Hair Loss Blood Test UK: Which Markers to Check — and What Your Results Actually Mean

1. Why a blood test is the first step for hair loss

Hair follicles are among the most metabolically active cells in the body. They divide rapidly, require a constant supply of iron, zinc, and amino acids, and are exquisitely sensitive to hormone fluctuations and nutrient depletion.

When the body is under metabolic stress — whether from iron deficiency, thyroid dysfunction, or elevated androgens — hair is one of the first systems to be deprioritised. The body redirects resources to vital organs, and follicles shift from their growth phase (anagen) into a resting and shedding phase (telogen) prematurely.

A blood test identifies the underlying cause. Without one, you're guessing — and most over-the-counter hair supplements are expensive guesses. The British Association of Dermatologists lists blood tests as a standard investigation for anyone presenting with diffuse hair thinning.

2. Types of hair loss and their blood test clues

Not all hair loss is the same. Understanding the pattern helps determine which blood markers to prioritise.

Many people have overlapping causes. A comprehensive blood panel catches all of them in a single test.

3. Ferritin: the most overlooked hair loss marker

Ferritin is your body's iron storage protein. Your GP will flag it only if it drops below 15 μg/L (the diagnostic threshold for iron deficiency anaemia), but hair follicles start struggling at much higher levels.

A landmark study published in the British Journal of Dermatology found that women with unexplained hair loss had significantly lower ferritin than controls, even when levels were technically “normal.” Multiple dermatology reviews now recommend a ferritin target of at least 70 μg/L for optimal hair growth — nearly five times the NHS's lower threshold.

Iron is also an acute-phase reactant, meaning ferritin can rise temporarily during infection or inflammation, masking a true deficiency. Testing ferritin alongside CRP gives a clearer picture.

4. Thyroid function: TSH, Free T4, Free T3

The thyroid gland regulates metabolism in every cell — including hair follicle cells. Both hypothyroidism (underactive) and hyperthyroidism (overactive) cause hair loss, and thyroid disease is one of the most common treatable causes in the UK, affecting roughly 1 in 20 people according to the NHS.

The hallmark sign is diffuse thinning across the entire scalp, often accompanied by thinning of the outer third of the eyebrows. Hair becomes dry, brittle, and slow to grow. In hypothyroidism, the hair follicle cycle slows dramatically — more follicles enter the resting phase, and fewer new hairs replace those that shed.

Most GPs test only TSH. That catches overt thyroid disease, but misses subclinical cases where TSH is technically “in range” (say 3.5–4.0 mIU/L) but Free T4 and Free T3 are at the bottom of their ranges. Hair loss can start well before a formal diagnosis is made.

Testing TSH, Free T4, and Free T3 together gives a complete thyroid picture. If thyroid antibodies (TPO, TgAb) are also elevated, it may indicate Hashimoto's thyroiditis — the most common cause of hypothyroidism in the UK and a condition that frequently presents with hair loss before other symptoms appear.

5. Testosterone, SHBG, and DHT-driven hair loss

Androgenetic alopecia — the most common type of hair loss in both men and women — is driven by dihydrotestosterone (DHT), a potent metabolite of testosterone. DHT binds to receptors on genetically susceptible hair follicles, causing them to miniaturise over time until they stop producing visible hair.

In men, the pattern is classic: receding temples and crown thinning. In women, it presents as diffuse thinning at the parting line. The NICE Clinical Knowledge Summary notes that androgenetic alopecia affects roughly 50% of men over 50 and up to 40% of women by menopause.

The key blood markers are total testosterone, sex hormone-binding globulin (SHBG), and the free androgen index (FAI). Low SHBG means more free testosterone is available for conversion to DHT — even if total testosterone is “normal.”

In women, elevated DHEA-S alongside low SHBG may point toward polycystic ovary syndrome (PCOS) — one of the most common causes of female hair loss and a condition frequently diagnosed through blood tests rather than imaging.

6. Vitamin D and the hair follicle cycle

Vitamin D receptors are present on hair follicle keratinocytes, and vitamin D plays a direct role in the anagen (growth) phase of the hair cycle. Low vitamin D is associated with telogen effluvium and alopecia areata — a connection confirmed by multiple systematic reviews, including a 2019 meta-analysis in the Dermatology and Therapy journal.

The UK is one of the worst countries in Europe for vitamin D deficiency. Public Health England estimates that 1 in 5 adults has a serum level below 25 nmol/L, and a far larger proportion sits between 25–50 nmol/L — technically “sufficient” by NHS standards but well below the 75–125 nmol/L range associated with optimal health outcomes.

If you're losing hair in the UK, especially between October and April when UVB exposure is virtually zero, vitamin D should be one of the first markers you check.

7. Zinc, biotin, and B12

Zinc is essential for hair follicle protein synthesis and cell division. A study in the Annals of Dermatology found significantly lower serum zinc in patients with all types of hair loss compared to healthy controls. Zinc deficiency is common in vegetarians, vegans, and people with gut disorders that impair absorption.

Biotin (vitamin B7) is aggressively marketed for hair growth, but true biotin deficiency is rare in people with normal diets. The evidence for biotin supplementation in non-deficient individuals is weak. However, biotin supplements interfere with many blood test assays — the MHRA has issued safety warnings about biotin causing falsely normal thyroid results and falsely low troponin readings. Stop biotin supplements at least 48 hours before any blood test.

Vitamin B12 deficiency impairs red blood cell production, reducing oxygen delivery to hair follicles. It's especially common in vegans, older adults, and those on long-term proton pump inhibitors. A B12 level below 300 pmol/L — even though the NHS lower limit is 148 pmol/L — may contribute to hair thinning.

8. Cortisol: when stress causes shedding

Telogen effluvium — sudden, diffuse hair shedding 2–4 months after a stressor — is one of the most common reasons people seek help for hair loss. The trigger can be physical (surgery, illness, crash dieting) or psychological (bereavement, job loss, prolonged work stress).

A 2021 study published in Nature demonstrated for the first time that corticosterone (the mouse equivalent of cortisol) directly inhibits hair follicle stem cell activation, keeping follicles locked in the resting phase. The research showed that removing the stress hormone source caused hair to regrow — providing biological proof of what many people already suspected.

Blood cortisol has limitations (it fluctuates throughout the day), but a morning cortisol test can flag overt adrenal dysfunction. If your cortisol is elevated alongside hair loss, it suggests stress as a contributing or primary factor.

9. Full blood count and anaemia screening

A full blood count (FBC) checks haemoglobin, red cell count, MCV (mean cell volume), and other markers that reveal anaemia — a direct cause of reduced oxygen delivery to hair follicles.

Iron deficiency anaemia is the most common type in women of reproductive age, affecting roughly 1 in 4 women in the UK according to NHS data. But anaemia can also be driven by B12 deficiency (macrocytic anaemia) or chronic disease — which is why an FBC alone is not enough. You need ferritin, B12, and folate alongside it to understand the cause.

Low MCV with low ferritin points to iron deficiency. High MCV with low B12 or folate suggests megaloblastic anaemia. Normal MCV with low ferritin is the sneaky one — your haemoglobin may still be “fine,” but your iron stores are depleted and your hair is paying the price.

10. NHS ranges vs optimal ranges for hair health

This is where the gap between “clinically normal” and “optimal for hair growth” is widest:

You can sit comfortably inside every NHS range and still have ferritin at 22, vitamin D at 30, and TSH at 3.8 — a combination that many dermatologists would flag as a probable contributor to hair loss. This is why testing against optimal ranges matters, not just diagnostic cutoffs.

11. Who should get a hair loss blood test?

Consider a blood test if you're experiencing any of the following:

• Noticeable increase in hair shedding lasting more than 6 weeks
• Visible thinning at the parting, temples, or crown
• Hair loss alongside fatigue, weight changes, or mood shifts (suggesting thyroid or iron issues)
• Post-partum shedding that hasn't resolved after 6 months
• Hair loss following a period of significant stress, illness, or crash dieting
• Family history of pattern baldness combined with early signs of thinning
• Vegetarian, vegan, or restrictive diet (higher risk of iron, B12, and zinc depletion)

The NICE guidelines for hair loss recommend a full blood count, ferritin, thyroid function, and (in women) androgens as initial investigations. Most of these are available through your GP — though getting all of them in a single appointment can require persistence.

12. GP blood test vs Helvy: what you actually get

13. What to do with your results

Hair loss treatment depends entirely on the cause your blood test reveals. Here are the most common findings and their evidence-based responses:

• Low ferritin (<70 μg/L): Iron supplementation (ferrous fumarate or bisglycinate for better absorption), typically 3–6 months to rebuild stores. Retest at 3 months. The NICE guideline on blood transfusion (NG24) covers oral iron dosing for deficiency.
• Underactive thyroid (TSH >4.2 or subclinical 2.5–4.2 with symptoms): See your GP for further investigation. Levothyroxine is the standard treatment for confirmed hypothyroidism. Hair typically improves 6–12 months after thyroid levels normalise.
• Low vitamin D (<75 nmol/L): Supplement with D3 (cholecalciferol). The NHS recommends 400 IU/day for maintenance, but correction doses of 2,000–4,000 IU/day are common in clinical practice for deficiency. Retest at 3 months.
• Elevated androgens / low SHBG: Discuss anti-androgen treatment with your GP or dermatologist. In men, finasteride or minoxidil may be appropriate. In women, spironolactone or combined oral contraceptives may help. Blood monitoring is recommended.
• Low B12 or folate: B12 supplementation (sublingual methylcobalamin or intramuscular injections for severe deficiency). Folate via diet or folic acid supplement. Both are straightforward to correct.
• Stress-related (elevated cortisol, normal everything else): The hair will typically regrow on its own once the stressor resolves. This can take 6–12 months. Addressing sleep, exercise, and stress management accelerates recovery.

14. Frequently asked questions