Blood Test for Mood Swings: Which Biomarkers to Check

Mood swings are one of those symptoms people learn to live with — blaming stress, poor sleep or simply "one of those days." But when the emotional rollercoaster becomes a daily fixture, there is usually something measurable driving it. Hormones, vitamins and thyroid function all influence neurotransmitter production, and a straightforward blood test can reveal whether any of them are out of range.

Why mood swings deserve investigation

Around 3 in 10 women in the UK report mood disturbances severe enough to affect daily life during their menstrual cycle, and the figure rises sharply during perimenopause — typically between 40 and 55. Men are far from immune: declining testosterone after 30, thyroid dysfunction and nutrient deficiencies all produce irritability, low motivation and emotional volatility that is frequently dismissed or misdiagnosed as depression.

The challenge is that mood swings sit at the crossroads of several body systems. A GP might check one marker — often TSH alone — and declare everything "normal." A broader panel is what actually separates a vague complaint from a clear, actionable finding.

Hormonal fluctuations: the biggest driver

Women — oestradiol and progesterone

Oestradiol (the most active form of oestrogen) and progesterone work in opposition throughout the menstrual cycle. When the ratio between them shifts too far in either direction, serotonin and GABA signalling are disrupted. In the luteal phase (days 15–28), progesterone should rise to roughly 15–90 nmol/L. If it stays below 5 nmol/L, the classic premenstrual irritability, tearfulness and anxiety intensify.

During perimenopause, oestradiol levels become erratic — sometimes surging above 1,000 pmol/L before crashing to post-menopausal levels within the same cycle. This volatility, rather than a steady decline, is what makes perimenopausal mood swings so unpredictable. Measuring oestradiol, progesterone, FSH and LH across the cycle gives a clear hormonal picture.

Men — testosterone

Total testosterone in men typically sits between 8.64 and 29 nmol/L, but the fraction that matters most is free testosterone — the portion not bound to SHBG or albumin. When free testosterone drops below around 0.2 nmol/L, men frequently report irritability, flat mood, brain fog and a loss of drive that looks almost identical to clinical depression. SHBG (sex hormone-binding globulin) often rises with age, stress and liver strain, effectively locking more testosterone away from receptors.

Thyroid dysfunction: the hidden mood disruptor

The thyroid gland sets the metabolic pace for every cell in the body, including neurones. Hypothyroidism (underactive thyroid) causes low mood, sluggishness and cognitive fog; hyperthyroidism triggers anxiety, agitation and restlessness. Both conditions are remarkably common in the UK — approximately 2% of the population has a diagnosed thyroid disorder, and many more go undetected because screening often relies solely on TSH.

A TSH between 0.27 and 4.2 mIU/L is technically "normal," but a TSH above 2.5 with symptoms often warrants further investigation. Free T4 (12–22 pmol/L) and Free T3 (3.1–6.8 pmol/L) show whether the thyroid is actually producing and converting hormones effectively. Thyroid antibodies (anti-TPO and anti-TG) reveal autoimmune thyroiditis — a condition where the immune system attacks thyroid tissue, causing hormones to swing between high and low before the gland eventually burns out.

Vitamin B12 deficiency

Vitamin B12 is essential for myelin synthesis and neurotransmitter production. Deficiency is more common than most people realise: around 6% of adults under 60 and nearly 20% of those over 60 in the UK are deficient. Early symptoms include mood changes, irritability and difficulty concentrating — long before the classic neurological signs of tingling or numbness appear.

The key marker is Active B12 (holotranscobalamin), not total B12. Total B12 includes inactive forms bound to haptocorrin, which means someone can have a "normal" total B12 of 300 ng/L while their active, usable B12 is critically low. Active B12 below 25.1 pmol/L is deficient; between 25.1 and 37 pmol/L is insufficient and often symptomatic.

Vitamin D deficiency

Vitamin D receptors are found throughout the brain, particularly in areas governing mood regulation. In the UK, Public Health England estimates that around 1 in 5 adults has vitamin D levels below 25 nmol/L — the threshold for frank deficiency. Optimal levels for mood and immune function sit between 75 and 150 nmol/L, meaning the majority of the UK population is suboptimal for much of the year.

Low vitamin D has been consistently linked to seasonal affective disorder, general depression and increased anxiety. It also interacts with thyroid function — vitamin D deficiency is significantly more prevalent in people with Hashimoto's thyroiditis.

Iron deficiency and ferritin

Iron does not just carry oxygen. It is a cofactor in the synthesis of serotonin, dopamine and noradrenaline. When ferritin (the body's iron storage protein) drops below 30 µg/L, neurotransmitter production slows — even if haemoglobin remains within range and full-blown anaemia has not yet developed. This is why many women with "normal" blood counts still experience mood disturbances, fatigue and brain fog: their ferritin is low enough to impair brain chemistry.

Menstruating women are particularly vulnerable. Heavy periods can push ferritin below 15 µg/L, a level at which irritability and low mood become almost inevitable. A full iron panel — ferritin, serum iron, transferrin saturation and TIBC — gives the complete picture.

Which blood tests should you order?

A single-marker approach rarely explains mood swings. The most useful panels combine hormones, thyroid, vitamins and iron into one draw:

• Hormones: Oestradiol, Progesterone, FSH, LH, Testosterone, Free Testosterone, SHBG
• Thyroid: TSH, Free T4, Free T3, Anti-TPO, Anti-TG
• Vitamins and minerals: Active B12, Folate, Vitamin D, Ferritin
• Full blood count: to check for anaemia and macrocytosis (enlarged red cells from B12/folate deficiency)

Recommended Lola Health tests

Rather than ordering individual markers, a comprehensive panel saves money and ensures nothing is missed:

• Female Hormones 7 — £95 — Covers oestradiol, progesterone, FSH, LH, testosterone, SHBG and prolactin. The ideal starting point for women experiencing cyclical mood changes, PMS or early perimenopause symptoms.
• Male Hormones Clarity 14 — £110 — Includes total and free testosterone, SHBG, oestradiol, prolactin, DHEA-S and thyroid markers. Built for men who suspect hormonal changes are behind mood shifts, fatigue or reduced motivation.
• Core Health 45 — £120 — The broadest option: 45 biomarkers spanning hormones, thyroid (full panel with antibodies), vitamins (B12, D, folate), iron studies, liver, kidney and full blood count. If you want one test to investigate mood swings thoroughly, this is it.

When to test

Timing matters, particularly for women. Oestradiol and progesterone are best measured on day 21 of the cycle (or 7 days before the expected period) to capture the luteal peak. FSH and LH are most informative on day 2–5. If you are in perimenopause with irregular cycles, test on any day — the irregularity itself is diagnostically useful.

For men, testosterone follows a circadian rhythm and is highest in the morning. A blood draw before 10am gives the most accurate reading.

What to do with your results

Once you have your results, the path forward depends on what the numbers show. Low progesterone may warrant a conversation with your GP about cyclical progesterone support. Subclinical hypothyroidism (TSH 4–10 with symptoms) is increasingly being treated rather than simply monitored. B12 and vitamin D deficiencies respond well to supplementation — though the form and dose matter. Low ferritin may require iron infusion if oral supplements are not tolerated or not raising levels quickly enough.

The point is this: mood swings are not a character flaw or an inevitable part of ageing. They are frequently a symptom of something measurable and correctable. A blood test is the fastest way to find out what is actually going on — and to stop guessing.