Blood Test for Hair Loss: Which Markers to Check

Hair loss affects roughly 8 million women and 6.5 million men in the UK. Most people reach for minoxidil, biotin supplements, or expensive shampoos without ever checking whether there's a treatable medical cause. That's a mistake — because the most common drivers of hair thinning are measurable in blood, and many are straightforward to fix once identified.

Before spending hundreds on topical treatments, get a blood test. Here's what to check and why.

When Hair Loss Is Nutritional

Ferritin: The Hair Growth Threshold Your GP Doesn't Know About

This is the single most important blood marker for hair loss that most GPs overlook. The NHS reference range for ferritin starts at 12 µg/L. If your result is 15, you'll be told it's normal. But dermatologists and trichologists have known for decades that ferritin needs to be above 70 µg/L for optimal hair growth.

A landmark study published in the European Journal of Dermatology found that women with unexplained hair loss had significantly lower ferritin than controls, even when haemoglobin was normal. The hair follicle is one of the fastest-dividing cell types in the body — it demands a steady supply of iron. When ferritin drops below 70, the body diverts iron to more critical functions, and hair growth slows or stops.

This doesn't show up as anaemia on a standard FBC. You need ferritin measured directly, and you need to know that the "normal" range isn't the same as the "optimal for hair" range.

Iron and Transferrin Saturation

Alongside ferritin, check serum iron and transferrin saturation. Transferrin saturation below 20% suggests iron-deficient erythropoiesis — your body is struggling to make red blood cells efficiently. This compounds hair loss by reducing oxygen delivery to the scalp.

Vitamin D

Vitamin D receptors are present in hair follicles, and research published in the British Journal of Dermatology has linked vitamin D deficiency to alopecia areata and telogen effluvium (diffuse hair shedding). In the UK, where 1 in 5 adults is clinically deficient in winter, this is a common contributing factor.

Optimal vitamin D for hair health: above 75 nmol/L. Many people with hair loss have levels below 50.

B12 and Folate

Both B12 and folate are essential for DNA synthesis in rapidly dividing cells — including hair follicle cells. Deficiency in either can cause diffuse hair thinning alongside fatigue, pallor, and neurological symptoms. Vegetarians, vegans, and people taking metformin or proton pump inhibitors are at higher risk of B12 deficiency.

Zinc

Zinc is required for hair protein synthesis and cell division. Deficiency causes telogen effluvium and makes existing hair brittle and thin. While not included in standard blood panels, zinc can be added as a supplementary test. Levels below 10 µmol/L warrant supplementation.

When Hair Loss Is Hormonal

Thyroid Dysfunction

Both underactive and overactive thyroid conditions cause hair loss. Hypothyroidism tends to cause diffuse thinning — hair becomes dry, brittle, and falls out evenly across the scalp. Hyperthyroidism causes finer, softer hair that sheds more easily.

A full thyroid panel — TSH, Free T3, and Free T4 — is essential. TSH alone can miss subclinical thyroid problems that are already affecting hair growth. Thyroid antibodies (anti-TPO, anti-TG) should be checked if autoimmune thyroid disease is suspected, as Hashimoto's thyroiditis is a common cause of hair loss in women.

Androgens in Women: PCOS and Female Pattern Hair Loss

In women, elevated androgens — particularly testosterone and its potent derivative DHT — drive hair thinning at the temples and crown. This is the mechanism behind female pattern hair loss (androgenetic alopecia), which affects around 40% of women by age 50.

Key markers to check:

• Total testosterone — elevated levels suggest androgen excess
• SHBG (sex hormone-binding globulin) — low SHBG means more free testosterone available to act on hair follicles
• Free testosterone — calculated from total testosterone and SHBG; the most clinically useful measure

Polycystic ovary syndrome (PCOS) is the most common cause of androgen excess in women of reproductive age, affecting roughly 1 in 10 women in the UK. Hair thinning, acne, and irregular periods are the classic triad.

Testosterone in Men: Low T and Hair Changes

Male pattern baldness is largely genetic and driven by DHT sensitivity at the follicle level. However, unusually rapid or early hair loss in men can signal hormonal imbalance. Low testosterone with elevated SHBG, or low DHEA-S, can alter the hormonal environment and affect hair growth patterns.

A male hormone panel including testosterone, SHBG, free testosterone, and oestradiol gives a clearer picture than testosterone alone.

How to Tell What's Causing Your Hair Loss

The pattern of loss often gives clues:

• Diffuse thinning all over — nutritional deficiency (iron, B12, vitamin D) or thyroid dysfunction
• Thinning at the temples and crown (women) — androgen excess, likely PCOS-related
• Receding hairline and crown (men) — androgenetic alopecia (genetic + DHT)
• Patchy bald spots — alopecia areata (autoimmune), check CRP and thyroid antibodies
• Sudden shedding 2–3 months after illness, stress, or weight loss — telogen effluvium, often resolves but nutritional markers should still be checked

Which Blood Test to Order

For a nutritional investigation, the Blood Health 6 test (£89) covers ferritin, iron, transferrin saturation, B12, folate, and a complete blood count — the core markers for nutritional hair loss.

If you suspect a hormonal cause, women should consider the Female Hormones 7 test (£95), which includes testosterone, SHBG, and key reproductive hormones. Men should look at the Male Hormones Clarity 14 test (£110) for a full androgenic profile.

For the broadest coverage — nutritional, thyroid, and hormonal markers in one test — the Core Health 45 (£120) covers ferritin, iron, full thyroid panel, vitamin D, B12, folate, and more. It's the best single test for investigating unexplained hair loss.

Don't guess at the cause of your hair loss. A blood test can tell you whether the problem is iron, thyroid, hormones, or something else — and point you toward a treatment that actually addresses the root cause. Order your test today and get results within 2–3 working days.