Hair loss: which blood tests to run

You notice it first on the brush — more hair than usual. Then on the pillow. In the shower, it comes away in clusters rather than single strands. You start paying attention in a way you never had to before, and one question keeps surfacing: is this normal, or is something wrong?

Some hair loss every day is entirely normal. The hair follicle lives in cycles: a growth phase (anagen, two to six years) transitions into a rest phase (telogen, two to three months), after which the hair sheds and a new one begins to grow in its place. A healthy scalp loses fifty to a hundred hairs a day through this process — but they are continuously replaced, so the overall density stays stable.

The problem starts when that balance breaks down. More follicles enter telogen simultaneously — a condition called telogen effluvium — or the growth cycle shortens, or follicles gradually miniaturise under the influence of androgens in androgenetic alopecia. Telling these processes apart without investigation is not possible.

The threshold for screening: persistent heavy shedding for more than three months, visible thinning or widening of the parting, or a receding hairline — any of these warrants a blood panel. A screening blood test is the first step that helps rule out medical causes before looking for dermatological treatment. Hair loss often accompanies general fatigue; if that applies to you, the fatigue blood tests article covers the combined screening approach.

The screening minimum for hair loss

The core panel covers the most common, correctable causes. Here are the eight markers worth checking first.

Marker	What it measures	What an abnormal result suggests
Ferritin	Iron stores in the body	Low → the single most common correctable cause in women; follicle matrix cells can’t divide without iron
Haemoglobin / CBC	Oxygen transport and full blood picture	Low haemoglobin → iron-deficiency anaemia; microcytosis / anisocytosis → chronic iron depletion
TSH	Thyroid function	High → hypothyroidism; low → hyperthyroidism; both cause diffuse telogen shedding
Vitamin D	25(OH)D in serum	Low → deficiency; associated with accelerated shedding and impaired follicle regeneration
Zinc	Serum micronutrient level	Low → impaired keratin synthesis; particularly relevant in plant-based diets
Testosterone (total + free)	Androgenic status (men; women with hyperandrogenism signs)	Elevated free testosterone → androgenetic alopecia; in women, may indicate PCOS
DHEA-S	Adrenal androgen precursor (women)	Elevated → adrenal-origin hyperandrogenaemia
Ferritin: hair threshold	Optimal level for follicle cycling	Dermatology literature discusses 30–70 ng/mL as potentially needed for follicle function — the exact threshold is debated (higher than the standard laboratory “normal” lower bound)

One practical note: ferritin, CBC, TSH, and vitamin D can all be measured from a single venous blood draw. Zinc and sex hormones are drawn separately, if needed after the core panel.

Iron deficiency — the most common cause in women

Among women of reproductive age, iron deficiency remains the most common single correctable cause of diffuse hair shedding worldwide. The mechanism is direct: iron is required for rapid cell division in the follicle matrix — the most mitotically active tissue in the human body. When iron stores are depleted, the body redistributes what remains to higher-priority functions (haemoglobin, heart, brain), and the follicle works with whatever is left over.

At-risk groups: women with heavy periods, women who are pregnant or recently postpartum, people following vegetarian or vegan diets, and those with chronic gastrointestinal conditions affecting iron absorption.

The critical detail about ferritin thresholds: the standard lower limit of normal (often 10–15 ng/mL in laboratory reference ranges) is the threshold for anaemia diagnosis — not the threshold for optimal follicle function. Dermatology literature discusses functional ferritin thresholds in the 30–70 ng/mL range as potentially needed for optimal follicle function — the exact threshold is debated, so the standard laboratory “normal” lower bound is not always the same as the functional threshold. A result labelled “normal” at 18–25 ng/mL combined with symptoms is worth discussing with a doctor, not filing away as reassuring.

A full breakdown of ferritin reference ranges, what the number actually means, and how to interpret serial results is in the Ferritin: Iron Stores and What the Test Shows article.

Thyroid dysfunction

The thyroid gland regulates the metabolic rate of every cell in the body — and the hair follicle is no exception. In hypothyroidism (elevated TSH), the growth cycle slows: the anagen phase shortens, follicles stall in telogen, and two to four months after thyroid function first declines, diffuse, evenly distributed shedding becomes apparent. A useful additional sign: the outer third of the eyebrows also thins.

Hyperthyroidism (low TSH) causes telogen effluvium through the opposite mechanism — accelerated metabolic turnover — but the end result for the scalp looks similar: diffuse shedding and reduced density.

The important reassurance: when thyroid function is corrected with treatment, hair regrowth follows — but it takes six to twelve months for the new follicles to complete a full anagen cycle. Progress is real but gradual.

TSH is the first and most sensitive screening test. Free T3 and T4 are added when TSH is outside the reference range. A full guide to interpretation, subclinical hypothyroidism, and when treatment is indicated is in the TSH Test: What It Shows and What Is Normal article.

Hormonal imbalance

Androgenetic alopecia

Androgenetic alopecia is the most common form of persistent, progressive hair loss in both men and women. The mechanism: the enzyme 5-alpha-reductase converts testosterone into dihydrotestosterone (DHT), which binds to androgen receptors within the follicle and progressively shortens the growth cycle. With each successive cycle the hair shaft becomes finer and shorter, until the follicle becomes dormant.

In men, the pattern is characteristically fronto-temporal: the hairline recedes at the temples and the crown thins. In women, it presents as diffuse thinning over the crown with the frontal hairline preserved. Genetic susceptibility is the primary driver, but the level of DHT-sensitive androgens determines how fast the process unfolds.

The female hormonal context

Extended hormonal testing is appropriate for women when there are additional signs of hyperandrogenaemia — irregular menstrual cycles, acne, unwanted facial or body hair growth — or when PCOS is suspected:

Total and free testosterone — free testosterone is the more clinically sensitive marker, as it represents the biologically active fraction.
DHEA-S — elevation points to an adrenal rather than ovarian source of excess androgens.
Prolactin — elevated prolactin suppresses ovarian function and can indirectly contribute to hair changes.

Postpartum telogen effluvium is a separate, physiological, and self-resolving condition. During pregnancy, elevated oestrogen locks follicles in anagen — hair appears thicker. After delivery, oestrogen drops sharply, the retained follicles synchronise into telogen, and two to four months later they all shed at once. Peak loss typically occurs at three to six months postpartum; full recovery usually follows by twelve months. Blood tests at this stage are typically normal.

Menopause shifts the oestrogen-to-androgen ratio in favour of androgens, accelerating follicle miniaturisation in genetically susceptible women.

Vitamin D and zinc deficiency

Vitamin D

Vitamin D receptors are found directly in hair follicle cells — evidence that vitamin D plays an active role in regulating the growth cycle, not merely a passive one. Studies consistently show that mean 25(OH)D levels are significantly lower in women with telogen effluvium and female pattern hair loss compared to matched controls.

The test is 25(OH)D in serum. Vitamin D deficiency has been linked to changes in the hair growth cycle, though causality is still under discussion. For the 25-OH D reference ranges and the distinction between deficiency, insufficiency, and adequate levels (in both nmol/L and ng/mL), see the dedicated vitamin D guide. Across most of Central and Eastern Europe, seasonal deficiency at the end of winter affects an estimated 40–70% of adults.

More on reference ranges, seasonal patterns, and supplementation approaches is in the Vitamin D Levels: What’s Normal and What’s Not article.

Zinc

Zinc is involved in the catalytic activity of approximately a hundred enzymes and is directly required for keratin synthesis — the structural protein of the hair shaft. When zinc is deficient, the follicle matrix produces weaker, thinner hair shafts that break more easily and transition to telogen sooner.

At-risk groups: vegetarians and vegans (plant-based zinc is less bioavailable due to phytates), people with inflammatory bowel disease or other malabsorption conditions, and those on long-term proton pump inhibitors.

The test is fasting serum zinc. Reference range: typically 70–120 mcg/dL in routine labs; the NIH ODS classifies values below 70 mcg/dL in women or 74 mcg/dL in men as indicating inadequate status (healthy reference often cited as 80–120 mcg/dL). One caveat: serum zinc is a relatively late marker — it drops only after intracellular stores are substantially depleted. A normal result does not fully rule out functional deficiency in high-risk individuals.

If blood tests are normal

Normal results are still results. They rule out the most common medical causes and allow a more targeted understanding of what is actually happening.

Telogen effluvium after stress or illness. Any significant physiological shock — a severe infection, major surgery, extreme caloric restriction, or intense psychological stress — can synchronise large numbers of follicles into telogen simultaneously. The characteristic delay: noticeable shedding begins two to four months after the triggering event, by which time the cause may no longer be obvious. The condition is reversible — hair recovers after the stressor resolves — but the process takes six to nine months.

COVID-19-associated telogen effluvium. Documented separately in the literature: viral illness is one of the strongest known triggers of telogen effluvium. The mechanism combines fever, a systemic inflammatory response, and metabolic stress. By the time shedding is noticed, blood tests are typically back to normal.

Early androgenetic alopecia. If testosterone and DHEA-S are within the reference range but there is genetic susceptibility and a pattern-consistent distribution (crown thinning in women, temple recession in men), standard blood tests will not identify the cause — because it is genetic, not biochemical. This is where a dermatologist or trichologist referral is useful: dermoscopy can directly visualise follicle miniaturisation and distinguish androgenetic alopecia from other forms.

Dietary restriction. Crash diets, prolonged caloric restriction, and mono-diets are common triggers that do not appear in standard panels but respond predictably to normalising nutrition.

If shedding is accompanied by persistent fatigue, low energy, or brain fog — these may be connected, and a combined workup makes sense. The fatigue blood tests article covers the screening panel for that symptom cluster.

How HealthLab helps

Hair loss rarely resolves with a single test. Did ferritin climbing from 12 to 45 ng/mL after iron supplementation actually stop the shedding? How many months after TSH normalised on thyroxine did density visibly improve? These answers only become visible in a trend.

HealthLab automatically recognises biomarkers from PDF lab reports issued by any laboratory — ferritin, TSH, vitamin D, zinc, haemoglobin, testosterone — and builds a trend chart over time. You see at a glance whether a marker is moving toward or away from the target range after treatment begins. No manual data entry, no spreadsheets.

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Frequently Asked Questions

Is shedding 50–100 hairs a day normal?

Yes — this is within the physiological range. The follicle cycles continuously between growth and rest, and at any given moment roughly 10–15% of all follicles are in the telogen (resting) phase. Counting hairs on a brush is an unreliable diagnostic method: the result varies with hair density, how often you wash, and how carefully you count. The more useful signal is a change relative to your own personal baseline over eight to twelve weeks, combined with visible thinning or a change in the hairline — those are the indicators worth acting on.

How long does recovery take after a deficiency is corrected?

It depends on the cause. With iron deficiency, active shedding typically stabilises within three to four months of ferritin reaching an adequate level; density recovery takes six to twelve months — the full length of an anagen cycle. With hypothyroidism, the timeline after TSH normalisation is similar: six to twelve months. With vitamin D or zinc deficiency, positive change becomes visible within three to six months of adequate repletion. Recovery is real, but hair follicles are slow-moving structures. Consistent treatment and serial testing are more useful than watching the mirror week by week.

Should women have testosterone tested?

Not routinely, but yes in the right clinical context. Testosterone (and DHEA-S) are worth checking when there are additional signs of hyperandrogenaemia: irregular periods, acne, unwanted facial or body hair, or when the core screening panel (ferritin, TSH, vitamin D) does not explain the shedding. Female androgenetic alopecia with an androgen-driven component is common but often under-recognised — the diagnosis requires clinical context rather than a single number on a lab report.

Do hair vitamins work without testing first?

When a genuine deficiency is confirmed, targeted supplementation addresses the root cause — and the effect is real. When levels are already adequate, most “hair vitamins” do not meaningfully accelerate growth: additional iron or zinc beyond sufficiency does not speed up follicle cycling, and excess amounts can be harmful. The logic is straightforward: test first, then decide. Without measurement you do not know what — if anything — is actually missing, and you risk spending time and money correcting the wrong problem.

This material does not replace clinical advice. Interpreting blood test results always requires clinical context.