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Dark mornings, dark afternoons, and a mood that sinks with the light. Many people feel flatter through the winter months. For some, it is more than that: seasonal affective disorder, or SAD, is a recognised type of depression.
If you are searching for a SAD blood test, the honest answer is that there is not one. This guide explains what SAD is, according to the NHS, and why it is diagnosed by talking to a doctor rather than by a blood test. It then covers the physical conditions that can look very similar and are worth ruling out: an underactive thyroid, low vitamin D, iron deficiency and low vitamin B12.
To be clear from the start: a blood test cannot diagnose depression or SAD, and correcting a deficiency is not a treatment for depression. What a blood test can do is check for a physical contributor that may be making you feel worse, so that nothing treatable is missed.
What is SAD?
The NHS describes seasonal affective disorder as a type of depression that comes and goes in a seasonal pattern. The main symptom is depression that returns each winter and goes away in the summer. A smaller number of people have it the other way round, which the NHS says is less common.
Symptoms listed by the NHS include:
- Low mood
- A lack of interest in things you usually enjoy
- Feeling guilty, hopeless or worthless
- Feeling restless or irritable
- Difficulty concentrating
- Being more hungry and eating more than usual
- Being more tired and sleeping more than usual
- Thoughts of suicide or self-harm
The exact cause is not known. The NHS says it is thought to be linked to less daylight in winter, which may affect brain chemicals such as melatonin and serotonin that influence mood and sleep patterns (the body’s internal clock).
Winter blues or SAD?
Feeling a little lower, slower and keener on staying in during winter is common, and most people carry on with daily life. SAD is different: it is depression, it affects how you function, and it tends to come back at the same time each year. If you are not sure which you are dealing with, that is exactly what a conversation with your GP is for.
How SAD is diagnosed
There is no blood test for SAD, or for depression. The diagnosis is clinical. According to the NHS, a GP will usually ask about your mood and how it changes over time, and may ask you to fill in a questionnaire. SAD is usually diagnosed when depression has come at the same time of year for two years or longer. The NHS advises seeing a GP if you think you might have SAD.
Sometimes a doctor will also want to rule out a physical cause, especially if your symptoms include marked tiredness, weight change or feeling the cold. That is where blood tests come in, and it is the reason for the rest of this article.
The conditions that can look like SAD
Several common, measurable conditions share symptoms with SAD, including tiredness, sleeping more, poor concentration and low mood. Finding one of them does not mean you do not also have depression: both can be true at the same time, and each needs its own treatment.
An underactive thyroid
An underactive thyroid (hypothyroidism) slows the body down. The NHS lists extreme tiredness, feeling colder than usual, weight gain, constipation, trouble concentrating, low mood or depression, and dry skin among its symptoms, which is a striking overlap with SAD. It usually develops slowly and gets worse over time, so it is easy to blame on the season. Unlike SAD, it does not ease when the days get longer.
NICE guideline NG145 says TSH is the usual first test for adults, with free T4 added on the same sample if TSH is above the reference range. See TSH and free T4. An overactive thyroid can also affect how you feel, with restlessness, anxiety and muscle weakness among the NHS-listed features. Our guide to why you might always feel cold explains the thyroid connection in more detail, and our thyroid blood test page covers the markers.
Low vitamin D
In the UK, the NHS says we do not make enough vitamin D from sunlight between October and early March, so low levels are common in exactly the months SAD appears. The NHS advises everyone to consider a daily 10 microgram supplement during autumn and winter. On the 25-OH vitamin D test, UK clinical guidance treats a result below 25 nmol/L as deficient, 25 to 50 nmol/L as possibly inadequate for some people, and above 50 nmol/L as sufficient for most. Deficiency is best known for causing bone problems, including bone pain in adults.
Whether low vitamin D causes low mood is not established, and vitamin D is not a treatment for SAD or depression. It is still worth correcting a deficiency for its own sake. Our guides to checking vitamin D in winter and vitamin D deficiency symptoms cover it in full.
Iron deficiency
Low iron stores cause tiredness and lack of energy, shortness of breath and palpitations, which can easily feel like the lethargy of a dark winter. The NHS notes that heavy periods and pregnancy are very common causes. Ferritin is the key marker: British Society of Gastroenterology guidance says a level below 30 µg/L is generally indicative of low iron stores, and that ferritin can look normal in someone who is iron deficient when there is inflammation. See our guide to what low ferritin feels like.
Low vitamin B12 or folate
The NHS lists extreme tiredness, a lack of energy, pins and needles, a sore tongue, and mood or thinking changes, including problems with memory and judgement, among the symptoms of B12 or folate deficiency. NICE guideline NG239 treats a total B12 below 180 ng/L as confirming deficiency, with 180 to 350 ng/L indeterminate and needing interpretation in context. Deficiency is more likely on a vegan diet without supplements, in older adults, and in people taking proton pump inhibitors or metformin. Folate can be checked too.
Other things a doctor may consider
Mood changes in your 40s can be part of perimenopause, and in men, low mood and low energy can accompany low testosterone. Poor sleep, alcohol, some medicines and the after-effects of a viral illness can all play a part as well. Our guide to feeling tired all the time in winter goes through the wider list.
What a blood test can and cannot tell you
| A blood test can | A blood test cannot |
|---|---|
| Check your thyroid function, vitamin D, iron stores and B12 | Diagnose SAD, depression or anxiety |
| Find a treatable physical contributor to how you feel | Measure how low your mood is |
| Give you a baseline to compare against next winter | Replace a conversation with your GP about your mood |
If a deficiency is found and corrected, the symptoms it was causing may improve. If depression is also present, it still needs its own assessment and treatment. And a normal result does not mean your symptoms are not real: it means the cause is not one of these markers. If you are taking an antidepressant, do not stop or change it without speaking to your prescriber.
Which panel covers these checks
Our General Wellness panel (£279) covers all four areas in one draw: a full blood count, iron studies including ferritin, vitamin D, B12 and folate, thyroid function (TSH, FT3 and FT4), CRP and ESR, and zinc. It also includes liver and kidney function, a lipid profile (cholesterol) and HbA1c.
If you are also noticing more hair shedding, the Hair Loss Essentials panel (£249) covers the same four areas, including folate and the full thyroid set, with zinc and CRP.
Both panels need a fast of at least 12 hours beforehand (water only, no food or other drinks), and you should hold B12, folate, B-complex or multivitamin supplements before testing because recent supplementation can mask a real deficiency. Your booking confirmation gives the full instructions. Other markers can be added through a custom panel, with the price confirmed in writing before booking.
Every result is reviewed by a GMC-registered doctor, who reads your numbers against your symptoms. Results are released as each marker is reviewed, with the expected timeframe confirmed at booking. You will receive a written report you can take to your GP, and we would encourage you to do that, especially if your mood is part of the reason you are testing.
How SAD is treated
The NHS describes several treatment options for SAD:
- Self-help, such as getting as much natural daylight as you can, regular exercise, keeping to your usual routines and sleep times, staying in touch with other people, and avoiding too much alcohol
- Light therapy, using a special lamp. The NHS says it is not usually available on the NHS because there is not enough evidence about how well it works, and advises speaking to a GP before buying a lamp
- Talking therapies, such as cognitive behavioural therapy (CBT)
- Antidepressant medication
If you are 18 or over (16 or over in some areas), you can refer yourself directly to an NHS talking therapies service without seeing a GP first.
Where to get help with your mood
- Your GP. The NHS advises seeing a GP if you think you might have SAD, or if you have been diagnosed and treatment is not helping.
- NHS 111, online or by phone, if you need urgent help that is not an emergency.
- Samaritans, on 116 123. Free to call, from any phone, day or night.
- If you are in crisis, call 999 or go to A&E. Do this if you have seriously harmed yourself or you feel you may be about to.
Common questions
Can a blood test tell me if I have SAD? No. SAD is diagnosed by a doctor from your symptoms and their seasonal pattern. A blood test can only check for physical conditions that cause similar symptoms.
Will taking vitamin D fix my winter low mood? Vitamin D is not a treatment for SAD or depression. The NHS advises everyone to consider a vitamin D supplement in autumn and winter, and a deficiency is worth correcting in its own right. If your mood is low, talk to your GP as well.
My blood tests were normal, but I still feel low. What now? Your symptoms are still real. A normal panel rules out several treatable physical causes, which is useful information, and it points the next step towards your GP, a talking therapy or the other treatments the NHS describes.
Do I need a GP referral for a blood test? No. You can book directly at our Harley Street clinic. If low mood is one of your symptoms, we would still encourage you to speak to your GP alongside testing.
Related reading
- Tired After Christmas? When a Blood Test Helps: when to test after the festive period.
- How Often Should You Have a Health Check?: age-by-age guidance on health checks and blood tests.
Sources
- NHS: Seasonal affective disorder (SAD): overview, symptoms, diagnosis and treatment. nhs.uk
- NHS: Help for suicidal thoughts. nhs.uk
- NHS: Underactive thyroid (hypothyroidism); Iron deficiency anaemia; Vitamin B12 or folate deficiency anaemia; Vitamin D. nhs.uk
- NICE Guideline NG145: Thyroid disease: assessment and management. nice.org.uk
- NICE Guideline NG239: Vitamin B12 deficiency in over 16s: diagnosis and management. nice.org.uk
- British Society of Gastroenterology: Guidelines for the management of iron deficiency anaemia in adults (Snook et al, Gut 2021). bsg.org.uk
- Royal Osteoporosis Society: Vitamin D and bone health: a practical clinical guideline for patient management. theros.org.uk
- Samaritans: Contact a Samaritan. samaritans.org
This article is for general information only and does not constitute clinical advice. If you have concerns about your mood or your symptoms, speak to your GP. If you are in crisis, call 999 or go to A&E, or call Samaritans on 116 123.