Hair Loss Causes: A Doctor's Guide to Why You Are Losing Hair (2026)

When hair starts falling out, the instinct is to search for solutions. But the most important step is not choosing a treatment. It is understanding why your hair is falling out in the first place.

Hair loss is a symptom, not a diagnosis. It has over a dozen distinct causes, each requiring different treatment. A patient with iron-deficiency hair loss needs iron supplementation, not PRP. A patient with thyroid-driven thinning needs thyroid medication before any regenerative treatment will work. A patient with androgenetic alopecia needs DHT-blocking therapy alongside regenerative treatment.

Getting the diagnosis right is not just helpful. It is the single most important step in effective hair loss treatment. At The London PRP Clinic by The Wellness, our GMC-registered doctors begin every patient journey with a thorough diagnostic assessment, because we have seen too many patients waste years and thousands of pounds treating the wrong cause.

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The Major Causes of Hair Loss

Androgenetic Alopecia (Genetic Pattern Hair Loss)

Prevalence: 85% of men, 50% of women over their lifetime.

This is the most common cause by a wide margin. It is driven by genetic sensitivity to dihydrotestosterone (DHT), a hormone derived from testosterone. DHT binds to receptors on susceptible hair follicles, causing them to gradually miniaturise over successive growth cycles.

In men: Presents as a receding hairline at the temples, thinning at the crown, or both. Follows the Norwood-Hamilton scale (Stages I to VII). Can begin in the late teens or early twenties.

In women: Presents as diffuse thinning along the part line and crown. The hairline is usually preserved. Follows the Ludwig scale (Stages I to III). Often becomes noticeable during or after menopause.

Treatment: Combination therapy (finasteride for men, minoxidil, PRP, microneedling, supplements). The London PRP Clinic achieves 87% success rate with comprehensive protocols.

Iron Deficiency

Prevalence: The most common nutritional cause of hair loss in women.

Iron is essential for oxygen delivery to rapidly dividing follicle cells. When iron stores (measured by ferritin) drop below approximately 30 ng/mL, hair follicles are among the first tissues affected because hair is a "non-essential" structure the body deprioritises during scarcity.

Iron deficiency is particularly common in women of reproductive age (due to menstruation), pregnant and postpartum women, vegetarians and vegans, and patients with gastrointestinal absorption issues.

Diagnosis: Blood test measuring both haemoglobin and ferritin. Normal haemoglobin does not rule out iron-related hair loss; ferritin can be low enough to cause shedding while haemoglobin remains within normal range.

Treatment: Iron supplementation guided by blood results. Hair improvement typically begins within 2 to 3 months of ferritin correction.

Thyroid Disorders

Prevalence: Thyroid dysfunction affects approximately 1 in 20 people in the UK.

Both hypothyroidism (underactive) and hyperthyroidism (overactive) disrupt the hair growth cycle. Thyroid-related hair loss is typically diffuse (affecting the entire scalp rather than following a pattern) and may be accompanied by other symptoms such as fatigue, weight changes, cold sensitivity, or skin changes.

Diagnosis: Blood test measuring TSH, free T3, and free T4.

Treatment: Thyroid medication to restore normal hormone levels. Hair regrowth typically follows within 3 to 6 months of thyroid stabilisation. PRP can accelerate recovery once thyroid levels are normalised.

Telogen Effluvium (Stress and Trigger-Related)

Prevalence: Extremely common. Most adults experience at least one episode.

A specific trigger causes a large number of follicles to simultaneously enter the resting (telogen) phase. Hair loss appears 2 to 3 months after the trigger. Common triggers include severe illness or high fever, major surgery, emotional trauma or chronic stress, crash dieting or rapid weight loss, childbirth (postpartum hair loss), and medication changes (including starting or stopping birth control).

Diagnosis: Clinical history identifying the triggering event, combined with examination showing diffuse thinning without a pattern.

Treatment: Identify and address the trigger. Nutritional support. PRP can accelerate follicular recovery. Most cases resolve within 6 to 12 months.

Hormonal Causes (Beyond Thyroid)

PCOS (Polycystic Ovary Syndrome) elevates androgen levels in women, causing thinning that resembles male pattern hair loss. Affects approximately 10% of women of reproductive age.

Menopause reduces oestrogen levels, shifting the hormonal balance toward androgens. Hair thinning during and after menopause is extremely common.

Postpartum oestrogen withdrawal triggers telogen effluvium in up to 50% of new mothers. (See our dedicated postpartum hair loss guide.)

Other Causes

Alopecia areata: Autoimmune patchy hair loss affecting approximately 2% of the population.

Traction alopecia: Caused by tight hairstyles pulling on follicles.

Medication side effects: Certain blood thinners, antidepressants, acne medications, and others can trigger hair loss.

Vitamin D deficiency: Endemic in the UK and linked to multiple types of hair loss.

Zinc and B12 deficiency: Both impair follicle cell division and hair production.

Why Proper Diagnosis Changes Everything

Consider two patients who both present with "thinning hair."

Patient A has low ferritin (18 ng/mL) and subclinical hypothyroidism. If she starts PRP without addressing these causes, she will see limited results and conclude PRP does not work. If she corrects her iron and thyroid levels first, then adds PRP, she achieves excellent outcomes.

Patient B has androgenetic alopecia with normal blood work. He needs DHT-blocking medication alongside PRP. If he only takes supplements and uses thickening shampoo, his thinning will progress unchecked.

The treatment is only as effective as the diagnosis is accurate. This is why every patient at The London PRP Clinic receives comprehensive assessment before any treatment recommendation.

Your Diagnostic Consultation at The London PRP Clinic

Our free consultation with a GMC-registered doctor includes detailed clinical examination of your hair loss pattern, hair pull test to assess active shedding, blood work recommendations (iron/ferritin, thyroid, vitamin D, B12, zinc, hormonal panel), differential diagnosis based on clinical and laboratory findings, and personalised treatment plan targeting your specific causes.

PRP from £545/session. ExoRevive from £445. 87% success rate. 187+ five-star reviews. Marylebone and Canary Wharf.

Book your free diagnostic consultation > WhatsApp | Email: team@thewellnesslondon.com | Call: +44 20 3951 3429

Medical Disclaimer: Hair loss has multiple potential causes requiring medical assessment. This article is for informational purposes only. All consultations at The London PRP Clinic are conducted by GMC-registered doctors. Last reviewed March 2026.