What restless legs syndrome actually is
RLS is a clinical diagnosis — there is no blood test or scan that confirms it. Clinicians look for five features together: an urge to move the legs, usually with unpleasant sensations; onset or worsening during rest or inactivity; partial or complete relief with movement; symptoms worse in the evening or night than during the day; and no better explanation such as leg cramps, positional discomfort or habitual foot-tapping.
Two things about that list matter. First, the relief-with-movement feature is the giveaway: night-time leg cramps are a sudden painful knot that movement does not fix, while RLS is a restless discomfort that walking eases within seconds. Second, the evening pattern is what makes RLS a sleep disorder rather than a leg disorder. The AASM guideline describes insomnia as the primary morbidity of RLS, with difficulty falling or staying asleep present in roughly 90 percent of people who have it. If you are reading this because you cannot fall asleep, our sleep hub and guide to menopause insomnia are the natural companions to this page.
How common is it? The AASM defines clinically significant RLS as symptoms occurring at least twice a week with at least moderate distress, and puts that at 2 to 3 percent of adults. Milder, occasional symptoms are considerably more common. RLS is roughly 50 percent more prevalent in women than in men, some of which is explained by pregnancy. About half of people with RLS have a first-degree relative with it — this is a strongly heritable condition, so a mother or sister with "jumpy legs" is a real clue.
Why does iron matter so much in restless legs?
The pathophysiology is only partly understood, and it is worth saying that plainly. What is reasonably well supported is that the brains of people with RLS show reduced iron indices. MRI, transcranial doppler and cerebrospinal fluid studies all point the same direction: less available iron in specific brain regions, particularly the substantia nigra, where iron is a required cofactor for dopamine synthesis. That is the working model for why dopamine-related drugs affect RLS at all.
The crucial and under-appreciated point is that brain iron and blood iron are not the same thing. Someone can have a completely normal haemoglobin, no anaemia whatsoever, and still have insufficient iron crossing into the brain. This is why the RLS thresholds sit so much higher than the haematology ones — and why "your blood count was fine" is not the same as "your iron was checked."
Supporting evidence comes from who gets RLS: conditions that cause systemic iron deficiency, including pregnancy and end-stage kidney disease, carry a markedly increased prevalence. In one study of female blood donors — a group in which iron deficiency is common — RLS prevalence one week after donation was around 7 percent, and the affected donors were more likely to report heavy periods.
Why ferritin 75 and not 15? The threshold most pages miss
Ferritin is the storage form of iron and the best single blood marker of iron reserves. For diagnosing iron deficiency anaemia, laboratories typically flag ferritin below about 15 to 30 ng/mL. RLS guidance uses a completely different number.
The AASM 2025 guideline states, as a good practice statement, that in all patients with clinically significant RLS clinicians should regularly test serum iron studies including ferritin and transferrin saturation. It then relays the international consensus position: iron supplementation in adults with RLS should be instituted with oral or intravenous iron if serum ferritin is at or below 75 ng/mL or transferrin saturation is under 20 percent, and only with intravenous iron if ferritin sits between 75 and 100 ng/mL. The guideline adds the line that matters most for patients: "These iron supplementation guidelines are different than for the general population."
The reason for the split at 75 is practical. Oral iron is poorly absorbed once ferritin is above roughly 50 to 75 ng/mL, so above that level swallowing more tablets does little; if extra iron is warranted, it has to be given intravenously. The guideline is explicit that these consensus thresholds have not themselves been empirically tested — they are expert consensus built on physiology and clinical experience, not a randomised result. That is an honest limitation, not a reason to skip the test.
| Test | What it measures | How RLS guidance reads it | Important caveats |
|---|---|---|---|
| Serum ferritin | Stored iron in the body | 75 ng/mL or below supports considering iron treatment; 75–100 ng/mL may still be treated, but with IV iron only | Ferritin rises with infection, inflammation, liver disease and obesity, so it can look falsely reassuring. The AASM advises drawing the sample in the morning and avoiding iron-containing foods and supplements for at least 24 hours beforehand. |
| Transferrin saturation (TSAT) | How much of your iron-transport protein is actually carrying iron | Under 20 percent supports considering iron treatment even if ferritin looks acceptable | Calculated from serum iron and total iron-binding capacity — ask that both are ordered. Varies through the day. |
| Haemoglobin / full blood count | Whether you are anaemic | Normal results do not rule out iron-related RLS | The most common reason people are wrongly reassured. Anaemia is a late consequence of iron deficiency, not an early one. |
| Vitamin B12 and folate | Other deficiencies affecting nerves and blood | Often checked alongside, since B12 deficiency causes overlapping leg symptoms | Neuropathy from B12 deficiency can mimic or coexist with RLS. |
| Kidney function (eGFR, creatinine) | How well the kidneys filter | Advanced kidney disease is a recognised secondary cause of RLS | Dialysis populations have both high RLS rates and different iron targets. |
All lab ranges vary between laboratories, and a single reading is a snapshot rather than a verdict. Ferritin measures the iron you have in storage, while serum iron measures what is circulating at that moment — two numbers that are constantly confused with each other. If you want help interpreting a results printout in context, our lab results explainer tool walks through what each line means.
Why this matters specifically for midlife women
Two things stack in women's forties and fifties. First, heavy menstrual bleeding is extremely common in perimenopause as cycles become erratic, and in high-income countries it is the leading cause of iron deficiency in premenopausal women. In one study of adolescents and young women with heavy periods, around half had a ferritin below 20 ng/mL — nowhere near the RLS threshold of 75 — and the majority of those with iron deficiency were not anaemic. Second, RLS rates rise with age and are already higher in women.
So the common scenario is this: a woman in her late forties with heavy, unpredictable periods, poor sleep she attributes to perimenopause, and legs that will not settle at night. Her haemoglobin comes back normal, and iron is never mentioned again. If that is you, asking specifically for ferritin and transferrin saturation, and mentioning the RLS thresholds, changes the conversation. Our guides to low ferritin and iron deficiency in women cover the broader picture.
Pregnancy deserves its own note. RLS becomes substantially more common in pregnancy — reported prevalence in the third trimester is many times the background rate — and typically improves after delivery. Iron demands in pregnancy are high, and iron management there is firmly a clinician's call, since both deficiency and excess carry risks.
What else drives restless legs?
Iron is one lever, not the only one. The AASM's first management step is addressing exacerbating factors, and the recognised ones include:
- Medications. Sedating antihistamines such as diphenhydramine (found in many over-the-counter sleep aids), anti-nausea drugs that block dopamine, and several antidepressants — mirtazapine most consistently, and to a lesser and more variable degree SSRIs and SNRIs — are associated with worsening RLS. This is worth raising with your prescriber, but do not stop or change a prescribed medication on your own; stopping an antidepressant abruptly carries its own risks, and the trade-off is a clinical judgement.
- Untreated sleep apnoea. The guideline lists it alongside medications as an exacerbating factor worth identifying before anything else is added.
- Advanced kidney disease. RLS is markedly more common in end-stage renal disease, where iron targets differ from the general population.
- Caffeine, alcohol and nicotine. All three are commonly reported to worsen symptoms. The evidence here is observational rather than trial-based, but timing changes cost nothing to test — for many people, moving the last coffee to before midday and skipping the evening glass of wine is the cheapest experiment available.
- Sleep deprivation itself. Poor sleep worsens RLS, and RLS worsens sleep. Breaking that loop matters as much as any lab value.
How well does iron actually work?
Here is the honest grading. A 2019 Cochrane review pooled 10 trials of 428 adults and concluded that iron therapy probably improves restlessness and RLS severity compared with placebo, at moderate certainty, without a significant increase in side effects. The effect size was low to moderate, most of the evidence came from intravenous rather than oral iron, and the reviewers were uncertain about quality-of-life benefit. Notably, restricting trials to participants with low ferritin at baseline did not produce a clearly larger effect — a finding that should temper any promise that "if your ferritin is low, iron will fix this."
Since then the evidence for intravenous iron has strengthened enough that the AASM 2025 guideline gives IV ferric carboxymaltose a strong recommendation in patients with appropriate iron status, with conditional support for two other IV formulations and for oral ferrous sulfate. That same guideline made a widely reported reversal elsewhere: it now suggests against the standard use of the dopamine agonists pramipexole and ropinirole, because of augmentation — a gradual worsening in which symptoms start earlier in the day and spread to other body parts after months to years of treatment. The alpha-2-delta ligands (gabapentin enacarbil, gabapentin and pregabalin) each carry a strong recommendation instead. All of these are prescription decisions; we mention them so you know the landscape changed, not so you can request one.
Practical reality: oral iron can take months to move ferritin, is often poorly tolerated, and absorption is affected by timing and what else you take. Intravenous iron bypasses the gut entirely and is given as an infusion in a clinic rather than swallowed. If your clinician does recommend iron, our guides to iron supplements and timing iron for absorption cover what to expect. Do not start iron on your own on the strength of a symptom: iron overload is a genuine harm, and some people carry haemochromatosis genes without knowing it.
When to see a doctor
Book an appointment if:
- Leg symptoms disturb your sleep two or more nights a week, or you are tired enough during the day that driving or concentrating suffers.
- You have heavy periods — soaking through protection hourly, flooding, clots larger than a coin, or periods lasting more than seven days — alongside restless legs. Ask for ferritin and transferrin saturation, not just a full blood count.
- Symptoms started or worsened after a new medication.
- You are pregnant and symptoms are affecting sleep.
- You are already on a dopamine agonist and symptoms are appearing earlier in the day, coming on faster when you sit, or spreading to your arms — this pattern suggests augmentation and needs review, not a higher dose.
Seek urgent care if leg discomfort is accompanied by sudden one-sided swelling, redness, warmth or calf pain (possible blood clot), by new weakness, numbness or loss of bladder control (possible nerve compression), or by chest pain or breathlessness. And any bleeding after menopause is not a period — postmenopausal bleeding, or anaemia found after menopause, needs prompt evaluation to rule out uterine and gastrointestinal causes, regardless of how well it explains your legs. Taking iron before the source of the loss has been found can mask the very signal that would have led to the diagnosis. See postmenopausal bleeding and anaemia after menopause.
Chronic broken sleep wears mood down. If you are struggling with hopelessness or thoughts of self-harm, contact your clinician — and in the US you can call or text 988 for the Suicide and Crisis Lifeline, any time.
The takeaway is narrow and useful: if restless legs are costing you sleep, iron studies belong in the workup, ferritin is the number to ask for, and 75 ng/mL — not 15 — is the figure sleep specialists work from. Getting tested tells you whether this particular lever is available to you. It may not be. But it is the one most often left unpulled.
]]>
