Blood Tests for Hair Loss: What's Worth Checking

Why Blood Tests Matter for Hair Loss

Hair follicles are among the most metabolically active structures in the body. They require a constant supply of nutrients, hormones in the right balance, and adequate iron to sustain the growth phase (anagen). Any disruption — even months prior — can trigger telogen effluvium, where follicles prematurely shift into the resting phase and hair sheds in large quantities 2–4 months later.

This delayed relationship between cause and shedding is why blood tests taken at the time of shedding may look normal for some markers — the deficiency already occurred. Catching these imbalances and correcting them is the only way to restore hair density, but it requires knowing which tests to request.

The Core Hair Loss Blood Panel

Biomarker	Optimal Level	Why It Matters
Ferritin	70–150 ng/mL	Most common correctable cause of diffuse hair loss
TSH	1.0–2.5 mIU/L	Both hypo and hyperthyroidism cause hair shedding
Free T3	3.5–4.5 pg/mL	Active thyroid hormone — normal TSH can mask low T3
TPO antibodies	<35 IU/mL	Hashimoto's causes hair loss even before TSH shifts
Vitamin D	50–80 ng/mL	Vitamin D receptors are expressed in hair follicles
Zinc	80–120 µg/dL	Required for hair follicle cell proliferation
Total testosterone (women)	15–70 ng/dL	Elevated androgens drive androgenetic alopecia
DHEA-S	Age-appropriate range	Adrenal androgen excess can cause female hair loss
SHBG	40–120 nmol/L (women)	Low SHBG = more free androgens reaching follicles
Fasting insulin	<7 µIU/mL	Insulin resistance elevates androgens in women with PCOS
Vitamin B12	400–900 pg/mL	B12 deficiency impairs DNA synthesis in follicle cells
CBC (full blood count)	Normal ranges	Rules out anaemia and detects iron deficiency

Ferritin: The Most Overlooked Cause

Ferritin is the single most important blood test in the investigation of diffuse hair loss, particularly in premenopausal women. Research shows hair shedding increases significantly when ferritin falls below 50 ng/mL, and that hair density can only be fully restored when ferritin is above 70–100 ng/mL.

The problem is that the lab reference range for ferritin starts as low as 12–15 ng/mL. A woman with ferritin of 18 ng/mL will be told her results are normal while experiencing significant shedding. Always ask for the actual number, not just "normal" or "abnormal."

Thyroid and Hair Loss

Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) cause diffuse hair shedding. In hypothyroidism, the entire hair cycle slows — follicles spend less time in the growth phase. In hyperthyroidism, follicles shift prematurely into the resting phase due to the metabolic acceleration.

A normal TSH does not rule out thyroid-related hair loss. Hashimoto's thyroiditis causes significant hair shedding in some people while TSH remains within the reference range, because TPO antibodies create autoimmune follicle damage. Free T3 is often low-normal in people with conversion issues — the active hormone that follicles need.

Hormonal Causes: Androgens and Hair Follicles

Androgenetic alopecia — pattern hair loss — is driven by dihydrotestosterone (DHT) binding to androgen receptors in follicles at the crown and temples, causing them to miniaturise over time. In women, this pattern is usually diffuse rather than the classic male pattern, and is associated with elevated free testosterone, low SHBG, or both.

PCOS is the most common cause of androgen excess in reproductive-age women. High insulin drives the ovaries to produce more testosterone, while low SHBG leaves more free testosterone available to act on hair follicles. Testing should include fasting insulin, testosterone (total and free), SHBG, DHEA-S, and LH:FSH ratio.

Nutritional Deficiencies and Skin Health

Beyond ferritin, zinc deficiency impairs keratinocyte proliferation and differentiation in the hair matrix. Zinc is often low in people following plant-based diets, those with gut absorption issues, or those under high stress. Vitamin D acts directly on follicle stem cells, and deficiency is associated with alopecia areata as well as telogen effluvium. Biotin deficiency is frequently cited in supplement marketing but is actually rare in adults eating a balanced diet; testing for biotin is generally not warranted unless there is a specific deficiency risk.

How to Get the Right Tests

Most GPs will order a basic thyroid panel and CBC if you present with hair loss, but may not automatically include ferritin, DHEA-S, free T3, zinc, or vitamin D. It helps to be specific: ask for a ferritin level, not just "iron studies." If a hormonal cause is suspected, a dermatologist or endocrinologist referral can ensure the full panel is run. Direct-to-consumer lab testing services also allow you to order these tests without a GP referral in many countries.