Blood Tests for Anxiety and Depression
Feeling anxious or depressed? Before assuming it is purely psychological, a blood test can reveal physical causes that mimic - or worsen - mental health conditions.
Why Check Blood Tests for Mental Health?
Here is a fact that surprises most people: up to 40% of patients referred to psychiatrists for depression have an underlying medical condition contributing to their symptoms. Thyroid disease alone accounts for a significant portion of these cases, and it is entirely treatable once identified.
The challenge is that many physical conditions produce symptoms that are identical to psychiatric disorders. An underactive thyroid does not feel like a thyroid problem - it feels exactly like depression. Reactive hypoglycaemia does not feel like a blood sugar issue - it feels exactly like a panic attack. Iron deficiency does not announce itself - it quietly drains your motivation, concentration, and joy.
This does not mean depression and anxiety are “not real” or “just physical.” It means that ruling out - or identifying - physical contributors is an essential first step. Treating an iron deficiency will not cure grief-related depression, but it will remove the physical burden that makes everything harder. And in some cases, correcting a deficiency resolves symptoms completely.
The 10 Essential Blood Tests for Anxiety and Depression
1. Thyroid Panel (TSH, FT4, FT3)
How this affects mood: The thyroid is the single most important blood test when mental health symptoms appear without an obvious cause. Hypothyroidism (underactive thyroid) mimics depression so closely that it is often misdiagnosed: fatigue, low mood, weight gain, brain fog, poor concentration, and loss of motivation. Hyperthyroidism (overactive thyroid) mimics anxiety almost perfectly: racing heart, tremor, insomnia, irritability, restlessness, and a feeling of impending doom.
Clinical detail: Even "subclinical" hypothyroidism - where TSH is mildly elevated (4-10 mIU/L) but Free T4 is still in range - can cause significant mood symptoms. A large meta-analysis found that subclinical hypothyroidism increases the risk of depression by 2.3 times. Thyroid antibodies (TPO antibodies) may also be worth checking, as Hashimoto thyroiditis can cause mood swings even before thyroid hormone levels become abnormal.
2. Iron Studies (Ferritin, Serum Iron)
How this affects mood: Iron is essential for producing dopamine, serotonin, and norepinephrine - the three neurotransmitters most directly involved in mood regulation. Low iron disrupts the synthesis of these chemicals at a fundamental level. The result is fatigue, poor concentration, irritability, and a flat, joyless mood that is clinically indistinguishable from clinical depression.
Clinical detail: Ferritin below 30 µg/L is associated with depression-like symptoms, even though labs often flag "normal" above 15-20 µg/L. Iron deficiency is particularly common in menstruating women, vegetarians, endurance athletes, and people with inflammatory bowel conditions. Correcting iron deficiency often produces a dramatic improvement in mood within weeks.
3. Vitamin B12
How this affects mood: Vitamin B12 deficiency causes direct neurological and psychiatric symptoms that can be severe. B12 is required for the production of myelin (the insulation around nerve fibres) and for the synthesis of serotonin and dopamine. Deficiency causes mood changes, irritability, confusion, memory problems, and in severe cases, paranoia and hallucinations. It is one of the few vitamin deficiencies that can cause genuine psychosis.
Clinical detail: B12 deficiency is common in vegans and vegetarians (B12 is found almost exclusively in animal products), people over 60 (reduced stomach acid impairs absorption), and those taking metformin, proton pump inhibitors, or H2 blockers long-term. Levels between 150-300 pmol/L are a grey zone - technically "normal" but potentially causing symptoms. Active B12 (holotranscobalamin) is a more sensitive test.
4. Folate
How this affects mood: Folate works closely with B12 in the methylation cycle, which is essential for producing neurotransmitters. Low folate is independently linked to depression, and there is growing evidence that folate status affects how well antidepressants work. The MTHFR gene variants (carried by up to 40% of the population) impair the conversion of folate to its active form (methylfolate), meaning some people need much higher intake to maintain adequate brain levels.
Clinical detail: Research shows that people with low folate are less likely to respond to SSRIs (the most commonly prescribed antidepressants). This has led to the development of L-methylfolate as an adjunct treatment for depression. Your doctor can check both serum folate (recent intake) and red cell folate (long-term status). Red cell folate below 340 nmol/L is associated with increased depression risk.
5. Vitamin D (25-OH)
How this affects mood: Vitamin D receptors are found throughout the brain, concentrated in areas involved in mood regulation (hippocampus, prefrontal cortex, and amygdala). Vitamin D influences serotonin synthesis and has anti-inflammatory effects in the brain. Multiple large studies have found a strong association between low vitamin D and depression, with the relationship showing a clear dose-response pattern: the lower the vitamin D, the worse the depression scores.
Clinical detail: Optimal levels for mental health are above 75 nmol/L (30 ng/mL). The seasonal pattern of depression (Seasonal Affective Disorder) maps closely to seasonal vitamin D fluctuations. In Australia, despite abundant sunshine, over 30% of adults are deficient - office workers, shift workers, and people with darker skin tones are at highest risk. Supplementation has shown modest but consistent benefits in depression trials.
6. Magnesium
How this affects mood: Magnesium is sometimes called "nature's relaxant" because of its role in regulating the nervous system. It modulates GABA receptors (the brain's primary calming neurotransmitter), regulates the hypothalamic-pituitary-adrenal (HPA) stress axis, and helps control cortisol production. Low magnesium leads to increased nervous system excitability, manifesting as anxiety, muscle tension, insomnia, irritability, and restlessness.
Clinical detail: Magnesium deficiency is difficult to detect accurately because only 1% of the body's magnesium is in the blood - the rest is in bones and tissues. Serum magnesium can be "normal" even when tissue stores are depleted. Red cell magnesium is a better marker but is not routinely available. If serum magnesium is low-normal (below 0.85 mmol/L), tissue depletion is very likely. Common causes: processed food diets, chronic stress (stress depletes magnesium), alcohol, and medications like proton pump inhibitors.
7. Cortisol
How this affects mood: Cortisol is your primary stress hormone. The HPA axis (hypothalamic-pituitary-adrenal) regulates its production, and dysfunction in this system is closely linked to both anxiety and depression. Chronically elevated cortisol (Cushing's syndrome or chronic stress) causes anxiety, mood swings, insomnia, and difficulty concentrating. Chronically low cortisol (adrenal insufficiency or HPA axis burnout from prolonged stress) causes a flat, exhausted depression with no ability to cope with even minor stressors.
Clinical detail: Morning cortisol (drawn at 9am) is the standard screening test. Levels below 200 nmol/L or above 600 nmol/L warrant further investigation with a more detailed cortisol assessment (24-hour urinary cortisol or dexamethasone suppression test). DHEA-S (tested alongside cortisol) provides context: a high cortisol-to-DHEA-S ratio suggests chronic stress, while low DHEA-S alone is associated with depression.
8. HbA1c / Fasting Glucose
How this affects mood: Blood sugar dysregulation is one of the most underdiagnosed causes of anxiety symptoms. When blood sugar drops rapidly (reactive hypoglycaemia), your body releases adrenaline and cortisol to compensate - producing shakiness, racing heart, sweating, irritability, and a sense of impending doom. These symptoms are identical to a panic attack. Diabetes also doubles the risk of clinical depression, partly through inflammation and partly through the psychological burden of chronic disease.
Clinical detail: HbA1c between 5.7-6.4% (39-47 mmol/mol) indicates pre-diabetes and is associated with mood instability. Adding fasting insulin to the panel can detect insulin resistance (the precursor to diabetes) years before blood sugar rises. If you experience anxiety that is worse 2-3 hours after meals or improves immediately after eating, blood sugar instability should be investigated.
9. CRP (C-Reactive Protein)
How this affects mood: This is one of the most exciting areas in modern psychiatry. Emerging research has established a strong link between chronic low-grade inflammation and treatment-resistant depression. Inflammatory cytokines can cross the blood-brain barrier and directly interfere with serotonin and dopamine production. High CRP levels are found in approximately 30% of people with depression, and this subgroup responds poorly to standard SSRIs but may respond better to anti-inflammatory approaches.
Clinical detail: High-sensitivity CRP (hs-CRP) above 3 mg/L is considered elevated and suggests systemic inflammation that may be contributing to mood symptoms. Some researchers propose that "inflammatory depression" is a distinct subtype characterised by fatigue, sleep disturbance, appetite changes, and poor concentration - essentially the "sickness behaviour" response. If your CRP is elevated alongside depression, addressing inflammation (through diet, exercise, omega-3 fatty acids, or treating underlying inflammatory conditions) may be more effective than antidepressants alone.
10. Full Blood Count (FBC)
How this affects mood: A full blood count is the foundation of any medical investigation. Anaemia (low haemoglobin) causes fatigue, poor concentration, irritability, and low mood that is clinically indistinguishable from depression. A high MCV (mean cell volume) suggests B12 or folate deficiency even before those vitamin levels drop below the reference range. Elevated white blood cells may indicate chronic infection or inflammation that is driving psychiatric symptoms.
Clinical detail: Haemoglobin below 120 g/L in women or 130 g/L in men indicates anaemia. An MCV above 95 fL is a red flag for B12 or folate deficiency. Low-normal haemoglobin (120-130 g/L in women) combined with symptoms may still warrant investigation with iron studies. The FBC is bulk billed, costs nothing, and catches a surprising number of physical causes of mood symptoms.
Deficiency vs Disorder: How to Tell the Difference
When symptoms overlap, these patterns can help distinguish whether a nutritional deficiency is the primary driver or whether a psychiatric condition is more likely. In many cases, both are present simultaneously.
| Clue | Nutritional Deficiency | Psychiatric Disorder |
|---|---|---|
| Onset | Often gradual, linked to dietary or lifestyle changes | May be triggered by life events, or appear without clear cause |
| Physical symptoms | Prominent (fatigue, numbness, muscle pain, hair loss) | Less prominent physical complaints |
| Response to supplements | Improves noticeably within 2-6 weeks | No change with supplementation |
| Family history | Less relevant (dietary/absorption factors) | Often present (genetic component) |
| Pattern | Constant, does not fluctuate with life events | Often reactive to stressors or seasonal |
| Sleep | Often excessive sleep but unrefreshing | Insomnia common, or early morning waking |
| Appetite | May crave specific foods (ice, dirt = iron) | Decreased or increased overall |
The Thyroid-Mood Connection: A Deep Dive
The connection between thyroid function and mental health deserves special attention because it is so common and so frequently missed. Thyroid hormones directly influence the production and sensitivity of serotonin, dopamine, and GABA - the three neurotransmitters most central to mood regulation.
T3 (triiodothyronine) is the active thyroid hormone and has a direct effect on serotonin receptors in the brain. When T3 is low, serotonin signalling is impaired - which is exactly what SSRIs are designed to fix. This is why some psychiatrists prescribe T3 as an adjunct to antidepressants in treatment-resistant depression, and why it is essential to check Free T3, not just TSH.
Hypothyroidism mimics depression
Low mood, fatigue, weight gain, brain fog, poor concentration, loss of motivation, constipation, dry skin, feeling cold
Hyperthyroidism mimics anxiety
Racing heart, tremor, insomnia, irritability, restlessness, sense of impending doom, weight loss, heat intolerance, diarrhoea
Hashimoto thyroiditis causes both
Mood swings alternating between anxiety and depression, as thyroid hormone levels fluctuate. Can occur with "normal" TSH if antibodies are elevated
The takeaway: If you are experiencing anxiety or depression and have not had a full thyroid panel (TSH + Free T4 + Free T3), this should be your first blood test. It is bulk billed, results are available within days, and treatment - when needed - is straightforward and effective.
What to Do Next: A Practical Plan
Here is a step-by-step approach to investigating physical contributors to anxiety and depression.
Step 1: Get tested
Ask your GP for a comprehensive panel: thyroid (TSH, FT4, FT3), iron studies, B12, folate, vitamin D, HbA1c, CRP, and FBC. Most are bulk billed when investigating mood symptoms. Mention your specific symptoms to help your GP code the request appropriately.
Step 2: Treat deficiencies
If any deficiencies are found, work with your GP to correct them. Iron, B12, vitamin D, and folate deficiencies are all straightforward to treat. Allow 4-8 weeks for supplements to take effect, then retest to confirm levels have normalised.
Step 3: Reassess at 3 months
After 3 months of treatment, honestly assess your symptoms. Have they improved? Partially? Not at all? If deficiencies have been corrected and symptoms persist, this is valuable information - it suggests the mood condition may need direct treatment.
Step 4: Pursue mental health care if needed
If symptoms persist after deficiencies are corrected, seek psychological support. A Mental Health Care Plan from your GP gives you 10 Medicare-subsidised psychology sessions per year. Blood tests and mental health care are complementary - not competing - approaches.
When to Seek Immediate Help
Blood tests are important, but they take time. If you or someone you know is experiencing any of the following, please seek help immediately - do not wait for blood test results.
Thoughts of self-harm or suicide
Feeling like a burden to others
Giving away possessions or saying goodbye
Severe panic attacks that feel like a heart attack
Inability to care for yourself (not eating, not sleeping for days)
Hearing voices or experiencing paranoia
Australian Crisis Services (Free, 24/7):
Lifeline: 13 11 14(Phone and online chat)
Beyond Blue: 1300 22 4636(Anxiety and depression)
Suicide Call Back Service: 1300 659 467(Phone, video, online)
Emergency: 000(Immediate danger)
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SmarterBlood provides educational health information. It is not a substitute for professional medical or psychological advice, diagnosis, or treatment. If you are experiencing anxiety or depression, please talk to your doctor. In a crisis, contact Lifeline (13 11 14) or Beyond Blue (1300 22 4636).