Symptom · Low mood
Depression is not caused by a blood result, and no blood test can diagnose or exclude it. What a test can do is find the handful of physical problems that produce symptoms which look identical from the inside: an underactive thyroid, low B12 or folate, low vitamin D, iron deficiency, falling oestradiol or testosterone, and a disrupted cortisol rhythm. Each of those causes flat mood, no energy, poor concentration and disturbed sleep in someone whose life is otherwise fine. Each is treatable. And NICE tells GPs to consider exactly these before settling on a diagnosis, which is why this is worth doing alongside, never instead of, talking to someone.
If you need help now
If you are struggling to cope, or having thoughts of harming yourself, please talk to someone today rather than waiting for a test result. Call your GP, or 111, or the Samaritans free on 116 123, any time, day or night. If you or someone else is in immediate danger, call 999. A blood test is not the right tool for a crisis, and nothing on this page should delay getting help.
Venous draw at 103 UK clinics or at home. Report within 48 hours of the lab receiving your sample. No GP referral.
Last reviewed 17 September 2026
The short answer
A blood test for low mood should cover a full thyroid panel with antibodies, vitamin B12 and folate, vitamin D, ferritin with iron studies, sex hormones (oestradiol and FSH in women, testosterone and SHBG in men), morning cortisol and DHEA-S, and HbA1c. A normal result does not mean nothing is wrong and an abnormal one does not explain everything, but finding a thyroid at 9, a B12 at 180 or a ferritin at 11 changes what happens next, and leaving them unmeasured means treating around a problem that would have responded to iron or levothyroxine.
What to test
| Marker | How it affects mood | What to look for | GP screen? | Panel |
|---|---|---|---|---|
| Thyroid (TSH, free T4, free T3, TPO antibodies) | An underactive thyroid produces flat mood, exhaustion, slowed thinking, weight gain and cold intolerance. It is the classic mimic, and it is treatable | TSH above range, or above 2.5 with symptoms; free T3 in the bottom fifth; positive antibodies | TSH usually | All panels |
| B12 and folate | Both are needed to make serotonin and dopamine. Low levels cause low mood, apathy and brain fog before any anaemia appears, and low folate is associated with a poorer response to antidepressants | B12 below 300 pmol/L is a grey zone worth treating with symptoms; folate low in range | Sometimes | All panels |
| Vitamin D | Receptors throughout the brain, and levels that fall through a UK winter alongside mood. The evidence for supplements treating depression is mixed, but correcting a genuine deficiency is worthwhile in its own right | Below 30 nmol/L deficient; below 50 insufficient | Rarely | All panels |
| Ferritin and iron studies | Iron is needed for dopamine synthesis and oxygen delivery. Deficiency causes exhaustion, poor concentration, tearfulness and low motivation, and is very common in women with heavy periods and after birth | Ferritin below 30 is deficiency; symptoms common below 50, whatever the blood count says | Rarely (blood count only) | All panels |
| Oestradiol and FSH (women), testosterone and SHBG (men) | Falling oestrogen in perimenopause causes low mood, irritability and anxiety, often years before periods stop. Low testosterone in men causes flat mood, low drive and fatigue | FSH rising with low oestradiol; testosterone in the bottom quarter on a morning sample | Rarely; NICE advises against FSH over 45 | Core hormones in Advanced; complete set in Ultimate and Signature |
| Morning cortisol and DHEA-S | The stress axis. A flattened rhythm goes with exhaustion and low motivation; a very high or very low result points to a hormonal condition that needs investigating in its own right | Morning cortisol at either extreme, drawn 8 to 9am; low DHEA-S | Rarely | All panels |
HbA1c is worth including too: unstable blood sugar produces mood swings and afternoon crashes that are easily mistaken for low mood. Thresholds are UK laboratory conventions and NICE guidance where it exists. Guides: thyroid, B12, vitamin D, ferritin, perimenopause, cortisol.
Being straight about this
It cannot
There is no blood marker for depression. Serotonin cannot be usefully measured in blood, and the idea that depression is simply a "chemical imbalance" you can test for is not how the condition is understood. Diagnosis is made by a clinician, from your history and how you are.
It cannot
Talking therapies and, where appropriate, medication are effective, and a blood test is not an alternative to either. If you are already being treated, keep going. The point of testing is to make sure nothing physical is working against the treatment.
It can
A TSH of 9, a B12 of 180, a ferritin of 11, a testosterone on the floor. Each of these produces low mood, each is treated in weeks, and none will improve with therapy alone. Finding one does not mean you were never depressed; it means part of it had a cause you can fix.
The two situations where this matters most are after having a baby, when iron deficiency and the underactive phase of postpartum thyroiditis peak at four to eight months, exactly when postnatal depression is usually diagnosed; and in women over 40, when falling oestradiol produces low mood that responds to HRT rather than to an antidepressant. In both, the right answer is usually both things at once: get the support, and check the blood. Postnatal blood test.Menopause blood test.
Which panel
Enough for most
Advanced
74 biomarkers · £269
Full thyroid with antibodies, B12, folate, vitamin D, ferritin with iron studies, morning cortisol and DHEA-S, core sex hormones, HbA1c, hs-CRP, liver and kidney. Every mimic in the table.
View AdvancedUltimate
114 biomarkers · £349
Adds oestradiol, FSH, LH, progesterone, prolactin, SHBG and free testosterone, plus fasting insulin. The panel when perimenopause or low testosterone is the likely story.
View UltimateSignature
230 biomarkers · £799
Processed by Randox. Adds homocysteine, a functional marker of B12 and folate status, plus autoimmune and digestive profiles including coeliac screening.
View SignatureClinic draw £19. At-home £39. Morning slot, fasted from 10pm, so cortisol and hormones are readable. Pay monthly available.
Honest comparison
NICE guidance on depression tells GPs to consider physical causes, and most will check a blood count, thyroid function and sometimes B12 before or alongside starting treatment. That is the right first step and it is free, so ask for it. What a standard screen usually leaves out is ferritin (the blood count is not a substitute), free T3 and thyroid antibodies rather than TSH alone, vitamin D, morning cortisol, and the sex hormones that explain so much low mood in women over 40 and men over 50. Those are the gaps a panel fills. But a blood test is not a substitute for the appointment: talking to a GP gets you the treatment that actually works for depression, and a panel is most useful as something you bring with you, so the conversation starts from a complete picture rather than a guess. What to ask your GP for.
After your result
Each marker read against the others, in plain English, with the treatable findings flagged first. Reviewed by a medical professional before it reaches you. It will not tell you whether you are depressed, and it does not try to.
A thyroid result, a very low B12, a hormone pattern or an extreme cortisol all need a prescriber, and several of them change what treatment makes sense. The report tells you which results to show them and why.
Iron, B12 and vitamin D take about twelve weeks to correct, and mood usually lifts as they do. The second report shows whether the numbers moved, which is useful evidence either way. Retest plans.
FAQs
No. There is no blood marker for depression, and serotonin cannot be usefully measured in blood. A blood test finds the physical conditions that produce similar symptoms, which is a different and still useful job. Diagnosis is made by a clinician.
A full thyroid panel with antibodies, B12 and folate, vitamin D, ferritin with iron studies, sex hormones, morning cortisol and DHEA-S, and HbA1c. Together they cover the physical causes worth ruling out.
Low vitamin D is associated with low mood, and it is very common in the UK in winter, but the evidence that supplements treat depression is mixed. Correcting a genuine deficiency is worth doing for bone, muscle and immune health regardless, and it is one of the simplest things on the list to fix.
Yes, and it is the commonest mimic. An underactive thyroid causes flat mood, exhaustion, slowed thinking and weight gain, and can be missed when only TSH is checked and the result sits inside a wide reference range. Free T3 and thyroid antibodies add the detail.
It can be, particularly if you have not felt the benefit you expected. Low B12, low folate, an untreated thyroid problem or iron deficiency all work against treatment, and low folate in particular is associated with a poorer response. Do not stop or change medication on the basis of a blood result; take it to your prescriber.
A morning appointment, fasted from 10pm, so cortisol, testosterone and fasting markers are readable. If you still have periods, days 2 to 5 give the clearest FSH and oestradiol. Full preparation guide.
Advanced and Ultimate reports are delivered within 48 hours of the laboratory receiving your sample. Signature takes around 12 working days.
Support, whatever your results say
Samaritans, free, 24 hours a day: 116 123. NHS 111, option 2, for urgent mental health support. Your GP can refer you for talking therapy, or in England you can self-refer to NHS Talking Therapies without seeing a GP first. Mind's information line is on 0300 102 1234. If you are in immediate danger, call 999.
Six physical causes of low mood in one morning venous draw, at 103 UK clinics or at home, with a report within 48 hours you can take to your GP.
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