Blood Tests for Fatigue: What to Actually Test

You're exhausted. Not just tired-after-a-long-day exhausted, but bone-deep fatigue that sleep doesn't fix. You drag yourself through mornings, crash in the afternoons, and wonder if this is just what life feels like now.

Your GP says your bloodwork is "normal". But something doesn't feel right. You're not imagining it — and the question that keeps nagging at you is: what else should I be testing?

Here's the truth: standard blood panels don't always catch everything. Some causes of fatigue show up clearly in routine tests. Others require looking deeper. The trick is knowing which tests are worth ordering — and which might leave you with more questions than answers.

Evidence Tier: Mixed

• Foundation tests (TSH, ferritin, B12, vitamin D, FBC): Guideline-Backed — your GP will know exactly what to do with these

• Extended tests (fasting insulin, hs-CRP, cortisol): Context-Dependent — recognised by some practitioners, not universally ordered

• Functional tests (organic acids, comprehensive hormone panels): Research-Oriented — popular in integrative medicine, limited mainstream adoption

Fatigue investigation involves tests across all three evidence tiers. Knowing which tier each test falls into helps you understand what to expect when you share results with your health practitioner.

What Blood Tests for Fatigue Actually Tell You

Fatigue has dozens of possible causes — from simple sleep deprivation to serious underlying conditions. Blood tests help narrow down the possibilities by checking for common, treatable causes that might otherwise be missed.

Think of it like troubleshooting a car that won't start. You check the obvious things first (battery, fuel) before investigating the complex stuff (engine internals). Similarly, fatigue testing starts with common causes before exploring deeper.

What Blood Tests CAN Tell You

• Iron deficiency — One of the most common causes of fatigue, especially in women. Ferritin shows your iron "savings account" — you can be depleted even with normal haemoglobin.

• Thyroid dysfunction — An underactive thyroid slows your metabolism, causing fatigue, weight gain, and brain fog. TSH is the standard screening test.

• Vitamin deficiencies — Low B12 causes fatigue, brain fog, and even nerve symptoms. Vitamin D deficiency is common in Australia despite our sunshine.

• Blood sugar issues — Diabetes and prediabetes can cause fatigue. HbA1c shows your blood sugar control over 3 months.

• Anaemia — Low red blood cells mean less oxygen delivery to tissues. A full blood count catches this.

• Inflammation — Chronic inflammation drains energy. CRP or ESR can indicate inflammatory processes.

What Blood Tests CAN'T Tell You

• Whether you're getting enough sleep (or good quality sleep)

• If stress, anxiety, or depression are contributing

• Chronic fatigue syndrome (ME/CFS) — diagnosed by exclusion, not a specific test

• How all your body systems are interacting together

Blood tests are one piece of the puzzle. They're powerful for ruling things in or out — but they don't capture everything that affects energy.

What Health Practitioners Say

Australian GPs follow RACGP guidelines that recommend a targeted, staged approach to investigating fatigue. The key insight: extensive testing rarely finds answers. According to RACGP research, only about 4% of patients investigated for fatigue receive a significant diagnosis from blood tests.

That doesn't mean testing is useless — it means targeted testing is more valuable than ordering everything.

The RACGP-Recommended Panel

When your GP investigates unexplained fatigue, they'll typically order:

• Full blood count (FBC) — Screens for anaemia and blood disorders

• TSH — Thyroid function screening

• Fasting glucose or HbA1c — Diabetes screening

• Electrolytes, urea, creatinine (EUC) — Kidney function and electrolyte balance

• Liver function tests (LFT) — Liver health screening

• ESR or CRP — Inflammation markers

A 2014 RACGP review found that a minimal panel of haemoglobin, ESR, fasting glucose, and TSH was nearly as effective as extensive testing for detecting serious pathology.

Tests Your GP May Add Based on Clinical Suspicion

• Ferritin — Iron stores (especially important for women)

• Vitamin B12 and folate — If dietary risk factors or neurological symptoms

• Vitamin D — Widely tested in Australia given high deficiency rates

• Coeliac serology — If gastrointestinal symptoms or unexplained iron deficiency

The GP approach is conservative by design: test for common, treatable causes first, avoid unnecessary tests that might generate false positives and anxiety.

What Integrative Practitioners Say

Naturopaths and functional medicine practitioners often take a different approach. They argue that "normal" lab results don't always mean "optimal function" — and that subtle imbalances can affect how you feel long before they become diagnosable conditions.

Extended Testing Often Recommended

• Full thyroid panel — Not just TSH, but Free T4, Free T3, and sometimes thyroid antibodies (TPO, TgAb). The theory: TSH alone can miss early thyroid dysfunction or conversion issues.

• Fasting insulin — Standard glucose tests can be "normal" while insulin is elevated (early insulin resistance). Some practitioners consider this an early warning sign.

• hs-CRP — High-sensitivity inflammation marker, considered more sensitive than standard CRP.

• Cortisol testing — Saliva or urine tests that track cortisol throughout the day (the "cortisol curve"). Popular for investigating adrenal fatigue, though this concept remains controversial in mainstream medicine.

• Magnesium — Serum magnesium often normal even when tissue stores are low. Some practitioners consider RBC magnesium more useful.

Functional Tests (Research-Oriented)

• Organic acids testing (OAT) — A urine test examining metabolic byproducts, claimed to reveal mitochondrial function, neurotransmitter balance, and nutrient status. Popular in functional medicine, limited mainstream adoption.

• Comprehensive hormone panels (DUTCH) — Measures hormone metabolites over 24 hours. More detail than single blood draws, but not validated for diagnosis in mainstream practice.

• Microbiome testing — Gut health increasingly linked to energy and mood. Research is evolving; clinical utility of consumer tests remains debated.

The integrative view: standard testing focuses on disease detection, not optimal function. These additional tests may reveal patterns that explain why you feel exhausted even when "everything is normal."

The limitation: many of these tests aren't recognised by GPs, so you may need to work with an integrative practitioner who knows how to interpret them — and who can translate findings into actionable recommendations.

The Evidence

Guideline-Backed Tests: Strong Evidence

Thyroid (TSH): RACGP guidelines recommend TSH as first-line thyroid screening. Australian data shows hypothyroidism affects 4-5% of the population, with fatigue as a primary symptom. Treatment with thyroid hormone reliably improves symptoms when deficiency is confirmed.

Iron (Ferritin): Iron deficiency is one of the most common causes of fatigue globally. Australian research shows that ferritin below 30 mcg/L is associated with fatigue symptoms even without anaemia. Treatment (iron supplementation or infusion) typically improves energy within weeks to months.

Vitamin D: Despite our sunshine, vitamin D deficiency is common in Australia — particularly in winter, in people with darker skin, and in those who work indoors. Systematic reviews show association between low vitamin D and fatigue, though supplementation results are mixed (some people improve significantly; others notice little change).

Vitamin B12: Deficiency causes fatigue, weakness, and neurological symptoms. RACGP guidelines recommend testing in vegetarians/vegans, people over 50, those on metformin, and anyone with unexplained fatigue or neurological symptoms. Treatment with B12 injections or supplements is highly effective when deficiency is confirmed.

Context-Dependent Tests: Emerging Evidence

Fasting Insulin: Not routinely ordered by GPs but increasingly recognised. Research shows elevated fasting insulin predicts type 2 diabetes years before glucose abnormalities appear. Some studies link insulin resistance to fatigue, though the relationship isn't as well-established as with thyroid or iron.

Free T3: Useful in specific scenarios (e.g., patients on thyroid medication, suspected conversion issues), but Endocrine Society guidelines don't recommend it for routine screening. Evidence for "subclinical" thyroid issues affecting energy is mixed.

hs-CRP: Validated for cardiovascular risk assessment. Chronic low-grade inflammation is associated with fatigue in research, but the clinical pathway (what to do about elevated hs-CRP) is less clear than with thyroid or iron.

Research-Oriented Tests: Limited Validation

Adrenal fatigue / cortisol testing: "Adrenal fatigue" is not a recognised medical diagnosis. The Endocrine Society states there is no scientific evidence supporting the concept. However, cortisol dysregulation (HPA axis dysfunction) is a legitimate area of research — the controversy is whether consumer testing meaningfully captures it.

Organic acids testing: Limited peer-reviewed validation. May provide interesting data points, but clinical utility and reproducibility are not well-established in mainstream literature.

Honest Limitations

• Most fatigue has no identifiable organic cause on blood testing

• Even comprehensive testing often returns "normal" results

• Sleep, stress, and lifestyle factors don't show up in blood tests

• Some people with abnormal results feel fine; some with normal results feel terrible

Blood tests are powerful but imperfect tools. They're best used to rule out treatable causes — not as definitive answers to complex symptoms.

Who Should Consider Fatigue Testing

Blood testing for fatigue may be worthwhile if:

• Fatigue persists beyond 2-4 weeks despite adequate sleep and rest

• You have risk factors for specific deficiencies (heavy periods, vegetarian diet, limited sun exposure, family history of thyroid disease)

• Your fatigue is severe enough to affect daily function — not just "I'm tired after a long week"

• You have other symptoms alongside fatigue — unexplained weight changes, hair loss, cold intolerance, brain fog, muscle weakness

• It's been a while since your last check-up — baseline testing can establish your personal reference points

Testing is probably not the answer if:

• You're not getting enough sleep (address that first)

• You're under acute stress that explains the fatigue

• You've had comprehensive testing recently with normal results

• You're looking for tests to explain fatigue that's clearly lifestyle-related

Next Steps — What to Do With This Information

Starting With Your GP

Your GP is the best first step. They can order foundation tests (often Medicare-rebated when clinically indicated) and help interpret results in context of your symptoms and history.

Questions to consider asking:

• "Given my symptoms, what tests would you recommend starting with?"

• "I'm particularly interested in checking iron/thyroid/vitamin D — would those be appropriate?"

• "If the standard panel is normal, what would you suggest as next steps?"

If Standard Tests Are Normal But You're Still Exhausted

Options to consider:

• Request specific add-ons — Ask your GP about ferritin (if not included), vitamin D, or B12 if you have risk factors

• Explore context-dependent tests — Fasting insulin, full thyroid panel (Free T4, Free T3), hs-CRP. Your GP may order these or you can access them privately.

• Consider integrative assessment — A naturopath or integrative doctor can order extended panels and interpret them from a different perspective. Just know that research-oriented tests require specialist interpretation.

• Address non-blood-test factors — Sleep quality, stress management, exercise, diet. Sometimes the answer isn't in the blood.

What to Expect From Results

• Clear abnormality found — Your GP will guide treatment (e.g., iron supplementation, thyroid medication, B12 injections)

• "Low-normal" results — Some practitioners will trial treatment; others prefer to monitor. This is where integrative practitioners often add value.

• Everything normal — Reassuring that serious causes are ruled out, but frustrating if you're still exhausted. This is the signal to look at lifestyle factors or consider extended testing.

Explore Evidence-Tiered Panels

If you're ready to investigate your fatigue with comprehensive testing, the Energy & Metabolism Panel includes both guideline-backed foundation tests and context-dependent markers — with honest evidence tier labels so you know what to expect.

• Energy & Metabolism Panel — TSH, ferritin, B12, vitamin D, HbA1c, fasting insulin, hs-CRP, and more

• Browse All Panels — See evidence tiers for every biomarker

All Clarity Labs panels include transparent evidence tier classifications, so you understand which markers are guideline-backed, context-dependent, or research-oriented — before you order.

This information is educational only and not medical advice. Always discuss results with your health practitioner. If you're experiencing severe fatigue, new symptoms, or symptoms that concern you, please consult a qualified health professional.