Blood Test for Anxiety: Could a Deficiency Be Driving Your Symptoms?

Anxiety disorders are the most common mental health condition in the UK, affecting approximately 8.2 million people. While psychological therapies and medication are the standard treatment approaches, a growing body of evidence shows that specific nutritional deficiencies and hormonal imbalances can directly trigger or worsen anxiety symptoms. In some cases, correcting a blood-level abnormality can resolve anxiety entirely — without the need for long-term medication.

When Anxiety Has a Physical Cause

The overlap between anxiety symptoms and certain medical conditions is remarkable. Heart palpitations, trembling, difficulty breathing, sweating and a sense of impending dread can all be caused by conditions that are identifiable through blood testing. Before concluding that anxiety is purely psychological, it is worth ruling out the physical drivers that are frequently missed in standard GP assessments.

Key Biomarkers to Investigate

Thyroid Function (TSH, Free T4, Free T3)

Hyperthyroidism — an overactive thyroid — is one of the most common physical mimics of anxiety. The excess thyroid hormone accelerates the body's metabolic rate, causing rapid heartbeat, tremor, sweating, irritability and a persistent sense of nervousness that is virtually indistinguishable from a panic disorder. In the UK, hyperthyroidism affects roughly 2 in 100 women and 2 in 1,000 men. A suppressed TSH (below 0.27 mIU/L) with elevated free T4 (above 22 pmol/L) or free T3 (above 6.8 pmol/L) strongly suggests hyperthyroidism.

Hypothyroidism can also worsen anxiety, particularly the subclinical form where TSH is mildly elevated (4.0–10.0 mIU/L) but free T4 remains within range. The resulting fatigue, brain fog and low mood can create a background of chronic unease.

Vitamin B12

Vitamin B12 is essential for the production of neurotransmitters including serotonin and dopamine. Deficiency (below 200 ng/L) causes neurological symptoms that frequently present as anxiety, irritability, confusion and cognitive impairment. The UK National Diet and Nutrition Survey found that around 6% of adults under 65 and 11% of those over 65 are B12 deficient. Vegans, vegetarians and individuals taking proton pump inhibitors or metformin are at heightened risk.

Vitamin D

Vitamin D receptors are present throughout the brain, including regions involved in mood regulation. A meta-analysis in the British Journal of Psychiatry found that low vitamin D levels were associated with a significantly increased risk of depression and anxiety. In the UK, roughly 1 in 5 adults has vitamin D levels below 25 nmol/L (classified as deficient), with this proportion rising substantially during winter months. Optimal levels are generally considered to be above 75 nmol/L.

Iron and Ferritin

Iron deficiency — even without full-blown anaemia — can cause restlessness, irritability, difficulty concentrating and heart palpitations, all of which overlap with anxiety. Ferritin below 30 µg/L is considered indicative of depleted iron stores. Iron deficiency is particularly common in women of reproductive age, affecting roughly 25% in the UK.

Magnesium

Magnesium plays a critical role in regulating the hypothalamic-pituitary-adrenal (HPA) axis — the body's central stress response system. Low magnesium increases the release of stress hormones and heightens nervous system excitability. Serum magnesium below 0.7 mmol/L is classified as hypomagnesaemia, though many researchers argue that levels below 0.85 mmol/L are suboptimal. Approximately 10–20% of the UK population is estimated to have inadequate magnesium intake.

Cortisol

Cortisol is the body's primary stress hormone, produced by the adrenal glands. While a single cortisol measurement has limitations (levels fluctuate throughout the day), a morning cortisol above 600 nmol/L or below 100 nmol/L can indicate adrenal dysfunction. Chronically elevated cortisol — as seen in Cushing's syndrome or prolonged high-stress states — drives persistent anxiety, insomnia and weight gain.

The Feedback Loop

What makes nutritional deficiencies particularly insidious in the context of anxiety is the self-reinforcing cycle they create. Anxiety reduces appetite and disrupts digestion, which worsens nutrient absorption, which deepens the deficiency, which amplifies anxiety. Breaking this cycle often requires identifying and correcting the deficiency directly.

Who Should Consider Testing?

• Anyone with new-onset anxiety that does not respond to standard interventions
• Individuals with anxiety accompanied by fatigue, palpitations or tingling
• Vegans and vegetarians (higher risk of B12 and iron deficiency)
• People taking medications known to deplete nutrients (PPIs, metformin, oral contraceptives)
• Those with a family history of thyroid disease or autoimmune conditions

Recommended Blood Tests

The Core Health 45 blood test (£120) provides a comprehensive baseline that includes a full blood count, iron studies, ferritin, B12, folate, vitamin D, thyroid function (TSH, free T4, free T3), magnesium, liver and kidney function — covering the most common physical drivers of anxiety in a single panel. If thyroid and hormonal function are your primary concern, the Thyroid & Hormonal Function blood test (£99) offers a focused assessment with extended thyroid markers alongside cortisol and key hormones.

The Bottom Line

Anxiety is not always "all in your head." Measurable, correctable deficiencies and hormonal imbalances can produce symptoms that are identical to generalised anxiety disorder and panic attacks. A blood test cannot diagnose an anxiety disorder, but it can rule out — or confirm — physical causes that deserve treatment in their own right.