Restless Legs Syndrome — Blood Tests Your GP Should Order
RLS is one of the most under-diagnosed sleep-disrupting conditions in Australia — and one of the most blood-test-treatable. The single most important number is your ferritin.
Quick Summary — What to Ask Your GP
Restless legs syndrome is a real, treatable medical condition — not just “jumpy legs”. The most important thing to know is the RLS-specific ferritin cutoff: ferritin should be above 75 mcg/L, not the standard 30 mcg/L. Many GPs will tell you your iron is “fine” at a ferritin of 35 because it sits inside the standard range — but if you have RLS, that is still too low.
Ask your GP for: ferritin and iron studies (and ask for the actual number, not just “normal”), full blood count, vitamin B12, folate, magnesium, vitamin D, HbA1c, kidney function (eGFR and urea), and TSH. If you are pregnant or your GP suspects sleep apnoea, additional investigations may be needed.
Once you have results, upload them to SmarterBlood and we will flag exactly which markers are working against you — with the RLS-specific cutoffs, not the generic ones.
8 Causes of Restless Legs Your Blood Can Reveal
Iron Deficiency (the RLS-specific cutoff)
How it causes RLS: This is the single most important cause to know about. Iron is the rate-limiting cofactor for tyrosine hydroxylase, the enzyme that converts the amino acid tyrosine into dopamine in the brain. Low brain iron means low dopamine, and dopamine is the neurotransmitter that suppresses involuntary movements at rest. The crucial point: brain iron deficiency happens before blood iron deficiency. The standard ferritin cutoff for the general population is 30 mcg/L, but for restless legs syndrome it is 75 mcg/L. Many GPs are not aware of this, which is why so many RLS patients are told their iron is "fine".
Typical pattern:
An irresistible urge to move the legs (sometimes the arms), worse in the evening or at rest, often with crawling, tingling, or pulling sensations deep in the legs. Movement provides temporary relief. Onset usually after 30, often associated with disturbed sleep, fatigue, brittle nails, hair loss, or heavy menstrual bleeding.
What to do next: Ask your GP for ferritin and a full iron panel. If ferritin is below 75 mcg/L (even if "in range"), ask about iron supplementation or, if intolerant of oral iron, an iron infusion. Symptoms can improve dramatically once ferritin is above 100 mcg/L. Iron supplements should be taken on an empty stomach with vitamin C if tolerated.
Chronic Kidney Disease
How it causes RLS: Restless legs syndrome is two to five times more common in people with chronic kidney disease than in the general population, and even more common in those on dialysis. Several factors contribute: uraemic toxins interfere with central dopamine signalling, kidney disease impairs erythropoietin production (causing anaemia), and abnormal calcium-phosphate balance disrupts nerve and muscle function. New-onset RLS in someone over 50 should prompt a kidney check.
Typical pattern:
RLS symptoms in someone with high blood pressure, diabetes, or family history of kidney disease. Often accompanied by fatigue, nausea, itchy skin, swollen ankles, foamy urine, or the need to wake at night to pass urine.
What to do next: Ask for eGFR (calculated from creatinine), urea, and a urine albumin-creatinine ratio (uACR). An eGFR below 60 mL/min/1.73m squared that persists for more than three months indicates chronic kidney disease. If kidney function is impaired, optimising blood pressure, treating anaemia, and addressing phosphate balance can improve RLS.
Diabetes and Insulin Resistance
How it causes RLS: People with type 2 diabetes have a roughly 70% higher risk of developing RLS. The link is partly through diabetic peripheral neuropathy (nerve damage from chronic hyperglycaemia), which alters sensation in the legs and disrupts the spinal pathways involved in RLS. Diabetic neuropathy can also coexist with B12 deficiency in patients on long-term metformin, compounding the problem.
Typical pattern:
RLS plus diabetes risk factors (overweight, family history, gestational diabetes), or established diabetes with neuropathic features such as numbness, tingling, or burning in the feet. Symptoms often worse with poorly controlled glucose.
What to do next: HbA1c is the key test. If above 6.5% you have diabetes; if 6.0 to 6.4% you have prediabetes. People on metformin should also have B12 checked. Improving glucose control with diet, exercise, weight loss, and medication slows neuropathy and often improves RLS.
Magnesium Deficiency
How it causes RLS: Magnesium is a natural NMDA receptor blocker, helping to regulate excitatory glutamate signalling in the nervous system. Low magnesium increases neuronal excitability and contributes to muscle cramps and twitching. It is also essential for normal sleep architecture. While trial evidence in RLS is mixed, low magnesium is biologically plausible as a contributor and is easy to correct.
Typical pattern:
RLS plus muscle cramps (especially nocturnal calf cramps), eyelid twitching, fatigue, and poor sleep. More common in people with chronic alcohol use, on proton-pump inhibitors, with diarrhoea or malabsorption, or who eat a low-magnesium diet.
What to do next: Ask for serum magnesium (normal 0.7 to 1.0 mmol/L). Note that serum magnesium can look normal even when total body magnesium is low; a red blood cell magnesium can give a more accurate picture. A trial of oral magnesium glycinate or citrate (200 to 400 mg at night) is usually safe in people with normal kidney function.
Vitamin B12 and Folate Deficiency
How it causes RLS: B12 and folate are both essential for the methylation cycle and for normal myelin and dopamine synthesis. Deficiencies cause peripheral neuropathy (which can mimic or worsen RLS) and reduce dopamine synthesis. B12 deficiency is particularly common in older adults, vegans, and people on metformin or proton-pump inhibitors. Folate deficiency is now uncommon in Australia thanks to bread fortification but still occurs in pregnancy and malabsorption.
Typical pattern:
RLS plus tingling, numbness, or burning in the feet. Often accompanied by fatigue, brain fog, balance problems, or a sore tongue. May coexist with iron deficiency in heavy menstrual bleeders or vegans.
What to do next: Ask your GP for vitamin B12 and folate levels, plus full blood count for MCV (large red cells suggest B12 or folate deficiency). If B12 is below 200 pmol/L, supplementation is warranted; injections are used for severe or symptomatic deficiency.
Pregnancy
How it causes RLS: Up to one in three pregnant women experience restless legs syndrome, especially in the third trimester. The combination of high iron demand from the foetus, plasma volume expansion that dilutes iron stores, hormonal changes (rising oestrogen and progesterone), and folate redirection all converge. Symptoms usually resolve within four weeks after delivery, but recurrence in subsequent pregnancies is common.
Typical pattern:
New onset of restless legs in pregnancy, often most severe in the third trimester. May significantly disrupt sleep at a time when sleep is already difficult. Personal or family history of RLS makes pregnancy-related RLS more likely.
What to do next: Routine pregnancy bloods will already include full blood count and ferritin. Ask specifically for the ferritin number, not just whether it is "normal". Aim for above 75 mcg/L. If low, oral iron supplementation is first-line; iron infusion can be used if needed and is safe in pregnancy. Continue prenatal folate and discuss any symptoms with your obstetrician or GP.
Thyroid Disorders
How it causes RLS: Both overactive and underactive thyroid have been linked to RLS, although the evidence is weaker than for iron and kidney disease. Hypothyroidism slows metabolism and can worsen sleep quality, anaemia, and peripheral neuropathy, all of which can drive or worsen RLS. Hyperthyroidism causes a hyper-aroused state that disrupts sleep and may exacerbate the symptom of restlessness.
Typical pattern:
RLS plus other thyroid features: weight gain, cold intolerance, dry skin, constipation (for hypothyroidism); or weight loss, heat intolerance, palpitations, anxiety (for hyperthyroidism).
What to do next: Ask for TSH and free T4. Bulk-billed under Medicare. Treatment of underlying thyroid dysfunction often improves sleep quality and may reduce RLS severity.
Sleep Disorders and Lifestyle Triggers
How it causes RLS: RLS interacts strongly with other sleep disorders. Obstructive sleep apnoea (OSA) and RLS commonly coexist, each worsening the other. Caffeine, alcohol, nicotine, and certain medications (some antidepressants, antihistamines, antinausea drugs, dopamine blockers) can trigger or worsen RLS. While these are not detected on a blood test, screening for the medical drivers above is essential before assuming the cause is purely lifestyle.
Typical pattern:
RLS plus loud snoring, witnessed pauses in breathing, daytime sleepiness, or morning headaches (suggesting OSA). Or RLS that flares with caffeine, alcohol, or starting a new medication.
What to do next: Cover the medical bases first (iron, kidney, thyroid, diabetes, B12). Then review medications with your GP; some can be substituted. Reduce caffeine after midday. If you snore heavily or are very tired during the day, ask about a sleep study, which is Medicare-rebated when ordered by a GP for clinical reasons.
Tests to Ask Your GP For
This is the comprehensive RLS panel. All are bulk-billed under Medicare when ordered for a clinical indication. The most critical test is ferritin — and getting the actual number, not just “normal”.
| Test | Why It Matters | Cost |
|---|---|---|
| Ferritin (with the RLS-specific cutoff) | The single most important RLS test. Aim for above 75 mcg/L, not the standard 30 | Bulk billed |
| Iron Studies (Iron, TIBC, Transferrin Saturation) | Confirms iron status alongside ferritin | Bulk billed |
| Full Blood Count (FBC) | Detects anaemia and abnormal red cell size | Bulk billed |
| Vitamin B12 and Folate | Both deficiencies cause neuropathy that can drive RLS | Bulk billed |
| Magnesium | Low magnesium contributes to muscle excitability and cramps | Bulk billed |
| Vitamin D (25-OH) | Often low in Australians; associated with RLS in observational studies | Bulk billed* |
| HbA1c | Screens for diabetes and prediabetes; diabetic neuropathy worsens RLS | Bulk billed |
| Kidney Function (eGFR, Creatinine, Urea) | CKD significantly increases RLS risk | Bulk billed |
| Urine Albumin-Creatinine Ratio (uACR) | Detects early kidney damage missed by eGFR alone | Bulk billed |
| Thyroid Function (TSH, Free T4) | Thyroid dysfunction can exacerbate RLS | Bulk billed |
* Vitamin D requires a documented clinical indication for bulk billing. Mention RLS, sleep disruption, fatigue, or muscle symptoms specifically.
Red Flags — When to Seek Urgent Help
Restless legs syndrome itself is not an emergency, but the symptoms below are not RLS — they suggest something else and need prompt assessment.
How to Interpret Your Results
Standard pathology reports use the general adult reference range, which is too generous for restless legs syndrome. SmarterBlood applies the RLS-specific cutoffs and explains every relevant marker in plain language.
Key reference ranges to remember for RLS:
- Ferritin (RLS-specific): aim for above 75 mcg/L, ideally 100. Standard range is 30 to 300.
- Transferrin saturation: above 20% is generally adequate.
- Haemoglobin: 120 to 160 g/L (women), 135 to 175 g/L (men).
- Magnesium: 0.7 to 1.0 mmol/L.
- Vitamin B12: above 200 pmol/L is the lower limit; symptoms can occur up to 300.
- Vitamin D: above 50 nmol/L is sufficient; aim for 75 to 150.
- HbA1c: below 6.0% is normal; 6.0 to 6.4% prediabetes; 6.5%+ diabetes.
- eGFR: above 60 mL/min/1.73m squared is normal; below 60 for three months indicates CKD.
Frequently Asked Questions
What is the most important blood test for restless legs syndrome?
Ferritin. The cutoff for RLS is much higher than the general anaemia cutoff: ferritin should be at least 75 mcg/L, not the standard 30 mcg/L. Brain iron deficiency is a primary driver of RLS even when blood iron looks normal.
Can low iron cause restless legs?
Yes. Iron is required for dopamine production in the brain, and dopamine controls the motor circuits that prevent involuntary leg movements. Low brain iron disrupts this circuit. Treating low ferritin (aim above 75, ideally 100 mcg/L) often dramatically improves or resolves RLS symptoms.
What blood tests should my GP order for restless legs?
Ask for ferritin and iron studies (with the higher cutoff in mind), full blood count, vitamin B12, folate, magnesium, vitamin D, HbA1c, kidney function (eGFR and urea), and TSH.
Why does restless legs syndrome happen during pregnancy?
Up to one in three pregnant women experience RLS, especially in the third trimester. The combination of high iron demand from the foetus, blood volume expansion diluting iron stores, hormonal changes, and folate redirection all contribute. Symptoms usually resolve within weeks of delivery.
Is restless legs syndrome a sign of kidney disease?
It can be. RLS is much more common in people with chronic kidney disease, particularly those on dialysis. Uraemic toxins disrupt brain dopamine signalling. Anyone with new-onset RLS, especially over 50, should have kidney function checked.
Can magnesium deficiency cause restless legs?
Possibly. Magnesium plays a role in muscle relaxation and nerve conduction. While the evidence for magnesium supplementation is mixed, low magnesium is reasonable to check and correct. Aim for serum magnesium between 0.7 and 1.0 mmol/L.
What medications make restless legs worse?
Common culprits include certain antidepressants (especially SSRIs and tricyclics), antihistamines, antinausea drugs, dopamine blockers (some antipsychotics, metoclopramide), and excessive caffeine. Always discuss with your GP before stopping any prescribed medication.
Related Reading
Stop the Restless Nights — Check Your Iron
Already have results? Upload your pathology PDF and SmarterBlood will flag your ferritin against the RLS-specific cutoff, plus check every other marker linked to restless legs — in plain language. Free and private.