Why Does My Hair Keep Falling Out? A Physician’s Guide to Nutritional Causes of Hair Loss

✍️ Written and reviewed by Prof. Dr Qazi Taqweemulhaq, FCPS Medicine — Professor of Medicine, Women Medical and Dental College, Abbottabad, Pakistan. Consultant Physician, 32 Years Clinical Experience

📅 Last Updated: August 2026 | References: NIH ODS, NCBI StatPearls (2025), American Academy of Dermatology, Journal of the American Academy of Dermatology, Cleveland Clinic, Merck Manual

⚡  Quick Answer: If you also wonder, “Why does my hair keep falling out?” then you are at the correct place. Hair loss with a normal blood test almost always means the wrong tests were ordered or the wrong threshold was used. The single most important and most commonly missed cause of diffuse hair loss in American women is iron deficiency—specifically low ferritin—at levels that appear normal on a standard report but are below the threshold required for healthy hair follicle function. The eight nutritional causes of hair loss are iron deficiency, zinc deficiency, biotin deficiency, Vitamin D deficiency, Vitamin B12 deficiency, selenium excess or deficiency, folate deficiency, and protein deficiency from severe caloric restriction. Each requires a specific blood test. The complete panel is in Section 4 of this article.

The Woman Whose Ferritin Was “Normal”

She was 34 years old. A nurse practitioner. She came to me after seeing two dermatologists and her family physician about hair loss that had been progressing for 18 months.

On washing her hair she was losing what she estimated to be 150 to 200 strands per day — she had counted. Her ponytail was noticeably thinner. She had developed a visible widening of her centre part. Her hairline above the temples had receded slightly.

Both dermatologists had diagnosed androgenetic alopecia — female pattern hair loss — and recommended minoxidil. Her family physician had ordered a blood count. The haemoglobin was 13.1 g/dL. Normal. She was told her hair loss was genetic.

She came to me because the minoxidil was not working after six months of consistent use.

I looked at her blood results. The haemoglobin was indeed normal. But ferritin had not been checked. I ordered it.

Her serum ferritin was 11 µg/L.

This value falls within the standard reference range at most US laboratories — typically listed as “normal” at anything above 6 to 10 µg/L. But the research is unambiguous: hair follicles require a ferritin above 30 µg/L for normal function. Below 30 µg/L, even in the apparent absence of anaemia, hair follicles enter a premature resting phase and shedding increases. Below 20 µg/L, the effect is pronounced. At 11 µg/L her iron stores were profoundly inadequate for hair growth even though her haemoglobin was technically normal.

Her Vitamin D was also 16 ng/mL. Her zinc was low-normal.

She had been told her hair loss was genetic. It was nutritional.

I stopped the minoxidil. I started ferrous sulphate 200 mg twice daily with Vitamin C to enhance absorption, Vitamin D3 3,000 IU, and zinc picolinate 15 mg. At four months her shedding had reduced significantly. At nine months her centre part had visibly narrowed and her ponytail thickness had returned to near normal.

💡  Clinical Insight:  The ferritin threshold for hair loss is the most important piece of clinical information most physicians dealing with hair loss have never been taught. A ferritin of 11 µg/L is not normal for a hair follicle even if it is technically within the laboratory reference range. Ask for your ferritin specifically. Ask for the actual number, not just “normal” or “abnormal.” And if it is below 30 µg/L and your hair is falling out, you have your answer. For the complete guide to nutritional deficiency diseases, read: Nutritional Deficiency Diseases: The Complete Doctor’s Guide.

Understanding Hair Loss — The Biology You Need to Know

Before understanding why nutrition causes hair loss, you need to understand how hair grows. The hair follicle operates on a cycle with three distinct phases.

The Hair Growth Cycle

Anagen (growth phase): Approximately 85 to 90% of your scalp hairs are in this phase at any time. It lasts 2 to 7 years. The longer the anagen phase, the longer your hair can grow. Nutritional deficiency shortens the anagen phase, producing hair that never reaches its previous length.

Catagen (transition phase): Lasting 2 to 3 weeks. The hair follicle shrinks and detaches from the dermal papilla. Approximately 1 to 2% of hairs are in this phase at any time.

Telogen (resting phase): Lasting 3 to 4 months. The old hair is retained in the follicle while a new hair begins growing beneath it. At the end of telogen, the old hair is shed. Approximately 10 to 15% of hairs are normally in telogen at any time. This accounts for the normal daily shedding of 50 to 100 hairs.

What Is Telogen Effluvium?

Telogen effluvium is the medical term for the most common type of diffuse hair loss — and nutritional deficiency is one of its primary causes.

When the body experiences a significant nutritional deficiency, physical illness, extreme stress, crash diet, or hormonal change, hair follicles receive a signal to exit anagen prematurely and enter telogen. This is a survival mechanism: the body is redirecting energy and nutrients away from non-essential functions (hair growth) toward essential ones (organ function and immune defence).

The critical clinical detail: telogen effluvium typically peaks 2 to 3 months after the triggering event. Hair follicles that entered telogen in response to a nutritional deficiency take 2 to 3 months to shed their hair. This is why patients almost never connect the cause to the timing.

A crash diet in January produces peak hair shedding in March or April. An iron deficiency that developed through heavy periods over 6 months produces shedding that the patient notices 3 months after the iron stores reached a critical low — by which point the cause feels unrelated.

Hair follicles are among the most metabolically active tissues in the body. They divide rapidly, consume large amounts of energy, and are highly sensitive to nutritional adequacy. When competing priorities arise, the body rations nutritional supply to hair follicles early and aggressively.

The Eight Nutritional Causes of Hair Loss

Cause 1: Iron Deficiency — The Most Common and Most Missed

Why it causes hair loss: Iron is required for cell division in the hair follicle matrix. The rapidly dividing cells of the hair bulb are among the highest consumers of iron in the body. When iron stores fall, these cells are among the first to be deprived. Follicles enter telogen prematurely and hair shedding increases.

The ferritin threshold — the most important number in this article:

Table 1. Ferritin thresholds and hair follicle function. Sources: Journal of the American Academy of Dermatology; Cleveland Clinic Hair Loss Clinic guidelines; NCBI StatPearls (2025).

The standard laboratory reference range for serum ferritin in most US laboratories begins at 6 to 10 µg/L for adult women. A patient with a ferritin of 18 µg/L will receive a report saying “normal.” But her hair follicles are functioning with approximately 25% of the iron stores they need for optimal growth. Her hair will be falling out and her doctor will tell her the tests are normal.

This is the single most common and most consequential clinical misunderstanding in the dermatological management of hair loss in the United States.

Who is most at risk: Women with heavy menstrual periods — this is the dominant cause of iron-related hair loss in premenopausal American women. Women who follow plant-based or low-meat diets. Women who have recently had a baby (postpartum hair loss is almost always a combination of the hormonal change plus iron depletion from pregnancy and delivery blood loss). Patients with coeliac disease or other malabsorptive conditions. Long-term proton pump inhibitor users whose gastric acid suppression reduces iron absorption.

The specific test: Serum ferritin. Ask for the numerical value. Anything below 30 µg/L in a patient with hair loss should be treated regardless of whether it falls within the laboratory reference range.

For the complete guide to iron deficiency including treatment protocols, read: Iron Deficiency Anaemia: Symptoms, Causes and Treatment.

Cause 2: Zinc Deficiency

Why it causes hair loss: zinc is required for hair follicle protein synthesis and cell proliferation. It also inhibits the regression of hair follicles (premature entry into catagen) and accelerates the recovery of damaged follicle sheaths. Zinc deficiency produces diffuse hair thinning across the scalp, characteristically accompanied by a scaly, slightly inflamed scalp.

The zinc-iron interaction: Zinc and iron compete for absorption using the same transporter protein (DMT-1) in the intestine. High-dose iron supplementation reduces zinc absorption. This is why patients treated for iron deficiency without concurrent zinc attention sometimes notice continued hair thinning despite normalising iron — the iron treatment has depleted zinc. When treating iron deficiency hair loss, always assess zinc status concurrently.

The symptom pattern: Diffuse hair thinning. Dull, brittle hair that breaks easily rather than shedding from the root. A scaly or slightly greasy scalp. Loss of taste and smell if zinc deficiency is more significant. Slow wound healing.

The specific test: Serum zinc. Below 70 µg/dL is deficient. Reference range varies between laboratories — request the numerical value and compare to 70 µg/dL as the lower threshold for hair function.

For the complete guide, read: Zinc Deficiency: Symptoms, Causes and Treatment.

Cause 3: Biotin (Vitamin B7) Deficiency — What the Supplement Industry Does Not Tell You

Why it causes hair loss: Biotin is required for keratin synthesis. Keratin is the structural protein that makes up the hair shaft. Biotin deficiency impairs keratin production, weakening the hair shaft structure and causing diffuse alopecia alongside the pathognomonic rash — periorificial dermatitis around the mouth, nose, and eyes.

The critical distinction: Biotin deficiency causes hair loss. Biotin supplementation in people who are NOT biotin deficient does not prevent or reverse hair loss. This distinction — obvious in pharmacology — has been completely obscured by decades of supplement marketing.

The evidence is unambiguous. A 2017 systematic review in Skin Appendage Disorders found that every published report of biotin improving hair growth involved patients with either confirmed biotin deficiency or a specific condition causing biotin depletion (such as biotinidase deficiency or prolonged raw egg consumption). No randomised controlled trial has shown benefit from biotin supplementation in biotin-sufficient individuals with hair loss.

The safety warning: High-dose biotin supplements at 5,000 to 10,000 mcg — commonly sold for hair growth in the US — interfere with laboratory immunoassays. The FDA issued a specific safety warning about this in 2017. Thyroid function tests, cardiac troponin, and Vitamin D assays can all return falsely abnormal results in patients taking high-dose biotin. Stop biotin supplements for 48 to 72 hours before any blood test.

Who actually has biotin deficiency: People consuming raw egg whites regularly (avidin in raw eggs binds biotin and prevents absorption). Patients on long-term anticonvulsant medications. Patients on broad-spectrum antibiotics for extended periods. Patients with biotinidase deficiency (identified on newborn screening in all 50 US states).

The diagnostic clue: True biotin deficiency produces a specific triad — diffuse hair loss, periorificial dermatitis (rash around the mouth, nose, and eyes), and neurological symptoms. If all three are present, check serum biotin. If hair loss is the only symptom, biotin deficiency is unlikely.

For the complete guide, read: Vitamin B7 Biotin Deficiency: Hair Loss, the FDA Warning and Treatment.

Cause 4: Vitamin D Deficiency

Why it causes hair loss: Vitamin D receptors are present in hair follicle cells. Vitamin D plays a direct role in initiating the hair follicle growth cycle — activating follicle stem cells and stimulating the shift from telogen back to anagen. When Vitamin D is deficient, this reactivation is impaired. Follicles remain in telogen longer than normal, reducing overall hair density over time.

Several studies have confirmed lower serum 25(OH)D levels in patients with telogen effluvium and alopecia areata compared to controls. A 2019 study in Skin Pharmacology and Physiology found a significant inverse correlation between serum Vitamin D and severity of hair loss.

Who is most at risk: Any American or European adult who does not supplement Vitamin D and does not spend significant time outdoors in summer months. NIH data shows 42% of Americans are Vitamin D deficient. Women who cover their skin for cultural reasons. People with darker skin living in northern climates.

The Vitamin D-magnesium interaction: Magnesium is required to convert Vitamin D to its biologically active form. Supplementing Vitamin D without ensuring adequate magnesium produces a partial response. If Vitamin D supplementation is not improving hair loss after 3 months, check magnesium simultaneously.

The specific test: Serum 25-hydroxyvitamin D [25(OH)D]. Below 20 ng/mL is deficient. Between 20 and 29 ng/mL is insufficient. For hair follicle function, aim for levels above 40 ng/mL.

For the complete guide, read: Vitamin D Deficiency: Symptoms, Causes and Treatment.

Cause 5: Selenium — Unique Because Both Too Little and Too Much Cause Hair Loss

Selenium is unique among nutritional causes of hair loss because deficiency and toxicity produce identical symptoms — diffuse alopecia. This makes selenium the only nutrient where supplementing without knowing your level can worsen the very symptom you are treating.

Selenium deficiency and hair loss: Selenium is required for selenoprotein production, including the enzymes that protect hair follicle cells from oxidative damage. In deficiency, oxidative stress in follicle cells increases, accelerating their transition into telogen.

Selenium toxicity (selenosis) and hair loss: Chronic selenium intake above 400 mcg per day from any source causes selenosis — hair loss, brittle nails, garlic-like breath odour, and peripheral neuropathy. This is the most specific pattern: if hair loss is accompanied by a persistent garlic-like odour on the breath without eating garlic, selenium toxicity is the diagnosis.

Brazil nuts are a common hidden source of selenium overdose. A single large Brazil nut from high-selenium soil can contain over 500 mcg of selenium — more than the daily upper limit from a single nut. People eating multiple Brazil nuts daily as a “health food” can unknowingly consume several times the upper limit.

The rule: Before supplementing selenium for hair loss, check your serum selenium level. Supplementing blindly when selenium is already adequate or elevated will cause the hair loss to worsen.

The specific test: Serum selenium. Below 70 ng/mL indicates deficiency. Above 150 ng/mL in the context of hair loss and garlic breath suggests toxicity.

For the complete guide, read: Selenium Deficiency: The Thyroid and Hair Connection.

Cause 6: Vitamin B12 Deficiency

Why it causes hair loss: Vitamin B12 is essential for DNA synthesis and cell replication. The rapidly dividing cells of the hair follicle matrix are highly dependent on B12 for normal cell turnover. Deficiency impairs follicle cell replication, reducing hair production and increasing shedding.

B12 deficiency hair loss tends to present alongside other symptoms that make the diagnosis more tractable: fatigue, tingling in the hands and feet, difficulty concentrating, and a smooth sore tongue. The combination of diffuse hair loss and these neurological features in a patient who is vegan, over 60, or on long-term metformin should prompt immediate B12 testing.

The metformin connection: Metformin — taken by millions of Americans for type 2 diabetes — blocks B12 absorption in the gut. Hair loss in a diabetic patient on long-term metformin should trigger B12 measurement as a priority. This interaction is documented, common, and almost never discussed at prescribing.

The specific test: Serum B12. Below 180 pg/mL is deficient. Between 180 and 300 pg/mL is borderline — add methylmalonic acid (MMA) to confirm.

For the complete guide, read: Vitamin B12 Deficiency: Nerve Damage and Pernicious Anaemia.

Cause 7: Folate Deficiency

Why it causes hair loss: Folate is required for the rapid cell division that characterises hair follicle activity. Folate deficiency impairs the production of new follicle cells, reducing hair growth rate and density. Folate works closely with B12 in the methylation cycle — deficiency in either impairs both pathways.

Folate deficiency hair loss is most commonly seen in women taking the oral contraceptive pill (which depletes folate), people with alcohol use disorder, patients on methotrexate for rheumatoid arthritis or psoriasis (methotrexate is a folate antagonist), and patients on anticonvulsant medications.

The specific test: Serum folate and red cell folate. Serum folate reflects recent dietary intake. Red cell folate reflects chronic status over the preceding 3 months.

For the complete guide, read: Folate Deficiency: The Pregnancy-Critical B Vitamin.

Cause 8: Protein and Caloric Restriction — The Crash Diet Effect

Severe caloric restriction and protein deficiency — whether from an extreme weight loss diet, anorexia nervosa, or gastric bypass surgery — trigger some of the most dramatic hair loss seen in clinical practice.

Hair is made almost entirely of keratin — a protein. When dietary protein is severely restricted, the body prioritises protein allocation to vital organs and immune function. Hair follicle protein synthesis is reduced or stopped. The clinical result is diffuse shedding that begins approximately 2 to 3 months after the period of restriction and can involve 30 to 50% of scalp hair density.

This is the explanation for the dramatic hair loss that many patients experience after gastric bypass surgery — a combination of surgical stress, caloric restriction, and the multiple nutritional deficiencies that follow bypass including iron, zinc, B12, copper, and Vitamin D.

It also explains why very low calorie diets (below 1,200 calories per day in women) reliably produce hair loss as a side effect even when they are nutritionally supplemented — the caloric deficit itself signals the body to downregulate follicle activity.

The Hair Loss Blood Panel — What to Ask For

This is the panel I recommend for any patient presenting with unexplained diffuse hair loss. It is ordered in a single blood draw and identifies the treatable nutritional cause in the majority of cases.

Table 2. Complete blood panel for hair loss. Source: Clinical experience; Journal of the American Academy of Dermatology; NIH ODS; NCBI StatPearls (2025).

The Single Most Important Number

When you receive your results, find the ferritin value. Look at the actual number — not just whether it is marked normal or abnormal. If it is below 30 µg/L and your hair is falling out, you have found a significant contributing cause regardless of what the lab report says.

The laboratory reference range was established from population data and is designed to identify deficiency severe enough to cause anaemia. It was not established to identify the threshold for optimal hair follicle function. These are different thresholds and they are not the same number.

Medications That Cause Hair Loss Through Nutrient Depletion

Several commonly prescribed medications cause hair loss as an indirect consequence of nutrient depletion. This category is almost never discussed when medications are prescribed.

Table 3. Medications causing hair loss through nutritional depletion. Sources: NIH ODS; NCBI StatPearls (2025); Journal of the American Academy of Dermatology.

When Hair Loss Is Not Nutritional

Not all hair loss has a nutritional cause. These conditions need to be identified because their treatment is different.

Androgenetic alopecia (female and male pattern hair loss) is the most common overall cause of hair loss in adults of both sexes. In women it produces diffuse thinning predominantly over the crown with preservation of the frontal hairline. In men it follows the classic Hamilton-Norwood pattern of temporal recession and vertex thinning. It has a genetic basis and is driven by dihydrotestosterone (DHT) acting on hair follicle androgen receptors. Nutritional optimisation helps maximise what remains but does not reverse androgenetic hair loss. Minoxidil and finasteride are the evidence-supported treatments.

Alopecia areata is an autoimmune condition producing patchy, circular areas of complete hair loss rather than diffuse thinning. The scalp skin within the bald patches is smooth and normal in appearance. Exclamation mark hairs at the periphery of the patch are pathognomonic. It can affect any hair-bearing site. Vitamin D and zinc deficiency are associated with increased severity and frequency of alopecia areata episodes, but the condition is autoimmune rather than primarily nutritional.

Thyroid disease — both hypothyroidism and hyperthyroidism — causes diffuse hair loss through direct follicle effects. The thyroid hormone receptors in hair follicles regulate the growth cycle directly. Importantly, selenium is required to convert T4 to active T3. A patient with normal TSH but low free T3 from selenium deficiency may have thyroid-related hair loss despite a technically normal thyroid function test. Always check free T3 alongside TSH in hair loss patients.

Traction alopecia results from consistent mechanical tension on the hair shaft from tight hairstyles — braids, extensions, tight ponytails. It presents as hair loss at the hairline and temples corresponding exactly to the direction of tension. It is not nutritional and does not respond to supplementation.

Postpartum telogen effluvium is a specific and very common form of hair loss affecting women in the 2 to 5 months after delivery. The high oestrogen levels of pregnancy prolong the anagen phase, producing the thick, full hair of pregnancy. After delivery, oestrogen falls, and all those follicles that were held in anagen simultaneously enter telogen. Massive shedding follows 3 to 4 months later. This is normal and self-limiting. However, it is frequently made significantly worse by the iron and nutritional depletion of pregnancy and delivery blood loss. Checking ferritin and treating iron deficiency postpartum reduces the severity and duration considerably.

Treatment and Recovery

The treatment of nutritional hair loss is straightforward once the cause is identified. The principle: correct the deficiency at an adequate dose and allow sufficient time for the hair cycle to recover.

Table 4. Treatment and timeline for nutritional hair loss. Source: Clinical experience; Journal of the American Academy of Dermatology; NIH ODS.

Why Recovery Takes Longer Than You Expect

Patients frequently ask why, if their nutritional deficiency is corrected in 6 to 8 weeks, their hair takes 6 to 12 months to recover. The answer lies in the hair cycle.

When a deficiency is corrected, follicles in telogen begin to reactivate and re-enter anagen. But the new anagen hair must then grow from the follicle, through the scalp, and to a visible length above the skin surface. This takes time. A hair that enters anagen today will not be visible above the scalp surface for approximately 6 to 8 weeks. It will not reach a length that contributes to visible density for 3 to 6 months.

The first sign of recovery is not increased density. It is reduced shedding. Shedding decreases as fewer follicles enter telogen. Density follows months later as the new anagen hairs reach visible length. Patience is essential. Most patients who abandon treatment at 3 months because “it is not working” abandon it exactly when the recovery is beginning.

Practical Tips for Maximising Recovery

Take iron and Vitamin C together. Vitamin C doubles non-haem iron absorption. Take iron at a different time from calcium-containing foods, tea, coffee, and other mineral supplements — all of which reduce iron absorption.

Correct multiple deficiencies simultaneously. Hair loss from iron deficiency will not fully resolve if zinc or Vitamin D remains low. The hair follicle needs all its nutritional inputs optimised at once.

Check ferritin every 3 months until it reaches 50 µg/L or above. Do not stop supplementing when it normalises at 30 µg/L — continue to 50 to 70 µg/L for optimal hair follicle function.

Avoid crash diets during treatment. Caloric restriction will undo nutritional repletion and retrigger telogen effluvium even while supplementing.