Hair is a low priority tissue for the body. When sleep, fuel, iron or thyroid signalling slip, the scalp is often one of the first places it shows, and the shedding usually appears months after the trigger rather than during it. That lag is why the cause is rarely obvious from the mirror alone.
The most common reasons for thinning hair are not lab findings at all. They are genetics, a recent physiological stressor and the way you have been eating, sleeping and training over the past few months.
Pattern hair loss is the single most frequent reason adults notice less density. It is driven by an inherited sensitivity of scalp follicles to androgens, which gradually shortens the growing phase and shrinks the hair shaft over successive cycles[1]. In men it tends to start at the temples and crown. In women it more often shows as a widening part and thinner ponytail with the frontal hairline preserved[2]. It is gradual, patterned and usually progresses over years rather than weeks.
Diffuse shedding is a different pattern. Here a large share of follicles are pushed into the resting phase at once, and the hair falls out a few months later when new growth pushes it free. Common triggers include a high fever or infection, surgery, childbirth, a sharp drop in calories, rapid weight loss, a period of poor sleep, heavy alcohol use and emotional stress that runs for weeks rather than days. Low protein intake and underfuelling relative to a heavy training load belong in the same category, because both reduce the raw material and the energy available for non essential tissue.
Medications are worth tracking too. Several drug classes are associated with shedding, including some used for blood pressure, mood, acne, anticoagulation and hormonal contraception. If the timing lines up with a new prescription, that is a conversation for the prescriber rather than a reason to stop anything on your own.
Life stage matters as well. Postpartum shedding is extremely common and usually self limiting. Perimenopause and menopause shift the balance between oestrogen and androgen signalling at the follicle, which is why many women first notice thinning in their forties and fifties[1]. In men, pattern loss can begin in the twenties.
Finally, mechanical and scalp causes are easy to miss. Tight braids, extensions and repeated heat or chemical processing cause traction loss along the hairline. Scalp inflammation, flaking and fungal infection can all reduce density, and patchy smooth bald areas suggest something different from pattern loss. These are things a clinician identifies by looking at the scalp, not by reading bloodwork.
Blood tests do not identify the cause of hair thinning on their own. They narrow the field by showing whether iron stores, thyroid signalling, nutrient status or androgen levels could be contributing alongside what the scalp exam and history already suggest.
| Marker | Why it is linked to this symptom | What a result can and cannot tell you |
|---|---|---|
| Ferritin | Reflects stored iron, and low stores are associated with increased shedding | A low result flags depleted iron stores worth discussing. It cannot confirm that iron is the reason, and it rises with inflammation |
| TSH | Thyroid hormone sets the pace of the hair cycle, and both underactive and overactive thyroid states are linked to diffuse thinning | An out of range result points toward thyroid function as a contributor. One value is not a diagnosis and usually needs repeat or additional testing |
| Vitamin D (25-OH) | Vitamin D receptors are present in the hair follicle and low status is reported more often in people with hair loss | Shows your current vitamin D status. It cannot establish that correcting it will change hair density |
| Zinc | Zinc supports keratin formation and follicle turnover, and deficiency is associated with hair loss | Indicates whether zinc status is low. Serum zinc is an imperfect reflection of body stores and shifts with inflammation and recent meals |
| Testosterone Total | Androgens drive the miniaturisation that causes pattern thinning | Gives the circulating total. Most people with pattern loss have ordinary levels, because follicle sensitivity matters more than the number |
| Dihydrotestosterone | DHT is the more potent androgen acting directly on the scalp follicle | Shows systemic DHT. It cannot measure what is happening inside the follicle, where local conversion occurs |
Typical lab reference ranges give a rough sense of where results sit.
| Marker | Commonly reported reference range |
|---|---|
| Ferritin | About 15 to 150 ng/mL (female), about 30 to 400 ng/mL (male) |
| TSH | About 0.4 to 4.5 mIU/L |
| Vitamin D (25-OH) | About 30 to 100 ng/mL |
| Zinc (serum) | About 60 to 120 µg/dL |
| Testosterone Total | About 300 to 1000 ng/dL (male), about 15 to 70 ng/dL (female) |
| Dihydrotestosterone | About 30 to 85 ng/dL (male), about 4 to 22 ng/dL (female) |
These are typical lab reference ranges, and they vary by laboratory, assay, age and sex. Always read your result against the range printed on your own report.
Low ferritin is one of the most frequently checked explanations for diffuse shedding, and research suggests iron status is associated with hair loss, although the evidence is not uniform across studies[3].
The mechanism is plausible. The hair matrix is one of the fastest dividing tissues in the body, and rapid cell division depends on iron dependent enzymes. When stores fall, the body prioritises red blood cell production and other essential functions, and follicles can shift earlier into the resting phase. Ferritin is the usual marker because it reflects stored iron rather than the iron circulating at that moment, so it drops before haemoglobin does. A systematic review of micronutrients in androgenetic alopecia found that iron and ferritin are among the most studied markers in this area, while noting that study quality and findings vary[4].
Plenty of things move ferritin besides iron intake. Heavy or prolonged menstrual bleeding, pregnancy, blood donation, gastrointestinal blood loss, coeliac disease and low intake on a plant heavy diet all pull it down. High volume endurance training can too. In the other direction, ferritin is an acute phase reactant, so infection, inflammation, obesity and liver issues can raise it into the normal range even when stores are genuinely low. That is why a ferritin result is read next to a full blood count and inflammatory markers rather than alone.
A low Ferritin result is worth discussing with a clinician, who can look for the reason stores are depleted instead of assuming the hair is the whole story.
Yes, thyroid dysfunction in either direction is a recognised cause of diffuse hair thinning, and TSH is the usual first test used to screen for it.
Thyroid hormone helps set the length of the growing phase of the hair cycle. When signalling is too low, hair growth slows, the resting fraction rises and the hair itself can become dry and brittle. When it is too high, the cycle speeds up and hair can shed more quickly than it is replaced. Thinning from thyroid causes is usually spread across the whole scalp rather than confined to the crown or part, and it often comes with other changes such as altered energy, temperature sensitivity, bowel habit, heart rate, mood or weight.
TSH is a pituitary signal, not a direct measure of thyroid hormone, which is why an out of range value is normally followed up with free T4 and sometimes free T3 and thyroid antibodies. Several things shift TSH without any thyroid disease. In people with autoimmune thyroid disease, levels can also drift over time.
An out of range TSH result is worth discussing with a clinician promptly, especially alongside symptoms. It is one possible reason for shedding rather than a confirmed explanation, and repeat testing is standard before anything is concluded.
Vitamin D is linked to the hair cycle through the vitamin D receptor, which is expressed in the follicle and appears to play a part in cycling the follicle back into its growth phase.
Reviews of vitamins and minerals in hair loss report that low vitamin D status is observed more often in people with several forms of hair loss, including pattern loss and alopecia areata, though the relationship is an association and does not establish cause[3]. A systematic review of micronutrients in androgenetic alopecia reached a similar conclusion, noting that studies are heterogeneous and that correcting a low level has not been shown to reliably restore density[4].
Vitamin D status moves with sun exposure, skin tone, latitude, season, body composition, malabsorption and some medications, so a low result is common and is not specific to hair. Testing is useful because it gives a clear picture of current status rather than a guess based on diet.
A Vitamin D (25-OH) result that sits below the standard range is worth reviewing with a clinician, who can consider the whole picture including bone health, immune function and energy, not the scalp alone.
Zinc deficiency is a recognised cause of hair loss, and it is one of the few nutrient links where the mechanism is fairly direct.
Zinc is a cofactor for the enzymes involved in protein synthesis and cell division, both of which the hair matrix relies on continuously, and it also has a regulatory role in the hair follicle. Reviews indicate that frank zinc deficiency is associated with hair loss along with skin changes, poor wound healing and altered taste[3]. Research on micronutrients and androgenetic alopecia has looked at zinc among other markers, with mixed findings across studies[4].
True deficiency is uncommon in people eating a varied diet, but it is more likely with malabsorption, inflammatory bowel disease, after bariatric surgery, with chronic alcohol use, in restrictive or very low variety diets and in people with high losses from sweat or diarrhoea. Serum zinc is an imperfect marker because levels shift with inflammation, recent meals, time of day and albumin, so the result is interpreted alongside those factors rather than read in isolation.
A low Zinc result is worth discussing with a clinician, who can look at intake, absorption and the other nutrient markers before deciding whether it is meaningful for your hair.
Total testosterone gives the circulating level of the main androgen, and androgens are the driver behind pattern hair loss, but the number is usually less informative than people expect.
In pattern hair loss, susceptible scalp follicles respond to androgens by shortening the growth phase and producing progressively finer, shorter hairs, a process called miniaturisation[5]. Most men with pattern thinning have testosterone levels that sit within the standard range, because the key variable is how sensitive the follicle is rather than how much hormone is circulating[2].
In women, the picture is different. A review of female pattern hair loss describes it as a condition where androgen status can be normal in many cases, so the diagnosis rests on the clinical pattern and not on a hormone value[1]. That said, when thinning appears alongside irregular cycles, new facial hair, acne or a deepening voice, androgen testing becomes more relevant and the result is worth discussing with a clinician promptly.
Testosterone also varies with time of day, sleep, acute illness, training load, body composition, alcohol and several medications, and the total figure includes hormone bound to carrier proteins. For that reason Testosterone Total is usually read alongside SHBG and other hormones rather than on its own.
Dihydrotestosterone is the more potent androgen at the scalp follicle, and it is produced locally when the enzyme 5 alpha reductase converts testosterone into DHT in the skin and follicle itself.
DHT binds the androgen receptor more tightly than testosterone and, in genetically susceptible follicles, shifts the hair cycle toward shorter growth phases and thinner shafts over successive cycles. Updated reviews of pathogenesis place this androgen receptor and 5 alpha reductase pathway at the centre of pattern hair loss, alongside inflammation and local signalling at the follicle[6]. The strongest indirect support for the model comes from the fact that reducing DHT production changes the course of the condition, which is why 5 alpha reductase inhibitors have been studied extensively for hair loss[7], including topical formulations currently under investigation[8]. Any prescription option in this category is a decision for a licensed provider, who evaluates whether it is appropriate for the individual after a full review.
A blood Dihydrotestosterone level reflects what is circulating, not what is happening inside the scalp follicle, where local conversion occurs. People with pattern loss often have ordinary systemic DHT, and people with high DHT do not necessarily lose hair. Levels also vary with age, body composition, medications affecting 5 alpha reductase and overall androgen status, so the result is context for the picture rather than an answer in itself. Therapy reviews consistently frame management as a long term, individualised decision made with a clinician[9].
A completely normal panel is common in hair thinning, and it does not mean nothing is happening. Blood tests measure systemic status, while hair loss is often a local problem at the follicle.
Several causes leave no trace in bloodwork. Inherited androgen sensitivity, receptor density and local 5 alpha reductase activity in the scalp cannot be read from a venous sample. Traction from tight styling, scarring forms of hair loss, scalp inflammation, seborrhoeic dermatitis and fungal infection are identified by looking at the scalp, sometimes with dermoscopy or a biopsy. Patchy, well defined bald areas suggest a different mechanism again and need a clinical assessment.
Timing also hides causes. Diffuse shedding typically appears a few months after its trigger, so by the time hair is falling the fever, the crash diet, the surgery or the stressful period may be long over and the markers back to normal. Low energy availability and low protein intake are history questions, not lab questions. Medication effects are identified by reviewing the timeline of prescriptions rather than by a blood result.
Finally, no blood test predicts how hair will respond to anything. Systematic reviews of management, including comparisons of scalp directed approaches such as platelet rich plasma, report variable results across study designs and protocols[10], and reviews comparing platelet rich plasma with established options note that methods and outcome measures differ widely between trials[11]. A meta analysis in women reached a similarly cautious conclusion, describing early and mixed evidence[12]. Much in this area remains uncertain, and a licensed provider is the person who decides whether any specific option fits a given individual.
Most hair thinning is not urgent, but some patterns should be assessed sooner rather than later. Book prompt care for sudden, heavy shedding that starts over days, for bald patches with smooth or scaly skin, and for any scalp pain, burning, pustules, crusting or visible scarring, because scarring loss is time sensitive.
Other reasons to be seen without waiting include thinning alongside unintentional weight loss, fever, night sweats, a racing or irregular heartbeat, marked breathlessness or new swelling. In women, hair loss appearing with new facial hair, acne, missed periods or voice deepening is worth assessing promptly. Signs of ongoing blood loss such as very heavy periods, black or bloody stools or persistent light headedness need attention in their own right, not just because of the hair. The same applies if shedding began soon after a new medication, which is a conversation for the prescriber before any change is made.
Seek urgent care for severe symptoms such as chest pain, fainting or difficulty breathing. Hair loss also affects mood for many people, and if it is driving persistent low mood, anxiety or withdrawal, that is worth raising with a clinician in its own right. If you are having thoughts of self-harm, call or text 988 (US) or your local emergency number now.
Nothing here is a diagnosis, and no result should be interpreted on your own. A clinician reads your labs alongside your history and an examination.
The Longevity Nutrients panel measures the markers described above, including ferritin, TSH, vitamin D, zinc, total testosterone and DHT, in a single blood draw, so the nutrient and hormone questions are answered together rather than one test at a time.
FitScript members have 150+ biomarkers tested per year, and results are followed over time rather than read once, which matters for hair because the trigger usually sits months behind the shedding. Atlas, FitScript's AI health advisor, explains each result in plain language, shows how it sits against the reference range and flags what is worth raising. A licensed clinician reviews the full picture and decides whether any treatment is appropriate for you.
This article is educational and is not medical advice. Discuss your results, your symptoms and any treatment with a clinician or licensed provider before acting on them.
Hair thinning means a visible loss of density, either because individual hairs are becoming finer or because more follicles are resting than growing. Everyone sheds hair daily as part of the normal cycle. Thinning is the point where the part widens, the ponytail feels smaller or the scalp becomes more visible.
Hair thinning in women most often comes from female pattern hair loss, which widens the part while sparing the frontal hairline, or from diffuse shedding after childbirth, illness, rapid weight loss or a stressful period. Low iron stores, thyroid dysfunction and the menopause transition are other common contributors worth checking with a clinician.
Hair thinning in men is usually pattern hair loss, driven by inherited follicle sensitivity to androgens, which typically recedes the temples and thins the crown. It is not the only cause. Diffuse shedding after illness, heavy training with underfuelling, thyroid dysfunction, low iron stores and some medications can all produce thinning that follows no pattern.
Ferritin and TSH are usually the two most informative starting points, because low iron stores and thyroid dysfunction are both associated with diffuse shedding. Vitamin D, zinc, total testosterone and DHT add context on nutrient and androgen status. Results are interpreted together with your history and a scalp examination, never alone.
Diffuse shedding usually appears a few months after its trigger rather than during it, because affected follicles must finish a resting phase before the hair releases. That delay is why people often cannot connect the shedding to the fever, surgery, crash diet or stressful stretch that preceded it. Timeline questions help a clinician identify it.
Rarely on their own. Low vitamin D status is associated with hair loss, and frank zinc deficiency is a recognised cause, but both links are uncertain and neither is a complete explanation. Testing clarifies your status, and a clinician decides whether the result is meaningful for your hair.
No. Normal results simply mean the systemic causes checked by those markers look unremarkable. Inherited androgen sensitivity, scalp inflammation, scarring forms of hair loss, traction from styling and the delayed effects of an earlier trigger leave no trace in bloodwork. A scalp examination and a detailed history fill that gap.
Seek prompt assessment for sudden heavy shedding, bald patches with smooth or scaly skin, scalp pain, pustules or visible scarring, and for thinning alongside unintentional weight loss, fever, palpitations or heavy bleeding. In women, thinning with new facial hair, acne or missed periods also warrants prompt review rather than waiting.