Her Ferritin Isn’t Low Enough to Call Anemia. It’s Low Enough to Explain Her Legs.
Her bloodwork came back normal, by the standard her primary doctor was taught to use. It isn’t normal by the threshold that actually matters for what she has — and that gap nearly meant starting a drug she didn’t need yet.
N.D., a 34-year-old woman, is six months postpartum with her second child, a daughter she had after a straightforward pregnancy and delivery, and is currently home on an extended parental leave from her job as a high school biology teacher. She has developed uncomfortable, hard-to-describe crawling sensations deep in both legs most evenings over the past two months, temporarily relieved by getting up and moving around, and clearly worse when she sits still to nurse her daughter before bed — a time of day she used to find genuinely peaceful and now dreads a little. She breastfed her first child as well, three years ago, and doesn’t specifically recall this happening then, though she’s honest that she was exhausted enough in that first postpartum year that a milder version of the same thing might simply have gone unnoticed.
She has no other medical conditions, takes a standard prenatal vitamin daily as she did with her first pregnancy, and her primary care physician, concerned enough to investigate, ordered a basic complete blood count that came back entirely normal — hemoglobin well within range, no evidence of anemia by any standard measure her physician was trained to check first.
Her physician, reasonably reading a normal CBC as ruling out an iron-related explanation, was preparing to discuss starting a gabapentinoid for symptom control when N.D. mentioned, almost in passing near the end of the visit, that a friend from her new-parent group with similar evening leg symptoms had been told her “iron levels” specifically were the issue, separate from her regular blood count — a distinction neither of them fully understood in the moment but that was enough to prompt a referral before starting any medication.
Referral visit, before any medication was started
This is a genuinely common and important gap, not a criticism of her primary care physician — a normal CBC and a normal hemoglobin only rule out overt anemia, they don’t rule out the iron deficiency that specifically drives RLS. Roughly half of primary RLS cases respond meaningfully to iron repletion, and the relevant lab isn’t hemoglobin, it’s ferritin measured against the RLS-specific threshold.
The IRLSSG’s own iron management guideline sets that threshold at ferritin above 75 ng/mL with transferrin saturation above 20% — a level most general lab reference ranges would call entirely normal, sometimes by a wide margin.
That’s a genuinely useful correction, and I’ll say directly — I was working from the anemia threshold because that’s the number I check reflexively, and I hadn’t separated “not anemic” from “has enough iron for this specific purpose” before today.
Postpartum status makes this especially relevant for her — pregnancy and delivery deplete iron stores substantially, and six months of breastfeeding on top of that is a real, ongoing drain, even with a normal CBC. I’d check ferritin and transferrin saturation now, before starting any medication, rather than treating symptoms pharmacologically while a likely underlying cause goes unaddressed.
Agreed: iron studies ordered against the RLS-specific threshold before starting any medication, with oral iron repletion planned if the results confirm what her postpartum, breastfeeding status already makes likely.
No real disagreement remained — the primary care physician’s candid acknowledgment of the threshold gap and the sleep physician’s explanation of the guideline converged directly into a shared plan, with both agreeing the medication conversation should wait for the more relevant lab result.