What causes restless legs? The common triggers, the iron link, and the tests that check them

Restless legs syndrome (RLS) is an urge to move your legs, usually with creeping, tingling, burning or aching feelings, that starts when you sit or lie still, eases while you move and is worst in the evening and at night. ²⁴ In most people no single cause is found and it often runs in families, but it is linked with low iron, pregnancy, kidney failure, nerve damage and some medicines, and alcohol, caffeine and lost sleep make it worse. ² The test that matters most is iron: sleep doctors read ferritin (the body's iron stores) and transferrin saturation (how much iron is in transit in the blood) against higher cutoffs than a standard lab report uses. ¹

What restless legs syndrome is, and what it isn't

Clinicians recognize RLS from four features, all of which need to be there. The NINDS lists them: an urge to move the legs, often with uncomfortable feelings; symptoms that start or worsen at rest, such as on a plane or watching a movie; relief while moving; and "Worsening of symptoms at night with a distinct symptom-free period in the early morning." ² There is no blood test or scan for RLS itself; a clinician diagnoses it from your description and history, then uses blood tests to look for causes. ²⁴

Most people with RLS also have periodic limb movements in sleep: twitches or jerks of the legs every 15 to 40 seconds that can last through the night, often noticed by a bed partner. ² RLS can start at any age but most often begins in middle age. ² An early-onset form, before 45, is more likely to run in families, while a late-onset form after 45 tends to progress faster and more often comes with another condition. ³⁶

If what you have is a sudden, painful, hard calf at night, that is a cramp rather than RLS, and muscle cramps covers it. Numbness, tingling and burning that don't change with movement or time of day point more to nerve damage; see tingling hands and feet. ¹³

Why the iron numbers for restless legs are different

Most lab reports flag ferritin as low only at the bottom of a wide range; MedlinePlus gives common normal ranges of 30 to 400 ng/mL for men and 13 to 150 ng/mL for women. ⁹ The 2024 AASM guideline sets a much higher bar for people with RLS: iron "should be instituted with oral or IV iron if serum ferritin ≤ 75 ng/mL or transferrin saturation < 20%, and only with IV iron if serum ferritin is between 75 and 100 ng/mL." ¹ The guideline is open that these consensus cutoffs "have not been empirically tested" and adds: "These iron supplementation guidelines are different than for the general population." ¹ So a woman with RLS and a ferritin of 45 has a result her lab calls normal and her sleep clinician may call low.

Do iron supplements help restless legs?

This is one of the most searched questions about RLS, and the guideline's answer is yes for some people. The AASM makes a strong recommendation for one intravenous iron (ferric carboxymaltose) in adults whose iron studies fall under its cutoffs, and supports oral or IV iron when ferritin is 75 ng/mL or less or transferrin saturation is under 20%. ¹ The choice between pills and an infusion depends on the numbers, which is why the test comes first: the guideline says analysis of iron studies "greatly influences the decision to use oral or intravenous (IV) iron treatment." ¹

Beyond iron, the guideline recommends gabapentin, pregabalin and gabapentin enacarbil, and now suggests against the standard use of the dopamine drugs pramipexole, ropinirole and rotigotine, pointing to side effects with long-term use, particularly augmentation, where RLS gets worse over time on the drug. ¹ Those are prescription decisions for your clinician. The first step the guideline names is simpler: "addressing exacerbating factors, such as alcohol, caffeine, antihistaminergic, serotonergic, antidopaminergic medications, and untreated obstructive sleep apnea." ¹

Restless legs at night and your sleep

RLS is worst in the evening and at night by definition, so its main cost is sleep. ² The NINDS lists daytime sleepiness, trouble concentrating, impaired memory, depression and anxiety among its effects. ² If you are lying awake with it, see can't sleep for the habits that help sleep in general. Over-the-counter sleep aids often contain a sedating antihistamine, which is one of the medicine types that can make RLS worse. ¹²

Getting a useful iron test

• Ask for ferritin and transferrin saturation together, not ferritin alone. ¹
• Book a morning draw, and avoid iron-containing supplements, including multivitamins with iron, and iron-rich foods for 24 hours before, as the guideline suggests. ¹
• Mention any recent illness or inflammation, which can raise ferritin and make stores look better than they are. ⁹
• List every medicine, including over-the-counter sleep and allergy products, with start dates. ²
• Mention heavy periods, pregnancy, blood donation or any bleeding. ¹¹
• One result is a snapshot; a retest after treatment shows the trend. See why trends beat single readings.

Questions to ask your clinician

• Can we check my ferritin and transferrin saturation, and read them against the RLS cutoffs rather than the lab range?
• Could any of my medicines, including over-the-counter ones, be making this worse?
• If my iron is low, what is causing it, and is oral or IV iron right for me?
• Should my kidney function, blood sugar or B12 be checked?
• Could sleep apnea be part of this?

This page is general education, not medical advice or a diagnosis. Restless legs often has more than one contributor, and the clinician who knows your history is the one to read your iron results and decide what to do with them.

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