Always tired? The blood tests that check the common causes.

If you are tired every day, a short list of blood tests covers most of the causes doctors can actually check: iron stores, thyroid function, vitamin B12 and folate, vitamin D, blood sugar and a full blood count. They will not explain every kind of tiredness, but they rule the common things in or out quickly.

The common medical causes of tiredness in women

Tiredness is one of the most common symptoms brought to general practice, and one of the least specific. The useful move is not to guess, but to work through the causes that have a number attached to them.

• Low iron, with or without anaemia. The single most common checkable cause in women who menstruate. Stores can be low for months before your full blood count looks abnormal, so ferritin matters more than haemoglobin here. Heavy periods, pregnancy, breastfeeding and a low-meat diet all drain stores.
• An under-active thyroid. Slow thyroid function shows up as tiredness, feeling cold, weight gain, brain fog, dry skin and heavier periods. Thyroid problems are several times more common in women than in men and are easy to miss, because every symptom is also blamed on being busy.
• Low vitamin B12 or folate. Both are needed to make healthy red blood cells and to keep nerves working. Deficiency is more likely if you eat little or no animal food, have had gastric surgery, are older, take long-term medicines that reduce stomach acid or B12 absorption, or have pernicious anaemia, an autoimmune condition that stops B12 being absorbed.
• Low vitamin D. Very common in Australia despite the climate, particularly if you work indoors, cover up, have darker skin, or live in the southern states through winter. It is linked with muscle aches, low mood and a general flatness rather than dramatic symptoms.
• Blood sugar running high. Persistent tiredness, thirst and needing to pass urine at night can point to HbA1c in the pre-diabetes or diabetes range. This is more likely with a family history, PCOS, or a history of gestational diabetes.
• The perimenopausal transition. Broken sleep from night sweats, plus mood and cycle changes, produces a fatigue that feels different from ordinary tiredness. Hormone results fluctuate here, so the panel is as much about ruling other things out.
• The postpartum year. Blood loss at birth, breastfeeding and interrupted sleep are the obvious causes, but postpartum thyroid changes and low iron are common and both are correctable once found.
• Ongoing inflammation, kidney or liver problems. Less common, but a raised CRP or an abnormal kidney or liver result gives a doctor a clear direction to investigate.

There are causes a blood test will not find at all: sleep apnoea, depression and anxiety, chronic pain, medication side effects, and the plain fact of carrying too much. Those matter just as much. Testing simply clears the checkable list first, so the conversation about the rest starts from solid ground.

The blood tests that check each cause

This is the mapping doctors work from. One collection can cover the whole table.

Possible cause	Blood test that checks it	What the result can point to
Low iron stores, iron deficiency anaemia	Iron studies (ferritin, serum iron, transferrin saturation) plus full blood count	Ferritin below about 30 ug/L in women suggests depleted stores; a low haemoglobin alongside it suggests anaemia has already set in.
Under-active or over-active thyroid	TSH, with free T4 (and antibodies if abnormal)	TSH above about 4.0 mIU/L suggests the thyroid is running slow; below about 0.4 mIU/L suggests it is running fast.
B12 or folate deficiency	Vitamin B12 and folate	Low results can explain fatigue, pins and needles or a sore tongue, and often show up with enlarged red cells on the full blood count.
Vitamin D deficiency	25-hydroxy vitamin D	50 nmol/L or above is generally considered sufficient in Australia; 30 to 49 nmol/L is mild deficiency, 12.5 to 29 is moderate and below 12.5 is severe.
Pre-diabetes or diabetes	HbA1c	6.5% (48 mmol/mol) or above is the diabetes threshold; 6.0 to 6.4% is commonly reported as high risk, and your doctor may suggest a glucose tolerance test to confirm.
Perimenopause and hormone changes	FSH, oestradiol, testosterone	Supportive rather than definitive: results swing month to month, so they are read with your age, cycle history and symptoms.
Infection, inflammation or organ strain	CRP, kidney function, liver function	Abnormal results do not name a cause on their own, but they tell your doctor where to look next.

Ranges vary by laboratory; your report shows the range your lab uses. Typical ranges on this page are drawn from Pathology Tests Explained (RCPA) and the sources listed below.

Two practical notes. Ferritin rises when you are unwell or inflamed, so a test taken during an infection can read falsely reassuring; if you have been sick, wait a couple of weeks. And high-dose biotin, sold widely for hair and nails, can interfere with thyroid assays, so pause it for 72 hours before collection.

Brain fog: what can and cannot be tested

Brain fog is not a diagnosis and there is no blood test for it. What exists is a list of causes that produce it, and several of them are measurable: low iron, an under-active thyroid, low B12, poorly controlled blood sugar and the perimenopausal transition all show up as difficulty concentrating, losing words and feeling one step behind.

So the honest framing is this. A "brain fog test" checks the causes of brain fog, not the fog itself. If those markers come back normal, that is genuinely useful information: it moves attention to sleep quality, stress load, mood, alcohol, and the cognitive changes that many women describe through perimenopause without any abnormal result at all.

Nothing about a normal set of results means you are imagining it. Fog is real, common and usually multi-factor.

What normal results mean, and when the answer is sleep, load or mood

A completely normal panel is not a dead end. It rules out the causes that would have needed action, and it tells you the answer is somewhere else. In practice, the somewhere else is usually one of five things.

• Sleep quantity and quality. Snoring, waking unrefreshed, morning headaches or a partner noticing pauses in your breathing point to a sleep problem worth investigating properly.
• Mood. Depression and anxiety very commonly present as exhaustion rather than sadness, especially in women who are still functioning at work.
• Load. Caring, shift work, study and long hours produce real physiological fatigue that no marker will capture.
• Medications and alcohol. Both are frequent, reversible contributors and worth reviewing with your GP or pharmacist.
• Post-viral fatigue. Tiredness that started after an infection and has lasted months deserves a proper clinical assessment rather than more tests.

The other outcome worth naming: a result that sits just inside the range. Ferritin of 32 ug/L is technically normal and still low for someone with heavy periods and hair shedding. TSH of 3.8 mIU/L with positive antibodies is normal today and worth repeating. Borderline numbers are exactly where a conversation with a doctor earns its place.

When to see a doctor promptly

Most tiredness is not an emergency. Some patterns are, and they should not wait for a self-ordered test.

• Unexplained weight loss, drenching night sweats or fevers that keep returning.
• Breathlessness at rest or on light activity, chest pain, or a racing or irregular heartbeat.
• Feeling faint, dizzy on standing, or very pale, especially alongside heavy bleeding.
• Bleeding that soaks through protection hourly, passes large clots, or lasts longer than a week.
• Blood in your stool or urine, or a new lump anywhere.
• Fatigue that came on suddenly and severely over days rather than months.
• New confusion, or tiredness with a severe headache or visual change.

If any of these apply, book with your GP or present to an emergency department rather than ordering a panel and waiting for results.

Your KnowLuna fatigue and energy check

The KnowLuna fatigue and energy check covers the whole table above in one blood collection, at an accredited Australian centre or with a home kit or a nurse visit if you prefer: iron studies, full blood count, TSH and free T4, vitamin D, vitamin B12, folate, HbA1c, kidney function, liver function and CRP.

Fasting is not required and morning collection is ideal. Your results come back as a plain-English report showing each marker against your laboratory's range, with the pattern read as a set rather than marker by marker. Anything abnormal is followed up by an AHPRA-registered doctor, and the 20-minute video consult is where borderline numbers get their context.

If you already suspect one cause, a single test may be all you need: iron studies, the thyroid panel or vitamin D can be ordered on their own.

Common questions

Sources and further reading

• Pathology Tests Explained (RCPA): ferritin
• Pathology Tests Explained (RCPA): thyroid function tests
• Pathology Tests Explained (RCPA): vitamin B12 and folate
• Pathology Tests Explained (RCPA): HbA1c
• Healthdirect: fatigue
• Healthdirect: iron deficiency
• Medical Journal of Australia: Vitamin D and health in adults in Australia and New Zealand, position statement (2012)

General information only, not medical advice. If your symptoms are severe or sudden, see a doctor promptly. This page does not replace a consultation with a doctor who knows your history; read our medical disclaimer.