What blood tests should I ask for if I have fatigue?
A thorough fatigue workup covers four clusters: iron status (CBC + ferritin + TSAT + serum iron/TIBC), thyroid function (TSH + free T3 + free T4 — not TSH alone), blood sugar regulation (fasting glucose + HbA1c + fasting insulin if available), and vitamins and minerals (vitamin D, B12, folate, magnesium). Standard panels typically cover a CBC and basic metabolic panel — ferritin, free thyroid hormones, and magnesium are frequently omitted and need to be requested specifically.
What is a TATT blood test?
TATT stands for "tired all the time" — a shorthand UK GPs use on referral and lab request notes rather than a specific test. There's no single "TATT test"; NICE's Clinical Knowledge Summary for tiredness/fatigue in adults recommends the same core workup covered here: full blood count, ferritin, TSH, fasting glucose or HbA1c, and coeliac screening as a minimum, with B12, folate, and vitamin D added depending on symptoms and risk factors.
Which blood test shows fatigue?
No single blood test shows fatigue — the cause is identified by looking at a combination of markers across four clusters. The most commonly abnormal findings in people with unexplained fatigue are: low ferritin (iron stores), low or borderline vitamin D, low or borderline vitamin B12, TSH above 2.5 mIU/L (borderline thyroid), elevated fasting glucose or HbA1c (blood sugar dysregulation), and low-normal magnesium. A standard CBC and metabolic panel catch only some of these — ferritin, free T3/T4, fasting insulin, and magnesium require specific ordering.
Why doesn't a standard blood panel cover fatigue properly?
Standard panels are designed for broad disease screening — they catch active pathology, not subclinical deficiencies or borderline dysfunction. Ferritin, the most sensitive marker of iron reserves, is not in a standard CBC. Free T3 is almost never ordered unless TSH is clearly flagged. Fasting insulin rarely appears on standard panels at all. The tests most relevant to fatigue as a primary symptom typically require specific requests.
What is the most commonly missed blood test for fatigue?
Ferritin is the most frequently missed. A standard CBC measures haemoglobin and red blood cell count — ferritin, which measures iron reserves, can be depleted for months before haemoglobin drops. NICE defines iron depletion at below 30 ng/mL, and clinical research found fatigue symptoms in women with ferritin below 50 ng/mL even without anaemia (Verdon et al., BMJ, 2003). Most doctors do not include ferritin unless specifically asked.
Can I order these blood tests without a doctor?
In most countries a doctor's order is required, but direct-to-consumer lab services exist. In the US: Ulta Lab Tests and Walk-In Lab offer full iron panels, thyroid panels, and nutrient tests. In the UK: Medichecks and Thriva offer similar panels. A full fatigue panel (iron + thyroid + nutrients) typically runs £60–120 in the UK and $80–150 in the US. (Provider availability and pricing last checked June 2026. Verify current pricing directly with each provider.)
Can my blood tests come back normal and still be the problem?
Yes, in two ways. First, the most relevant tests (ferritin, free T3, fasting insulin, magnesium) may not have been ordered. Second, standard reference ranges flag only clear abnormalities — values in the borderline zone can be clinically associated with fatigue without triggering a flag. A ferritin of 18 ng/mL is "normal" on most reports but research shows it can still explain fatigue in women. If your results say normal and you still feel exhausted, check where each value sits within its range, not just whether it passed.
How long does it take to feel better after addressing a deficiency?
It depends on the marker. B12 and vitamin D often produce early energy improvements within 4–8 weeks. Iron stores (ferritin) take 3–6 months to fully rebuild, though energy may begin improving after 4–8 weeks of iron repletion. Thyroid symptoms take 6–8 weeks to stabilise after treatment begins. Fasting glucose responds within 4–6 weeks of consistent dietary changes.
Is it worth getting all four clusters tested at once?
If fatigue is your main symptom and you haven't had a thorough workup, yes — testing all four clusters at once gives you a complete picture and avoids the frustration of addressing one deficiency only to find another three months later. Ask your doctor for: full iron panel (CBC + ferritin + TSAT), full thyroid panel (TSH + FT3 + FT4), metabolic markers (fasting glucose + HbA1c), and nutrient panel (vitamin D + B12 + folate + magnesium). That's 12–14 tests, a full morning blood draw, usually processed together.
What deficiencies cause extreme fatigue?
The deficiencies most often behind persistent fatigue are iron (measured by ferritin, not just haemoglobin), vitamin D, vitamin B12, and folate. Low magnesium can add muscle fatigue and poor sleep. Iron depletion is the most common — ferritin can be low for months before a CBC shows anaemia. Each of these can sit in a borderline zone that a standard panel doesn't flag, which is why where a value falls within its range matters as much as whether it passed.
What hormone is tested for fatigue?
Thyroid hormones are the main ones — TSH first, and ideally free T3 and free T4, since TSH alone can read normal while the active hormones are low. Beyond thyroid, cortisol (the stress/adrenal hormone) and the sex hormones (testosterone in men, and oestrogen/progesterone around the menopause in women) are tested when the history points that way. A standard panel usually checks only TSH, so the others need to be requested specifically.
What is a red flag for fatigue?
Fatigue alongside any of these warrants prompt medical review rather than a routine workup: unintentional weight loss, drenching night sweats, fever that won't settle, breathlessness or chest pain, new lumps or swollen glands, blood in stool or urine, or fatigue that is rapidly worsening. These can point to something beyond a deficiency. If you have any of them, see a doctor promptly rather than ordering tests yourself.
What disease causes extreme fatigue?
Many conditions can, which is why testing matters. Common ones include iron-deficiency anaemia, hypothyroidism, diabetes and insulin resistance, vitamin B12 or D deficiency, coeliac disease, sleep apnoea, depression, and chronic infections. Less commonly, fatigue is a feature of autoimmune disease, kidney or liver dysfunction, or ME/CFS (a diagnosis of exclusion). The blood-test clusters on this page screen for the most common and treatable of these first.
Can blood tests tell the difference between fatigue and depression?
Not on their own. Blood tests rule physical causes in or out — thyroid dysfunction, anaemia, and B12 or vitamin D deficiency can all produce fatigue that looks and feels like depression, and treating the marker often resolves the fatigue without treating mood separately. But a normal panel does not rule out depression, and depression itself can lower motivation to eat well, sleep, or exercise, which then worsens markers like ferritin and vitamin D. NICE guidance recommends checking thyroid function, FBC, and glucose before treating fatigue as a primary mood disorder, precisely because the physical causes are treatable and often missed first.
Are fatigue blood tests different for women?
The tests are the same; the thresholds and likely causes differ. Iron deficiency from menstrual blood loss makes ferritin the marker most often responsible for fatigue in women of reproductive age, and clinical research has linked fatigue to ferritin below 50 ng/mL in women even without anaemia (Verdon et al., BMJ, 2003) — a level most labs still mark "normal." Thyroid disease is also several times more common in women, and perimenopause introduces its own fatigue pattern tied to shifting oestrogen and progesterone. A full iron panel and thyroid panel matter more for women than the CBC alone.
What blood tests explain tiredness with weight gain?
Start with TSH plus free T3 and free T4 — hypothyroidism slows metabolism and causes both symptoms together, and TSH alone can miss a case where free T3 is already low. Fasting insulin and HbA1c catch insulin resistance, which drives fatigue and weight gain before glucose itself moves out of range. Cortisol is worth checking if weight gain is concentrated around the midsection alongside poor sleep. Vitamin D deficiency is linked to both fatigue and weight gain in observational studies, though the mechanism is less direct than thyroid or insulin.