Blood Test for Anxiety & Depression UK: 10 Biomarkers That Rule Out Physical Causes

1. Why a Blood Test Matters for Anxiety and Depression

Anxiety and depression are clinical diagnoses based on symptoms. The problem is that dozens of medical conditions produce the same symptoms — fatigue, brain fog, irritability, insomnia, weight change, low motivation, racing heart, muscle tension. Without blood work, there is no way to distinguish between a primary mood disorder and a physical condition masquerading as one.

A 2018 BMJ review found that thyroid disorders alone account for a significant proportion of patients initially diagnosed with depression or anxiety. Correcting the thyroid resolves the mood symptoms entirely in many of these cases — no antidepressant needed.

This is not about dismissing mental health. It is about checking the treatable physical causes first, so that psychological treatment is directed at the right problem.

2. What NICE Recommends

The NICE guideline CG90 (Depression in Adults) recommends that clinicians “consider undertaking a physical examination and baseline blood tests” when assessing depression. The purpose is to exclude medical conditions that can cause or contribute to depressive symptoms.

NICE does not prescribe a specific panel, but the clinical consensus includes:

• Full blood count — to check for anaemia
• Thyroid function (TSH, free T4) — to exclude hypothyroidism or hyperthyroidism
• HbA1c or fasting glucose — to exclude diabetes or blood sugar instability
• Vitamin D, B12, folate — nutritional deficiencies linked to mood disorders
• Liver and kidney function — metabolic causes of fatigue and cognitive symptoms

In practice, many GPs will order TSH and FBC but skip the nutritional markers. This guide covers the full picture.

3. Thyroid Dysfunction: The Most Common Physical Mimic

The thyroid gland controls your metabolic rate, energy production, body temperature and — critically — neurotransmitter synthesis. When it malfunctions, the psychological effects are often the first symptoms patients notice.

Read more: Thyroid Blood Test UK: What It Measures, Results Explained

4. Iron Deficiency: Fatigue, Brain Fog and Anxiety

Iron deficiency is the most common nutritional deficiency worldwide and the most common cause of anaemia in the UK. But what many people do not realise is that iron deficiency produces psychological symptoms before anaemia develops.

Low ferritin (your iron storage protein) is associated with fatigue, poor concentration, irritability and restless legs — a pattern commonly attributed to anxiety. A 2013 study in the European Journal of Clinical Nutrition found that iron supplementation in non-anaemic women with low ferritin significantly improved fatigue and cognitive function.

The critical marker is ferritin. The NHS considers anything above 15 µg/L as “normal,” but many clinicians now argue that optimal ferritin for symptom resolution is above 50–70 µg/L — particularly for women of reproductive age, who are most commonly affected.

Read more: Iron Deficiency Blood Test UK: Ferritin, Iron Studies & What Your Results Mean

5. Vitamin D Deficiency: The UK's Hidden Epidemic

The UK sits between latitudes 50°N and 60°N. From October to March, UVB radiation is insufficient for skin synthesis of vitamin D. Public Health England estimates that 1 in 6 UK adults have serum 25(OH)D below 25 nmol/L (deficient), and many more sit below the 50 nmol/L threshold considered sufficient.

Vitamin D receptors are found throughout the brain, including regions involved in mood regulation (the prefrontal cortex, hippocampus and amygdala). A 2014 meta-analysis in the British Journal of Psychiatry found a significant association between low vitamin D and depression, with the strongest effects in studies that measured 25(OH)D below 25 nmol/L.

Seasonal affective disorder (SAD), which affects up to 3% of the UK population, correlates with the October–March vitamin D trough. While light therapy remains the primary SAD intervention, checking and correcting vitamin D levels is a logical first step.

Read more: Vitamin D Deficiency in the UK: Symptoms, Testing & How to Fix It

6. Vitamin B12 and Folate: Neurological Essentials

B12 and folate are required for methylation — a biochemical process essential for producing neurotransmitters including serotonin, dopamine and noradrenaline. When either is deficient, neurotransmitter synthesis is impaired.

B12 deficiency produces a characteristic pattern: fatigue, brain fog, poor memory, numbness or tingling in hands and feet, irritability and low mood. These neurological symptoms can appear months or years before anaemia develops — which means a normal FBC does not rule out B12 deficiency.

Groups at higher risk include vegetarians and vegans (B12 is found almost exclusively in animal products), adults over 50 (reduced absorption), people taking metformin or proton pump inhibitors (which impair B12 absorption), and those with coeliac disease or Crohn's disease.

The NHS “normal” range for serum B12 starts at around 180–200 ng/L. However, neurological symptoms can occur at levels up to 300–400 ng/L. Active B12 (holotranscobalamin) is a more sensitive marker of functional deficiency.

Read more: Vitamin B12 Blood Test UK: What It Measures & Results Explained

7. Cortisol and the Stress Response

Cortisol is your primary stress hormone, produced by the adrenal glands in a diurnal rhythm — highest in the morning (the cortisol awakening response), declining through the day, lowest at midnight.

Chronic stress can dysregulate this rhythm. Persistently elevated cortisol is associated with anxiety, insomnia, weight gain (particularly abdominal), high blood pressure and impaired immune function. Conversely, a blunted cortisol response (low morning cortisol) — sometimes seen after prolonged stress — is associated with fatigue, low motivation and depressive symptoms.

A morning cortisol blood test (taken between 8–9 AM) provides a snapshot. The cortisol:DHEA-S ratio adds context: DHEA-S is a counterbalancing adrenal hormone, and a high cortisol-to-DHEA-S ratio suggests your stress response is outpacing your body's protective mechanisms.

Extreme cortisol abnormalities can indicate Cushing's syndrome (excess) or Addison's disease (insufficiency) — both of which have prominent psychiatric presentations and require urgent medical attention.

Read more: Cortisol Blood Test UK: Stress Hormone Guide

8. Blood Sugar Instability and HbA1c

Reactive hypoglycaemia — blood sugar crashing 2–4 hours after a carbohydrate-heavy meal — triggers an adrenaline surge that produces symptoms indistinguishable from a panic attack: racing heart, sweating, tremor, dizziness and intense anxiety.

At the other end, insulin resistance and pre-diabetes create a chronic low-grade inflammatory state that is independently associated with depression. A 2013 meta-analysis in Diabetes Care found that people with type 2 diabetes have a 2–3 times higher risk of depression compared to the general population.

HbA1c measures your average blood sugar over the past 2–3 months. A level of 42–47 mmol/mol indicates pre-diabetes; 48 mmol/mol or above indicates diabetes. But even levels at the high end of “normal” (39–41 mmol/mol) may indicate early insulin resistance worth investigating.

Read more: HbA1c Blood Test UK: Results Explained & Prediabetes Guide

9. Inflammation and the Brain: hs-CRP

The “inflammatory hypothesis of depression” has gained substantial evidence over the past decade. A 2020 meta-analysis in Molecular Psychiatry confirmed that people with depression have significantly elevated inflammatory markers, including C-reactive protein (CRP), interleukin-6 (IL-6) and tumour necrosis factor alpha (TNF-α).

High-sensitivity CRP (hs-CRP) is the most accessible marker. Levels above 3 mg/L suggest chronic low-grade inflammation; levels above 1 mg/L but below 3 mg/L are considered intermediate risk. While hs-CRP is primarily used for cardiovascular risk assessment, it also serves as a proxy for the systemic inflammation that may be contributing to depressive symptoms.

Sources of chronic inflammation include poor sleep, visceral adiposity, ultra-processed diets, chronic infections, autoimmune conditions and sedentary behaviour. Addressing these can lower hs-CRP and, in some patients, improve mood.

10. Magnesium: The Overlooked Mineral

Magnesium is a cofactor in over 600 enzymatic reactions, including those involved in GABA receptor function — your brain's primary inhibitory (calming) neurotransmitter. Low magnesium is associated with anxiety, insomnia, muscle tension and irritability.

A 2017 systematic review in Nutrients found that magnesium supplementation had a positive effect on subjective anxiety in vulnerable populations, though the authors noted that study quality was mixed.

The challenge with testing is that serum magnesium reflects only 1% of your body's total magnesium — the rest is inside cells and bones. RBC (red blood cell) magnesium is a better measure of intracellular status but is not widely available on the NHS.

Read more: Magnesium Blood Test UK: Serum, RBC & Ionised Magnesium Explained

11. Full Blood Count: The Baseline Check

A full blood count (FBC) is the most commonly ordered blood test in the UK. For mental health screening, it flags:

• Anaemia (low haemoglobin) — causes fatigue, breathlessness and poor concentration that mimic depression
• Macrocytosis (high MCV) — suggests B12 or folate deficiency, even before anaemia develops
• Elevated white blood cells — may indicate infection or chronic inflammation contributing to symptoms

An FBC alone will not diagnose the cause of anxiety or depression, but it is an essential first filter.

12. Sex Hormones and Mood

Both testosterone and oestrogen influence mood, motivation and cognitive function. When either drops significantly, psychological symptoms often dominate the clinical picture.

13. Getting Tested: NHS vs Private

If you visit your GP with anxiety or depression symptoms, you will likely get a TSH and FBC. You may also get HbA1c if there are risk factors for diabetes. However, you are unlikely to get vitamin D, B12, ferritin, cortisol, hs-CRP or sex hormones without specifically requesting them — and even then, your GP may decline based on local commissioning guidance.

A comprehensive private panel covers all of these in a single finger-prick home kit. Results typically arrive within 5 working days, with interpretation by a GMC-registered doctor.

14. How to Interpret Your Results

Blood test results are reported against reference ranges — but “normal” does not always mean “optimal.” A TSH of 4.5 mIU/L is technically within range for most labs, but many endocrinologists consider it suboptimal, particularly if you have symptoms.

The key patterns to look for:

• Multiple borderline results — each marker may be “in range,” but having thyroid at the high end, ferritin at the low end and vitamin D borderline together creates a compounding effect
• Single clear abnormality — one significantly out-of-range result (e.g., TSH of 8.0 or ferritin of 8) is a strong lead worth pursuing with your GP
• All clear — if every marker is optimal, that is valuable information too. It increases the probability that your symptoms are primarily psychological, and you can pursue therapy or medication with greater confidence

Read more: How to Read Your Blood Test Results: A UK Guide

15. When to See Your GP

See your GP urgently if you experience:

• Suicidal thoughts or self-harm urges — contact your GP same-day, call NHS 111, or the Samaritans (116 123, free, 24/7)
• Severe panic attacks that feel like a heart attack (chest pain, difficulty breathing)
• Inability to function at work, maintain relationships, or perform daily tasks

Book a routine GP appointment if:

• Blood results show a clear abnormality (thyroid, ferritin, B12, vitamin D) — bring your results to the appointment
• Symptoms have persisted for more than 2 weeks and are affecting your daily life
• Blood results are all optimal but symptoms continue — this points toward a primary mood disorder that may benefit from therapy (CBT/IAPT) or medication

Blood testing and mental health treatment are not either/or. The strongest approach is both: rule out physical causes and address psychological factors.

16. Frequently Asked Questions