Blood Tests and Mental Health: The Physical Causes of Low Mood, Anxiety, and Brain Fog That Get Missed

When someone visits their GP with low mood, anxiety, fatigue, or brain fog, the conversation often moves towards psychological explanations. Stress, sleep, work pressure, relationship difficulties. Those causes are real and common. But several common biochemical deficiencies produce symptoms that closely mimic depression and anxiety, and they don't always get checked.

I've seen this repeatedly in clinic. A patient referred for counselling who actually had a TSH of 12 and an underactive thyroid. A woman prescribed antidepressants whose ferritin turned out to be 8. A man told his fatigue was burnout when his vitamin D was 18 nmol/L. These aren't rare cases. They're common presentations where a blood test would have changed the direction of treatment entirely.

This article isn't about dismissing mental health conditions as "just" a vitamin deficiency. Depression and anxiety are real illnesses that deserve proper treatment. It's about making sure the physical causes have been ruled out before assuming the problem is entirely psychological.

Thyroid dysfunction: the great mimicker

An underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) both produce symptoms that closely mirror psychiatric diagnoses. NICE recommends checking thyroid function as part of the assessment for depression [NICE NG222, 2022].

Hypothyroidism symptoms include:

• Fatigue and sluggish thinking

• Weight gain

• Low mood and poor concentration

• Cold intolerance and constipation

Hyperthyroidism symptoms include:

• Anxiety and restlessness

• Palpitations and tremor

• Weight loss and insomnia

Thyroid disorders affect an estimated 5% of the UK adult population [British Thyroid Foundation], and they're more common in women, particularly after pregnancy and around the perimenopause. A TSH test is all that's needed for initial screening, and treatment for hypothyroidism (levothyroxine) is straightforward and effective.

Iron deficiency and ferritin: fatigue before anaemia

Iron isn't just needed for making haemoglobin. It plays a direct role in the production of dopamine and serotonin, two neurotransmitters involved in mood regulation. Low iron, even without anaemia, has been associated with increased rates of depression, anxiety, and cognitive impairment [Al-Naseem et al., Clinical Medicine, 2021]. A ferritin below 30 µg/L is strongly associated with fatigue, difficulty concentrating, irritability, and low motivation that looks very similar to depression.

This is particularly common in premenopausal women losing iron through monthly periods, vegetarians and vegans with lower dietary iron intake, and anyone with gut conditions that impair absorption. A full iron profile, not just haemoglobin, is essential when investigating mood or energy problems.

Vitamin B12: neurological symptoms that mimic depression

B12 is essential for myelin production (the insulation around your nerves) and for neurotransmitter synthesis. Deficiency can cause low mood, cognitive slowing, memory problems, irritability, and a characteristic brain fog that patients often describe as feeling like they're thinking through treacle. Neuropsychiatric symptoms can appear before any blood count changes, which means a normal full blood count doesn't rule out B12 deficiency [Green R et al., Nature Reviews Disease Primers, 2017].

Risk groups include vegans and strict vegetarians (B12 comes almost exclusively from animal products), people over 60 (gastric acid production declines with age, reducing B12 absorption), people taking metformin or long-term stomach acid medications (such as omeprazole and lansoprazole), and anyone with pernicious anaemia or malabsorption conditions.

Vitamin D: the winter mood connection

Vitamin D receptors are found throughout the brain, particularly in regions involved in mood regulation. Observational studies consistently link low vitamin D with increased rates of depression, and a 2023 meta-analysis of randomised controlled trials found that vitamin D supplementation reduced depressive symptoms compared with placebo, with effects strongest in people who were already deficient [Mikola et al., Critical Reviews in Food Science and Nutrition, 2023].

Vitamin D deficiency alone probably doesn't cause clinical depression. But in combination with other factors (short days, reduced exercise, social isolation), it can contribute to the low mood and fatigue that many people experience between October and March. Knowing your level lets you supplement appropriately rather than guessing.

Magnesium, folate, and HbA1c

Three other markers deserve mention. Magnesium helps regulate neurotransmitters including GABA (a brain chemical with a calming effect on the nervous system). Deficiency is associated with anxiety, insomnia, and irritability. Folate is involved in serotonin production, and low folate has been linked to poorer response to antidepressant medication [Papakostas et al., American Journal of Psychiatry, 2012].

And HbA1c matters because poorly controlled blood sugar causes energy crashes, irritability, and brain fog. Insulin resistance and pre-diabetes are common in the UK adult population and frequently go undiagnosed. An HbA1c between 42 and 47 mmol/mol indicates pre-diabetes, and many people in this range have no idea [NICE NG28, 2022].

When to See Your GP

If you're experiencing persistent low mood, anxiety, fatigue, or cognitive difficulties, please speak to your GP. The message here isn't to skip the GP and self-diagnose with a blood test. It's to make sure the physical causes have been covered alongside psychological assessment. Blood tests for thyroid function, iron studies, B12, and vitamin D are all reasonable investigations when someone presents with mood or energy problems, and NICE guidance supports checking thyroid function as part of a depression assessment [NICE NG222, 2022].

How Brooksby Medical Can Help

The Wellness Profile covers the key markers relevant to mental health symptoms: full blood count, iron with ferritin, active B12, folate, vitamin D, thyroid function, CRP, and HbA1c. For a focused nutrient check, the Nutrients and Energy Profile tests active B12, folate, iron profile, and vitamin D.

Every result comes with a GP-written report that interprets your numbers in the context of your symptoms, not just whether they fall inside a reference range.

References

1. NICE. Depression in adults: treatment and management. Guideline NG222 (2022). NICE NG222

2. Green R, Allen LH, Bjorke-Monsen AL, et al. Vitamin B12 deficiency. Nature Reviews Disease Primers. 2017;3:17040

3. Al-Naseem A, Sallam A, Choudhury S, Visser J. Iron deficiency without anaemia: a diagnosis that matters. Clinical Medicine. 2021;21(2):107-113

4. Mikola T, Marx W, Lane MM, et al. The effect of vitamin D supplementation on depressive symptoms in adults: a systematic review and meta-analysis of RCTs. Critical Reviews in Food Science and Nutrition. 2023;63(33):11784-11801

5. Papakostas GI, Shelton RC, Zajecka JM, et al. L-methylfolate as adjunctive therapy for SSRI-resistant major depression. American Journal of Psychiatry. 2012;169(12):1267-1274

6. NICE. Type 2 diabetes in adults: management. Guideline NG28 (2015, updated 2022). NICE NG28

Written by Dr James Coleman, GP and founder of Brooksby Medical. Dr Coleman is a practising General Practitioner who founded Brooksby Medical to give patients direct access to the blood tests and clinical interpretation they need, without waiting lists.

Medically reviewed: March 2026 | Next review due: March 2027

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, medical history, and clinical picture. If you have concerns about your health, please consult your GP.