Overview.
Headaches are the most common neurological complaint, affecting >50% of adults annually. While most are primary (tension, migraine, cluster) and don't require blood tests, persistent or atypical headaches can signal thyroid dysfunction, iron deficiency, magnesium depletion, vitamin deficiencies, or — in patients over 50 — giant cell arteritis (a medical emergency). Blood work is most valuable when headaches are new-onset, changing in pattern, or accompanied by systemic features.
Primary headaches (tension, migraine, cluster) are diagnosed clinically and don't typically require blood tests. Secondary headaches have an identifiable medical cause and may benefit from targeted lab workup. The clinical challenge is identifying which headaches need investigation.
Prevalence: Tension headache: ~40% annual prevalence. Migraine: ~15% (women 3x > men). Chronic daily headache: ~4%. Secondary headaches from medical causes: ~5-10% of presentations to primary care.
What to test.
First-line tests
- CBC — Anaemia (especially iron deficiency) is an underrecognised cause of headache. Polycythaemia can cause headache through hyperviscosity. WBC elevation suggests infection.
- TSH — Both hypothyroidism and hyperthyroidism cause headache. Hypothyroidism is associated with chronic tension-type headache; hyperthyroidism with pulsatile headache from increased cardiac output.
- CMP — Electrolyte abnormalities (hyponatraemia), kidney dysfunction (uraemia), and liver dysfunction all cause headache. Glucose abnormalities (hypoglycaemia) trigger migraines.
- Ferritin — Iron deficiency (even without anaemia) is associated with increased headache frequency and migraine severity. Ferritin <30 ng/mL warrants trial of supplementation.
Second-line tests
- Magnesium (RBC preferred) — Magnesium deficiency is present in 30-50% of migraine sufferers. It affects cerebrovascular tone, cortical spreading depression, and serotonin signalling. Magnesium supplementation (400-600mg glycinate/oxide daily) is an evidence-based migraine preventive.
- Vitamin D — Vitamin D deficiency is associated with increased headache frequency in observational studies. Receptors are present in trigeminal ganglia. Supplementation may reduce migraine days.
- Vitamin B12 / Folate — B12 deficiency causes neurological symptoms including headache. Elevated homocysteine (from B12/folate deficiency) is associated with migraine with aura.
- CRP / ESR — CRITICAL in patients >50 with new headache: ESR >50 mm/hr with temporal headache and jaw claudication = giant cell arteritis until proven otherwise. CRP elevated in most GCA. Normal ESR effectively rules out GCA.
Specialized tests
- Homocysteine — Elevated homocysteine (from B12/B6/folate deficiency or MTHFR variants) is associated with migraine with aura and may increase stroke risk in migraine patients.
Common causes.
- Hypothyroidism — Chronic, diffuse, tension-type headache with fatigue, weight gain, cold intolerance
- Iron Deficiency — Increased headache frequency and migraine severity, especially with exercise or exertion
Diagnostic patterns.
- New headache in patient >50 + elevated ESR (>50 mm/hr) + scalp tenderness — likely Giant cell arteritis (temporal arteritis). Next step: URGENT: start prednisolone immediately (don't wait for biopsy); temporal artery biopsy within 2 weeks
- Chronic headaches + low ferritin + fatigue — likely Iron deficiency contributing to headache frequency. Next step: Iron supplementation targeting ferritin >50 ng/mL; reassess headache frequency in 3 months
- Migraines + low RBC magnesium + muscle cramps — likely Magnesium deficiency contributing to migraine threshold. Next step: Magnesium supplementation 400-600mg/day (glycinate or oxide); expect benefit in 4-12 weeks
- Normal blood work + typical migraine features + family history — likely Primary migraine. Next step: Clinical diagnosis; migraine-specific management (triptans, preventives, lifestyle modification)
Lifestyle.
Non-medical causes
- Tension-type headache (stress, posture, screen time — most common headache type)
- Migraine (genetic, triggered by hormones, sleep changes, stress, foods)
- Medication overuse headache (>10-15 days/month of analgesics — paradoxically worsens headache)
- Caffeine withdrawal (onset 12-24 hours after last caffeine)
- Dehydration
- Poor sleep (both too little and too much trigger headache)
- Cervicogenic headache (neck dysfunction referring pain to head)
- TMJ dysfunction (jaw pain referring to temporal region)
Considerations
- Regular sleep schedule (most important modifiable migraine trigger)
- Adequate hydration (2-3L/day)
- Regular meals (fasting triggers migraine in many sufferers)
- Magnesium supplementation (400-600mg/day — evidence-based preventive)
- Limit caffeine to <200mg/day and keep consistent (don't fluctuate)
- Regular aerobic exercise (proven migraine preventive — 150 min/week)
- Screen time breaks (20-20-20 rule: every 20 min, look 20 feet away for 20 seconds)
FAQs.
Should I get blood work for headaches?
If you have typical tension headaches or migraines that respond to treatment — probably not. But blood work is valuable if: headaches are new-onset or changing pattern, you're over 50 with a new headache type, headaches are accompanied by systemic symptoms (fatigue, weight change), or standard treatments aren't working. A basic panel (CBC, TSH, ferritin, CMP, magnesium) is a reasonable screen.
Can magnesium really prevent migraines?
Yes — magnesium supplementation is one of the few evidence-based, low-risk migraine preventives. The American Academy of Neurology rates it 'probably effective' for migraine prevention. Dose: 400-600mg daily (magnesium oxide or glycinate). Benefit takes 4-12 weeks. It's most effective when combined with B2 (riboflavin 400mg/day) and CoQ10.
I'm over 50 with a new headache — should I be worried?
New headache onset after age 50 ALWAYS warrants investigation. The most important exclusion is giant cell arteritis (GCA) — an inflammatory condition of the temporal arteries that can cause permanent blindness if untreated. Your doctor should check ESR and CRP urgently. Scalp tenderness, jaw claudication, and vision changes are red flags.