Iron is the most important and most commonly missed part of managing restless legs syndrome. The reason it gets missed is specific and worth understanding: the threshold that matters here is higher than the one used to define normal.
You can be told your iron is fine and still be low by the standard that applies to restless legs. That single fact is why this article exists.
Why iron, specifically
The link isn't incidental, and knowing the mechanism explains the odd threshold.
Restless legs involves dopamine signalling, and iron is required to make dopamine — tyrosine hydroxylase, the rate-limiting enzyme in its synthesis, is iron-dependent. So iron availability in the brain constrains the system that appears to be at fault.
The important wrinkle is that brain iron and blood iron are not the same thing. Iron has to be transported across the blood-brain barrier, and research in people with restless legs has found reduced iron in the brain in some cases where circulating iron looked adequate. That is the reason the treatment threshold sits well above the level at which someone would be called iron deficient in general practice: you are not trying to correct anaemia, you are trying to get enough iron into a tissue that is harder to reach.
The numbers
From the 2025 American Academy of Sleep Medicine clinical practice guideline:
In adults, iron supplementation is indicated when:
- serum ferritin is 75 ng/mL or below, or
- transferrin saturation is under 20%
Between 75 and 100 ng/mL, the guideline recommends intravenous iron specifically rather than oral.
In children, the threshold is ferritin below 50 ng/mL.
For context, many laboratories flag ferritin as low only well beneath 75 — in some reference ranges the lower limit sits under 30. So a result that comes back entirely unflagged can still sit inside the range where this guideline says to treat. "Your iron is normal" and "your iron is adequate for restless legs" are different statements, and only one of them is usually being made.
What to ask for, and why both tests
Serum ferritin and transferrin saturation. Both, because they measure different things.
Ferritin reflects stored iron — the reserve. Transferrin saturation reflects how much of the iron-transport capacity in your blood is actually carrying iron, which is closer to a measure of iron availability right now. Either one crossing the threshold is sufficient under the guideline, and they don't always move together.
A full blood count alone will not answer this question. You can have entirely normal haemoglobin and depleted iron stores, because stores are depleted first and the blood count only changes once they're exhausted. The same point comes up in our guide to the four blood tests worth asking for when you have brain fog.
Say it plainly at the appointment: "I have symptoms of restless legs. I'd like ferritin and transferrin saturation checked, and I understand the threshold for restless legs is higher than the usual one."
Two practical details. Ferritin rises with inflammation and infection, so a result taken while you were unwell can read falsely reassuring — a CRP measured alongside helps interpret it, and repeating the test when well is reasonable. And if you are already taking an iron supplement, say so, because it affects how the result should be read.
Oral or intravenous
The guideline's strong recommendation is for intravenous ferric carboxymaltose, based on moderate-certainty evidence of improvement in symptom severity, sleep quality and quality of life. That is an unusually firm recommendation in this field and it reflects two advantages of the intravenous route: it bypasses the absorption bottleneck in the gut entirely, and it delivers a large dose in one or two visits rather than months of tablets.
Oral ferrous sulfate carries a conditional recommendation, as do some other intravenous formulations. Between ferritin of 75 and 100, only the intravenous route is recommended.
Which applies to you is a clinical decision. It depends on your numbers, how you tolerate oral iron, what's available locally, and cost.
If you are prescribed oral iron
Absorption is the whole problem with oral iron, and a few things make a real difference. All of this is worth confirming with whoever prescribed it rather than acting on alone:
- Take it on an empty stomach where you can tolerate it. Food substantially reduces absorption.
- Vitamin C alongside — or simply a glass of orange juice — improves uptake of non-haem iron.
- Keep it away from tea, coffee, calcium supplements and dairy, all of which inhibit absorption, ideally by a couple of hours.
- Alternate-day dosing is worth asking about. A growing body of evidence suggests that taking iron every other day can improve the fraction absorbed compared with daily dosing, because a dose raises hepcidin — the hormone that blocks iron absorption — for around a day afterwards. It often means fewer side effects too.
- Separate it from levothyroxine by several hours if you take thyroid replacement, since iron reduces its absorption. More on that here.
And give it time
This is where people give up. Correcting brain iron is slower than correcting a blood test, and symptom improvement in restless legs is typically measured in months, not weeks. Judging oral iron at a fortnight will tell you nothing except that iron tablets can upset your stomach.
Re-testing after a course, rather than assuming, is the sensible endpoint — and it also guards against the opposite problem of continuing indefinitely.
Do not self-supplement iron
This is the part to take seriously. Iron is not a benign supplement to take speculatively:
- Iron overload is a real harm, and some people carry a genetic predisposition to it (haemochromatosis) without knowing. Supplementing blind is exactly how that goes wrong.
- Too much oral iron causes significant gastrointestinal problems — constipation, nausea, cramping — which is also the most common reason courses get abandoned.
- Iron interferes with the absorption of several medications, levothyroxine and some antibiotics among them.
- Iron is genuinely dangerous to a small child. Between 1983 and 1990 it was the leading cause of death from medicines in children under six in the United States. Serious cases have fallen a long way since, following changes to packaging, labelling and awareness — but the underlying toxicity hasn't changed, and iron belongs out of reach in a house with young children.
Get tested, then treat to the result. That sequence is the whole point of this post.
One more thing worth knowing
If you are already being treated for restless legs with a dopamine agonist — pramipexole, ropinirole or levodopa — the 2025 guideline moved those off routine first-line use, because of a phenomenon called augmentation.
Augmentation is worth describing properly, because it's counter-intuitive and people don't spot it in themselves. It is the treatment making the condition worse over months to years: symptoms starting earlier in the day than they used to, becoming more intense, coming on faster when you sit down, and spreading to other parts of the body such as the arms or trunk. Because it develops gradually, and because the obvious interpretation of worsening symptoms is that you need a higher dose, it can go unrecognised for a long time — and raising the dose makes it worse.
Iron status is relevant here too: low iron is one of the factors associated with augmentation, which is another reason the test at the top of this article matters even for people already on treatment.
If any of that describes you, raise it at a review. Don't stop anything on your own — abrupt withdrawal can cause a severe rebound.
Why we're not selling you anything here
We don't sell iron. If we did, the section above about overload, interactions and child safety would still have to be in this post, which is a reasonable argument for the section.
Magnesium is the supplement most often recommended for restless legs online. We stock magnesium, and our own guide to what it does and doesn't do is blunt about the limits; the trial evidence for it in this condition is thin, and no guideline gives it a strong recommendation. Iron is the intervention with the evidence, and it is the one we don't sell.
If you're not sure whether what you have is restless legs at all, start with how to tell it apart from insomnia — the distinction is clearer than most people expect, and it turns on one specific feature. Restless legs and periodic limb movements also turn up more often in people with ADHD, which is one strand of why ADHD and sleep problems so often overlap.
Thresholds and recommendations from the 2025 AASM clinical practice guideline on the treatment of restless legs syndrome and periodic limb movement disorder. This article is not medical advice, and nothing here should be used to start, stop or change a medication or to begin iron supplementation without testing.