• Values >45% indicate the body's iron-binding capacity is being exceeded
• Unbound iron becomes toxic, depositing in organs (liver, heart, pancreas)
• The case report of hemochromatosis in RLS patients 4 demonstrates this exact scenario

Treatment Goals for This Patient

Given this patient's presentation:

• Starting ferritin: 30 ng/mL (low)
• Target ferritin: 75-100 ng/mL per AASM guidelines 1
• Transferrin saturation: Should remain <45%

The treatment should continue until ferritin reaches 75-100 ng/mL, but must be discontinued immediately if transferrin saturation reaches 50%, regardless of ferritin level, to prevent iron toxicity.

Assay Variability Caveat:

Be aware that ferritin measurements vary by assay method 5. A Beckman assay reading of 75 ng/mL equals approximately 121 ng/mL on a Roche assay. Clinicians should know which assay their laboratory uses and adjust targets accordingly, while also monitoring transferrin saturation as a complementary measure.

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