Can a ferritin level below 75 ng/mL cause leg cramps?

Low Iron Below 75 ng/mL Can Cause Leg Cramps, Particularly in Specific Populations

Low ferritin levels below 75 ng/mL can cause leg cramps, especially intradialytic leg cramps in hemodialysis patients, and may contribute to leg cramps in patients with restless legs syndrome (RLS).

The Evidence for Iron and Leg Cramps

The connection between low iron and leg cramps is most clearly established in two clinical contexts:

In Hemodialysis Patients

Research demonstrates that low serum ferritin concentration is significantly associated with intradialytic leg cramps 1. In a study of 154 hemodialysis patients with adequate hemoglobin levels (≥10.0 g/dL), those with ferritin <100 ng/mL experienced significantly more severe leg cramps during dialysis. Importantly, this occurred even when hemoglobin was normal, indicating that iron deficiency itself—not anemia—was the culprit. The study found that low transferrin saturation (<20%) was also independently associated with leg cramps 1.

In Restless Legs Syndrome

The American Academy of Sleep Medicine 2025 guidelines establish that iron supplementation should be instituted when serum ferritin ≤75 ng/mL in adults with RLS 2. While RLS manifests primarily as an urge to move the legs rather than "cramps" per se, the guideline's emphasis on this specific ferritin threshold (75 ng/mL) reflects the critical role of iron in leg symptoms. The guidelines recommend oral or IV iron supplementation at ferritin ≤75 ng/mL or transferrin saturation <20%, and IV iron alone when ferritin is 75-100 ng/mL 2.

Clinical Algorithm for Evaluation

When a patient presents with leg cramps and ferritin <75 ng/mL:

1. Check complete iron studies (ferritin, transferrin saturation, iron, TIBC) - ideally in the morning, avoiding iron supplements/foods for 24 hours prior 2

2. Assess for RLS symptoms: urge to move legs, worse at rest, worse in evening/night, relieved by movement

3. Consider patient population:

   • Hemodialysis patients: Higher risk for iron-related leg cramps 1
   • Women of reproductive age: Higher prevalence of iron deficiency 3
   • Patients with inflammation: May have falsely elevated ferritin

4. Initiate iron supplementation if ferritin ≤75 ng/mL, using:

   • Oral or IV iron for ferritin ≤75 ng/mL 2
   • IV iron preferred for ferritin 75-100 ng/mL 2

Important Caveats

The 75 ng/mL threshold is specific to RLS management and represents a higher cutoff than traditional iron deficiency thresholds 2. The American Society of Hematology 2026 guidelines propose a threshold of ≤20 ng/mL for general iron deficiency diagnosis 4, while emerging evidence suggests 30 ng/mL may be more appropriate for identifying iron deficiency without anemia 3.

Ferritin is an acute phase reactant: In patients with inflammation, infection, or chronic disease, ferritin may be elevated despite true iron deficiency. In these cases, transferrin saturation <20% becomes more important for diagnosis 2, 1.

The evidence linking ferritin <75 ng/mL specifically to leg cramps is strongest in hemodialysis patients and those with RLS. For general leg cramps without these conditions, the relationship is less well-established, though iron supplementation when ferritin is low may still provide benefit given the multiple symptoms associated with iron deficiency 1.