Symptom Guide

Hair Fall: Blood Tests That Find the Cause

Hair loss has many medical causes. Most are correctable once identified. Here's which blood tests to ask for and what each result means for your hair.²

Written by Suman Konda, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last updated: 1 September 2026 · How we check our content

Quick answer

Losing 50 to 100 hairs a day is normal; medically significant loss means visible thinning, a widening parting, or clumps coming out during washing. The commonest pattern is telogen effluvium, diffuse shedding after stress, illness or childbirth. Both underactive and overactive thyroid cause diffuse hair loss, and low ferritin is the commonest nutritional cause in women.

Cause group	Clues	Urgency
Telogen effluvium	Diffuse shedding after stress, illness or childbirth; the commonest cause	Routine workup
Thyroid disease	Diffuse thinning, outer eyebrow loss; TSH abnormal	Routine workup
Iron deficiency	Heavy periods, fatigue; ferritin below 30 ng/mL	Routine workup
Hormonal (androgenic)	Crown thinning in women; test testosterone, DHEA-S, LH:FSH	Routine workup
Alopecia areata	Smooth round patches; autoimmune	See dermatologist

Key points

50 to 100 hairs a day is normal shedding; clumps or visible thinning is medically significant.
Telogen effluvium, diffuse shedding after a trigger, is the commonest cause.
Always test TSH first: thyroid disease is the most correctable cause.
Ferritin below 30 ng/mL causes shedding even with normal haemoglobin.
Vitamin D and B12 deficiency contribute; both are common and treatable.
Hormonal pattern loss in women needs testosterone, DHEA-S and LH:FSH assessment.

When is hair fall medically significant?

Losing 50–100 hairs per day is normal. Medically significant hair loss (alopecia) means visible thinning, widening parting, receding hairline, or clumps of hair coming out during washing or brushing. Before treating hair loss, finding the cause is essential: the wrong treatment won't work.

Blood tests for hair fall

1. TSH (Thyroid): Most important first test

Both hypothyroidism (high TSH) and hyperthyroidism (low TSH) cause hair loss. Thyroid-related hair loss affects the entire scalp uniformly (diffuse thinning), including outer third of eyebrows in hypothyroidism. Hair regrows once thyroid is treated. Always test TSH first for hair loss: it's the most correctable cause.

2. Ferritin: Low iron stores cause hair shedding

Low ferritin is the most common nutritional cause of hair loss in women, especially those with heavy periods. Hair follicles need iron to function. Ferritin below 30 ng/mL can cause significant hair shedding, even when haemoglobin is still normal. This is why ferritin must be tested specifically for hair loss; a normal CBC does not rule it out.

3. Vitamin D

Vitamin D receptors are present in hair follicles. Deficiency (below 20 ng/mL) is associated with alopecia areata (patchy hair loss) and diffuse thinning. Very common where indoor lifestyles limit sun exposure. Correcting Vitamin D doesn't always immediately reverse hair loss but is an important part of treatment.

4. Vitamin B12

B12 is essential for rapidly dividing cells, including hair follicle cells. Deficiency causes premature hair loss. Especially common in strict vegetarians. B12 below 200 pg/mL needs treatment.

5. Testosterone / DHEA-S / LH:FSH (for women)

Hormonal hair loss (androgenic alopecia) in women is caused by excess androgens. Testing testosterone and DHEA-S, along with LH:FSH ratio, helps identify PCOS or adrenal causes. Pattern: diffuse thinning at the crown, no frontal hairline recession (unlike male pattern baldness).

6. Zinc and Biotin (less commonly tested)

Zinc deficiency can cause hair loss, especially in people on restrictive diets. Biotin deficiency is often marketed but is actually very rare as a cause of hair loss in otherwise healthy people. Most "biotin supplements for hair" lack strong evidence: treat proven deficiencies first.

Questions to ask your doctor

My ferritin is 25 ng/mL. Is that low enough to cause hair loss?
I have normal TSH but still losing hair. What else should we check?
Do I need to see a dermatologist for a scalp examination?
How long after treating the deficiency will my hair start regrowing?

References

Sources cited on this page. PubMed links open the original abstract.

Almohanna HM, Ahmed AA, Tsatalis JP, Tosti A. The Role of Vitamins and Minerals in Hair Loss: A Review. Dermatol Ther (Heidelb). 2019;9(1):51–70. PMID 30547302 · doi:10.1007/s13555-018-0278-6
Dakkak M, Forde KM, Lanney H. Hair Loss: Diagnosis and Treatment. Am Fam Physician. 2024;110(3):243–250. PMID 39283847

Telogen effluvium, the most common cause of hair shedding

Telogen effluvium is the most frequently misdiagnosed hair loss condition. Normal hair grows through three phases: anagen (active growth, 2–6 years), catagen (transition, 2–3 weeks), and telogen (resting/shedding, 3 months). At any time, about 85% of hair follicles are in anagen and 15% in telogen.

In telogen effluvium, a triggering event, physical stress, illness, surgery, rapid weight loss, childbirth (postpartum telogen effluvium is extremely common, typically peaking 3–4 months after delivery), or nutritional deficiency, causes a disproportionate number of follicles to simultaneously enter the telogen phase. When these hairs shed 3 months later, the patient notices alarming diffuse hair loss. The trigger is in the past, not the present.

This temporal mismatch is clinically important: blood tests taken at the time of hair loss often appear normal because the true trigger (e.g., an iron-deficient diet from 4 months ago) may have resolved. Detailed dietary history and thyroid function testing are the most useful initial investigations.

Blood tests that identify treatable causes of hair loss

Ferritin (iron stores): The most important blood test in hair loss investigation. Ferritin is an acute phase reactant and can be falsely elevated in inflammation, but a genuinely low ferritin (below 30–40 µg/L) is strongly associated with hair shedding, even when haemoglobin is normal. Many trichologists use a ferritin threshold of 70 µg/L as the target for optimal hair growth. Iron deficiency without anaemia causing hair loss is under-recognised.
TSH (thyroid function): Both hypothyroidism (producing diffuse thinning with dry, brittle hair) and hyperthyroidism (producing fine, fragile hair loss) cause hair shedding. TSH is always part of the standard hair loss screen.²
FBC: Anaemia from any cause, iron deficiency, B12/folate deficiency, haemolysis, causes telogen effluvium. Macrocytic anaemia with hair loss suggests B12 or folate deficiency.
Zinc: Zinc deficiency, common in restrictive diets, inflammatory bowel disease, and malabsorption, causes diffuse hair thinning.¹ Severe zinc deficiency also causes perioral and acral rashes and impaired wound healing.
DHEAS and testosterone (in women with suspected androgen excess): Androgenetic alopecia in women (female pattern hair loss) is associated with elevated androgens in some cases, particularly when accompanied by hirsutism (excess facial or body hair), acne, and menstrual irregularity suggesting PCOS.
ANA: Lupus can cause scarring alopecia (discoid lupus, permanent follicle destruction) or non-scarring diffuse loss in active systemic disease.

Medical Disclaimer: This page is for general educational purposes only and does not constitute medical advice. Always consult a qualified doctor before making any health decisions.