Fatigue: which blood tests to run to find the cause

You wake up after eight hours of sleep — and already feel drained. Coffee stops working. Everyday tasks that used to require no effort — a walk, a conversation, running errands — now leave you exhausted. You put it down to stress, the season, getting older. But if this has been going on for weeks or months, it may have a concrete medical explanation.

Chronic fatigue is one of the most common reasons people see a doctor. And in a significant share of cases, the underlying cause is straightforward, well-understood, and readily treatable: iron deficiency, hypothyroidism, low vitamin D or B12, or disrupted glucose metabolism. None of these announce themselves loudly — each quietly chips away at your energy reserves until the symptoms become impossible to ignore.

When fatigue has lingered, the first step isn’t to cycle through new supplements or wait it out. The first step is to rule out medical causes with a screening blood panel. One visit to the laboratory, concrete answers to concrete questions.

The screening minimum for fatigue

No doctor orders everything at once. But there is a core panel that covers the most common, correctable causes of fatigue. Here are the seven markers worth checking first.

Marker	What it measures	What an abnormal result suggests
Ferritin	Iron stores in the body	Low → latent iron deficiency (fatigue, brain fog, exhaustion)
Haemoglobin	Oxygen transport by red blood cells	Low → anaemia (breathlessness, palpitations, persistent weakness)
TSH	Thyroid function	High → hypothyroidism; low → hyperthyroidism
Vitamin D	25(OH)D in serum	Low → deficiency, often accompanied by fatigue and muscle weakness
B12	Vitamin B12 in serum	Low → neurological and haematological symptoms, fatigue
Glucose / HbA1c	Blood sugar level / 3-month average	High → prediabetes or type 2 diabetes (chronic fatigue is a common symptom)
Creatinine	Kidney function (eGFR)	High → chronic kidney disease, often symptom-free in early stages

One practical note: most of these can be measured from a single venous blood draw as part of a standard biochemistry panel. The logistics are minimal; the informational yield is high.

Deficiency-related causes

Iron deficiency and ferritin

Iron deficiency is the most common correctable cause of chronic fatigue in women of reproductive age worldwide. The paradox is that the classic marker — haemoglobin — can remain perfectly normal while the body’s iron stores (ferritin) are already depleted. This is the so-called “latent” phase: fatigue is real, but the blood count “looks normal.”

Ferritin is a storage protein: it reflects how much iron is held in reserve. If iron is the fuel, ferritin is the reserve tank and haemoglobin is the fuel currently running the engine. The engine keeps going until the tank is empty — but performance drops before it reaches zero.

A full breakdown of reference ranges and interpretation is in the Ferritin: Iron Stores and What the Test Shows article.

Vitamin D deficiency

By various estimates, 40 to 70 percent of adults in Central and Northern Europe have insufficient vitamin D levels at some point during the year. Deficiency produces no specific symptom — instead it gradually dampens energy, contributes to muscle weakness, and degrades sleep quality.

The test is 25(OH)D in serum. For the specific 25-OH D reference ranges and the distinction between deficiency, insufficiency, and adequate levels (in both nmol/L and ng/mL), see the dedicated Vitamin D Levels: What’s Normal and What’s Not article — it also covers seasonal patterns and supplementation.

Vitamin B12 deficiency

B12 is essential for DNA synthesis, red blood cell maturation, and normal neurological function. Deficiency causes: persistent fatigue, tingling in the hands and feet, difficulty concentrating and memory problems, and occasionally glossitis — an inflamed tongue.

Particularly vulnerable groups: vegans and vegetarians (B12 is found almost exclusively in animal products), people over 60 (absorption declines with age), and those taking metformin or proton pump inhibitors long-term. The test is simply vitamin B12 in serum.

A result in the 200–300 pg/mL range (148–221 pmol/L in SI units) is a grey zone — symptoms can occur even when the standard reference range labels the result normal. Methylmalonic acid (MMA) and homocysteine are confirmatory tests: both rise when intracellular B12 is functionally low, and either elevation supports a diagnosis of subclinical deficiency that may benefit from supplementation.

Oral supplementation (typically 1000 mcg/day of cyanocobalamin or methylcobalamin) is effective for most people, including most cases of dietary deficiency or borderline absorption. Intramuscular injections remain the standard for clinically confirmed pernicious anaemia, severe neurological symptoms, or documented severe malabsorption — but most patients do not need them.

Hormonal causes

Hypothyroidism

The thyroid gland regulates the metabolic rate of every cell in the body. In hypothyroidism, everything slows down: you feel cold, gain weight without explanation, your hair thins, your heart rate drops, your thinking feels sluggish — and you’re exhausted even after a full night’s rest.

The first test is TSH (thyroid-stimulating hormone). It’s the most sensitive screening marker: it responds to even subtle changes in thyroid function before T4 shifts. A full guide to TSH ranges, subclinical hypothyroidism, and when treatment is indicated is in the TSH Test: What It Shows and What Is Normal article.

Cortisol and sex hormones

If TSH, ferritin, vitamin D, and B12 are all within range but fatigue persists, a doctor may extend the workup to include morning cortisol, FSH, LH, oestradiol, or testosterone. These go beyond the standard screening minimum and require clinical context to interpret. More detail will follow in a dedicated hormonal profile article.

Metabolic causes

Glucose and HbA1c

Chronically elevated blood sugar — even in the prediabetes range — directly impairs energy levels. Cells receive less glucose due to insulin resistance, and the body is caught in a constant low-grade energy deficit.

Symptoms that often go unconnected to glucose: persistent fatigue after meals, difficulty concentrating, frequent urination, and slow-healing cuts. The tests are fasting glucose (or an oral glucose tolerance test) combined with HbA1c — glycated haemoglobin — which reflects the average blood sugar level over the past three months.

For reference ranges and a detailed explanation, see Blood Glucose Ranges: Normal, Prediabetes, and Diabetes.

Kidney function (creatinine, eGFR)

Chronic kidney disease in its early stages is typically silent, or causes non-specific fatigue, mild swelling, and reduced appetite. The test — serum creatinine with a calculated eGFR — is included in most standard biochemistry panels and requires no special preparation.

When blood tests are normal but fatigue remains

Normal results are still results. They rule out the most common medical causes and allow attention to shift toward functional factors.

Obstructive sleep apnoea. A person sleeps eight hours but breathing repeatedly slows or stops throughout the night. Sleep never truly restores. Classic signs: snoring, daytime sleepiness, waking with a headache. Diagnosis requires polysomnography — not blood tests.

Depression and anxiety. Mental health directly affects physical energy. Depression commonly presents as fatigue and low motivation rather than sadness in the textbook sense. This isn’t a character weakness — it is a condition with specific neurobiological mechanisms and effective treatments. There is no reason to delay seeking help if you suspect this is a factor.

Chronic stress. Prolonged psychological or physiological stress depletes the HPA (hypothalamic-pituitary-adrenal) axis. Sleep quality deteriorates, recovery from physical effort slows. Blood tests can be entirely normal throughout.

Sleep hygiene and dehydration. Small things that carry weight: irregular sleep timing, blue light exposure before bed, insufficient fluid intake. Even mild dehydration — one to two percent body mass — measurably impairs cognitive performance. Not every cause of fatigue is visible in a blood sample.

How HealthLab helps

A single test is a snapshot. A trend is understanding. Whether your ferritin climbed from 12 to 35 ng/mL after a course of iron supplements, or your TSH has been quietly creeping upward year by year — the story only becomes visible in a chart.

HealthLab automatically recognises biomarkers from PDF lab reports issued by any laboratory — ferritin, TSH, vitamin D, B12, glucose, haemoglobin — and builds a trend chart over time. You see at a glance whether a marker is moving toward or away from normal. No manual data entry, no spreadsheets.

Download HealthLab on the App Store

Frequently Asked Questions

Should I run all these tests at once?

In most cases, yes — it is both efficient and logistically convenient. Ferritin, haemoglobin, TSH, vitamin D, B12, glucose, and creatinine can all be measured from a single blood draw as part of a comprehensive biochemistry panel. Many laboratories offer ready-made “check-up packages” with exactly this composition. If cost is a constraint, prioritise ferritin, TSH, vitamin D, and B12 — these cover the most correctable causes.

How often should I retest during ongoing fatigue?

If a deficiency has been identified and treatment started, a follow-up test at 8–12 weeks is standard — that is how long ferritin and vitamin D take to respond to treatment. If results are normal and a doctor has ruled out medical causes, repeating the panel in 6–12 months (or sooner if symptoms worsen) is reasonable. Monthly monitoring without a clinical indication adds no value: most of these markers change slowly.

What if results are borderline?

“Borderline” does not mean “fine.” A ferritin of 16 ng/mL may be within the reference range, but combined with symptoms it is a reason to discuss with your doctor — not to file away and ignore. Context is everything: symptoms, diet, menstrual status (for women), chronic conditions. A clinician evaluates the clinical picture, not just the number. Borderline results are an invitation for a conversation, not a verdict and not a clearance.

How should I prepare for these tests?

Fasting glucose requires an 8–10 hour fast (water only). Ferritin, TSH, vitamin D, B12, haemoglobin, and creatinine follow standard morning collection — a light breakfast is acceptable if glucose is not on the panel. Avoid testing during an acute illness (fever, active infection): ferritin and some other markers can be falsely elevated by the inflammatory response. It is better to wait 1–2 weeks after recovery for a representative result.

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This material does not replace clinical advice. Interpreting blood test results always requires clinical context.