Every morning a few hairs on the pillow. A clump on the brush. A tangle clogging the shower drain. Sound familiar? Hair loss is one of the most emotionally distressing health complaints, especially for women. And almost always, it is a signal that something measurable has gone wrong inside the body.
The good news: in the majority of cases, the cause is visible on blood work. According to dermatology data, about 80% of women with diffuse hair loss show low ferritin, low vitamin D, thyroid dysfunction, or a combination of these factors. The less encouraging news: the investigation takes time, and hair recovery takes even longer — follicles react with a 3-4 month delay.
This article provides a complete list of the blood tests you should order for hair loss and explains why each one matters.
Disclaimer: This information is for educational purposes only and does not replace professional medical advice. If you see distinct bald patches, scarring, scalp itching, or redness, see a dermatologist or trichologist right away rather than waiting for lab results.
Why Hair Falls Out — The Medical Causes
The hair follicle is one of the body's most resource-hungry structures. It needs iron, zinc, protein, B-vitamins, thyroid hormones, and balanced sex hormones. The moment the body senses a deficit, follicles are among the first to be sacrificed — shifted into a shedding phase to conserve resources for more critical systems.
The main causes of diffuse hair loss (hair thinning evenly across the entire scalp):
- Iron and ferritin deficiency — cause number one in women of reproductive age.
- Thyroid disorders — both hypothyroidism and hyperthyroidism drive hair loss.
- Vitamin D deficiency — highly prevalent in northern latitudes and during winter months.
- Hormonal imbalance — elevated androgens, low estradiol, postpartum changes, or discontinuation of oral contraceptives.
- Chronic stress — a powerful trigger for telogen effluvium, often appearing 2-3 months after a stressful event.
- Protein, zinc, B12, and folate deficiency — especially common in vegetarians and after restrictive diets.
- Autoimmune processes — certain rheumatologic conditions cause hair loss as a symptom.
Androgenetic alopecia (hereditary pattern hair loss in men and women) is a separate category. It also requires blood work, but the pattern is different: thinning in specific zones rather than uniformly.
The goal of laboratory testing is to determine which category your problem falls into.
Essential Tests — The Minimum Panel
This is the baseline panel that every woman and man with persistent hair loss should order. At minimum, four tests.
1. Ferritin and Serum Iron
Ferritin is the most important blood test for hair loss. In roughly 60% of women complaining of shedding, it will be below optimal.
- Lab reference range: 15-150 ng/mL for women, 30-400 ng/mL for men.
- For hair growth, ferritin should be at least 50 ng/mL, ideally 70-100 ng/mL.
This is a critical distinction: ferritin of 25 ng/mL is technically "within range," but at that level, hair will not grow — it will fall out. Latent iron deficiency develops gradually: hemoglobin is still normal, the CBC looks clean, but iron stores are depleted and follicles are starving.
Order alongside serum iron, transferrin, and transferrin saturation (TSAT) for a complete iron metabolism picture.
2. TSH, Free T4, Free T3
Thyroid hormones regulate the speed of hair growth. In hypothyroidism, hair becomes thin, brittle, slow-growing, and sheds heavily. In hyperthyroidism, hair falls out in large quantities.
What to order:
- TSH — reference range 0.4-4.0 mIU/L. For hair-loss patients, many endocrinologists target below 2.5 mIU/L.
- Free T4 — reference range 0.8-1.8 ng/dL.
- Free T3 — the biologically active hormone.
- Anti-TPO antibodies — add this on the first workup to rule out Hashimoto's thyroiditis. With autoimmune thyroiditis, shedding can begin long before TSH goes out of range.
For a thorough breakdown, see our thyroid test interpretation guide.
3. Vitamin D (25-OH)
Vitamin D is not merely a vitamin but a hormone that directly influences the hair growth cycle. Its receptors exist on hair follicles themselves. Deficiency is linked to active shedding and alopecia.
- Deficiency: below 20 ng/mL.
- Insufficiency: 20-30 ng/mL.
- Optimal for hair: 50-80 ng/mL.
In northern latitudes during winter, 40-70% of people are deficient. If you have not been tested recently, chances are it is time.
4. Complete Blood Count (CBC)
A CBC is needed to:
- Detect overt anemia (hemoglobin, RBC count, MCV).
- Review the WBC differential for signs of hidden inflammation.
- Check platelets and ESR — general health markers.
If the CBC reveals anemia (hemoglobin below 12 g/dL in women, 13 g/dL in men), that alone explains hair loss. If the CBC is normal but ferritin is low, you have latent iron deficiency that still needs correction. For a detailed walkthrough, see our CBC interpretation guide.
The Hormonal Panel
If the baseline block is normal but hair keeps falling, the next step is hormones. The panels differ for women and men.
For Women
When to draw: cycle days 2-5 (with a regular cycle), except progesterone — drawn on day 21-23.
- Total and free testosterone — elevated levels often correlate with PCOS and androgenetic alopecia.
- DHEA-S — the main adrenal androgen. Elevation points to an adrenal source of excess androgens.
- SHBG (sex hormone-binding globulin) — the lower the SHBG, the higher the free androgens at the same total testosterone. Reference range: 27-146 nmol/L.
- Free androgen index (FAI) — calculated as (total testosterone / SHBG) x 100. Normal for women is below 5%.
- LH and FSH — an LH/FSH ratio above 2 is a common marker of polycystic ovary syndrome.
- Estradiol — the primary female hormone. Drops postpartum, before menopause, and under stress.
- 17-OH progesterone — to exclude non-classic congenital adrenal hyperplasia.
- Prolactin — elevation can cause hair loss and menstrual irregularity.
For Men
Male hair loss is more often genetic (androgenetic alopecia), but a hormonal check is still warranted:
- Total and free testosterone.
- DHT (dihydrotestosterone) — the main "enemy" of hair in androgenetic alopecia. Testosterone itself is not the culprit; the enzyme 5-alpha-reductase converts it to DHT, which damages follicles in genetically predisposed men.
- Estradiol — elevation in men can also drive shedding.
- TSH — hypothyroidism in men is not rare.
Additional Tests
These are added when indicated — when baseline labs are normal or a comprehensive picture is needed.
Zinc
Zinc is critical for cell division in hair follicles. Deficiency is common in vegetarians and after strict diets.
- Reference range: 60-120 mcg/dL (11-20 mcmol/L).
- Symptoms of deficiency: hair loss, brittle nails, slow wound healing, frequent infections.
Vitamin B12 and Folate
B12 and folate work together in DNA synthesis and red blood cell production. When deficient, rapidly dividing cells — including those in the hair matrix — suffer first.
- B12: reference range 200-900 pg/mL. Optimal for hair is above 400.
- Folate: reference range 3-17 ng/mL.
- Add homocysteine — an integrated marker of the B12/folate cycle.
Magnesium
Magnesium participates in over 300 enzymatic reactions. Its deficiency often masquerades as stress and fatigue but also affects hair. Reference range: 1.7-2.2 mg/dL.
Chemistry Panel
A standard metabolic panel (ALT, AST, bilirubin, creatinine, BUN, total protein, albumin) reveals liver, kidney, and protein status. Low albumin signals dietary protein deficiency, which directly weakens hair structure. For more, see our blood chemistry guide.
Autoimmune Markers
If baseline and extended labs are normal yet hair continues to shed aggressively, discuss further markers with your doctor: ANA (antinuclear antibodies), hsCRP, and condition-specific antibodies when an autoimmune disease is suspected.
How to Interpret Your Results
The hardest part of hair-loss labs is not getting the numbers but interpreting them in context. For example:
- Ferritin of 35 ng/mL is "normal" on the report, but for hair it is low — follicles are already starving.
- TSH of 3.5 mIU/L is "normal," but with shedding, weight gain, and cold intolerance, it is already a reason to investigate further.
- Vitamin D of 28 ng/mL is "insufficient," yet many labs do not flag it in red — even though it may be driving your hair loss.
Context matters: sex, age, symptoms, cycle phase in women, and how markers relate to one another. For a personalized breakdown, upload your results at Evallume — you will receive an interpretation that accounts for the reason you are getting tested and a prioritized list of what to discuss with your doctor.
When You Need a Trichologist
See a trichologist or dermatologist if:
- You have distinct bald patches with sharp borders (possible alopecia areata).
- Thinning follows a pattern — crown or temples in women, frontal recession in men.
- The scalp is itchy, flaking, or red.
- Hair falls out with white "bulbs" at the root (anagen effluvium — a serious signal).
- All blood work is normal yet shedding persists beyond 6 months.
A trichologist performs trichoscopy (examining follicles under a video microscope) and, if needed, a phototrichogram (objective assessment of hair growth phases). These are diagnostic tools that a primary care physician does not offer.
In most cases the algorithm is: get your blood work done, interpret it, and see your GP or endocrinologist. If labs are clean but hair keeps falling — then it is time for a trichologist. If fatigue accompanies the hair loss, also check the chronic fatigue blood test checklist — the overlap is substantial.
This article is for informational purposes only. It does not constitute medical advice, diagnosis, or treatment. Consult a healthcare professional for any medical concerns.
