The essentials

Losing 50 to 150 hairs a day is normal. Heavy shedding that starts 3 to 5 months after an illness, a birth or a crash diet is usually telogen effluvium, and it grows back. Thinning, bald patches or other symptoms deserve a visit and, depending on the pattern, blood tests such as CBC, ferritin and TSH.

Doctors first look at the pattern: diffuse shedding, patterned thinning (a widening part, a receding hairline) and round bald patches have different causes[3]. There is no routine panel: tests follow your history and the scalp exam[3]. Below: which tests, and what your ferritin level really says.

When is hair loss urgent?

When to see a doctor

  • Emergency: call 911 if sudden hair loss comes with vomiting, confusion, numbness or severe weakness, especially after possible contact with a poison such as arsenic, thallium or mercury[2].
  • Within 24 to 48 hours for a painful, swollen or oozing scalp patch (an infection that needs pills, not creams)[3], or patchy loss together with a new rash or a genital sore, which can point to syphilis[2].
  • At your next appointment for shedding lasting months, bald spots (scalp, beard, eyebrows), rapid loss in your teens or twenties, an itchy or scaly scalp, or thinning with acne, facial hair, irregular periods, fatigue or feeling cold[1].

Why is my hair falling out? Harmless causes first

  • Common Telogen effluvium (shedding). A fever, surgery, a birth, a crash diet or severe stress sends many hairs into their resting phase at once; they fall out 3 to 5 months later, all over the scalp[3]. It usually stops within 2 to 6 months, and regrowth shows within 6 to 12 months once the trigger is gone[3][4].
  • Common After pregnancy. Shedding peaks about 4 months after the birth; most women have their usual fullness back by the baby's first birthday[12].
  • Common Androgenetic alopecia (pattern hair loss). Hereditary and gradual: receding hairline or thin crown in men, widening part in women. About 80% of men show signs of it[1]. No blood test is needed[3].
  • Common Hair care. Tight braids or ponytails (traction alopecia), bleaching and relaxers[2].
  • Possible Medicines and supplements. Some medicines, and too much vitamin A or selenium, cause shedding. Ask the prescriber before stopping anything[2][7].

Which diseases and deficiencies cause hair loss?

  • Possible Thyroid disease. An under- or overactive thyroid can thin the hair; treatment can reverse it[2]. See hypothyroidism symptoms.
  • Possible Low iron stores. Iron deficiency is often checked, especially with heavy periods; its exact role is debated[6][7].
  • Possible Low vitamin D, zinc or protein. Very restrictive or rapid weight-loss diets are classic triggers[2][4].
  • Possible Androgen excess in women. PCOS is the most common cause; stopping some birth control pills can also play a role[2].
  • Possible Alopecia areata. The immune system attacks follicles, leaving smooth round patches that often regrow on their own[3].
  • Rare Syphilis, lupus, scarring alopecias, ovarian or adrenal tumors. Hair lost to syphilis often regrows after treatment[2][1]. Scarring alopecias need a dermatologist and often a biopsy[3][5].

Which blood tests check for hair loss causes?

For diffuse shedding, a common first round is CBC, ferritin and TSH, sometimes a metabolic panel[4][5]. The rest depends on your story.

Blood tests a doctor may order for hair loss
TestWhat it answersUsually when
Complete blood countIs there anemia?Diffuse shedding
Ferritin (± iron panel)Are iron stores low?Shedding, heavy periods
TSHIs the thyroid under- or overactive?Shedding, fatigue
25-hydroxy vitamin DIs vitamin D low?Shedding, alopecia areata, little sun
Zinc, vitamin B12Is there a specific shortage?Restrictive diet, gut disease
Testosterone, DHEA-SAre androgens high?Women with acne, facial hair, irregular periods
Syphilis test (RPR or treponemal)Is it syphilis?Patchy loss, rash, risk exposure

Pattern hair loss in men needs no test[3]. In women, hormone tests are added when there are signs of androgen excess[4]; testosterone is drawn in the morning, between 7 and 10 a.m.[11]. Routine zinc screening is not recommended for ordinary shedding[7]. A dermatologist may add a pull test or trichoscopy (a magnified scalp exam)[5].

Shedding for months and new results? BloodSense explains your whole report line by line, in plain words, so you know what to ask your doctor.

Explain my results

How do I read my hair loss blood test results?

Fictitious example of a lab report, woman aged 34 with shedding (United States format)
TestResultReference rangeFlag
Hemoglobin12.6 g/dL12.0-15.5 g/dL
Ferritin18 ng/mL13-150 ng/mL
TSH2.1 mIU/L0.4-4.5 mIU/L
Vitamin D, 25-hydroxy17 ng/mL30-100 ng/mLL
  1. Hemoglobin is normal: no anemia.
  2. Ferritin has no flag, yet 18 is near the bottom. Below about 30 ng/mL, iron stores are probably low[7].
  3. TSH is normal: a thyroid cause is unlikely.
  4. L means below this lab's range. The National Academies consider 20 ng/mL enough for most people; deficiency risk rises below 12[10].

Ferritin and hair: which level is "enough"?

Labs call women's ferritin normal from about 13 ng/mL[9]. Some specialists aim for more than 40, or 70 in severe shedding, without consensus[7]. In a Duke study of 457 women, ferritin of 40 or less was as common in women without hair loss as in those with it[6]. Iron is clearly treated with anemia; without anemia it is controversial, though some clinicians aim for the upper part of the range[4]. Inflammation and liver disease raise ferritin and can hide low stores[9] (see the iron panel).

Vitamin D, zinc and biotin

A 2023 meta-analysis of 23 studies found vitamin D deficiency 3 times more often with non-scarring hair loss[8], but whether vitamin D, zinc or biotin pills help without a deficiency is unclear[4]. Biotin supplements can falsely raise or lower many lab results, thyroid tests included: tell the lab if you take them[7].

What treatments exist?

Treatment follows the cause: correct a confirmed deficiency, treat the thyroid or an infection, remove a trigger. Pattern hair loss responds to minoxidil (lotion or, off-label, low-dose tablets) and, in men, finasteride; results take 6 to 12 months and fade when treatment stops[3][4]. Alopecia areata may be treated with steroid injections or, when extensive, JAK inhibitor tablets such as baricitinib[5]. Acute shedding usually needs only reassurance[4].

What should I note before the appointment?

  • Start date, and any illness, fever, surgery, birth, diet or major stress 2 to 5 months before
  • Pattern: all over, widening part, receding temples, round patches
  • New medicines, stopped birth control, supplements (biotin, vitamin A)
  • Periods (heavy or irregular), acne, facial hair
  • Fatigue, feeling cold, weight change, vegetarian or very low-calorie diet
  • Family history of baldness; scalp photos over time

Frequently asked questions

What vitamin deficiency causes hair loss?

Low iron stores most often, sometimes zinc, vitamin D or protein; biotin deficiency is rare with a normal diet[7]. Supplements help when a shortage is confirmed.

Will my hair grow back after telogen effluvium?

Yes, in most cases: the follicles are not damaged. Shedding slows once the trigger is gone, and new hair becomes visible within 6 to 12 months[4].

My blood tests are normal. Why is my hair still falling out?

Normal results make deficiencies and thyroid causes unlikely. The two most common causes, pattern hair loss and shedding after a trigger, do not show in blood tests: they are recognized from your history and the scalp exam[3].

References

  1. MedlinePlus Medical Encyclopedia. Hair loss. medlineplus.gov (accessed November 7, 2026)
  2. American Academy of Dermatology. Hair loss: who gets and causes. aad.org (accessed November 7, 2026)
  3. Phillips TG, Slomiany WP, Allison R. Hair loss: common causes and treatment. Am Fam Physician. 2017;96(6):371-378. aafp.org (accessed November 7, 2026)
  4. Bergfeld WF, Mendez D. Shedding: an updated guide to a common cause of hair loss. Cleve Clin J Med. 2026;93(10):616-622. doi:10.3949/ccjm.93a.26008 (accessed November 7, 2026)
  5. Walker CW, Champigny D, Hartman T, et al. Hair loss: a systematic approach to evaluation, diagnosis, and management. JAAPA. 2026;39(9):27-34. doi:10.1097/01.JAA.0000000000000405 (accessed November 7, 2026)
  6. Olsen EA, Reed KB, Cacchio PB, Caudill L. Iron deficiency in female pattern hair loss, chronic telogen effluvium, and control groups. J Am Acad Dermatol. 2010;63(6):991-999. doi:10.1016/j.jaad.2009.12.006 (accessed November 7, 2026)
  7. 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. doi:10.1007/s13555-018-0278-6 (accessed November 7, 2026)
  8. Chen Y, Dong X, Wang Y, et al. Serum 25 hydroxyvitamin D in non-scarring alopecia: a systematic review and meta-analysis. J Cosmet Dermatol. 2024;23(4):1131-1140. doi:10.1111/jocd.16093. PubMed 38010941 (accessed November 7, 2026)
  9. MedlinePlus Medical Encyclopedia. Ferritin blood test. medlineplus.gov (accessed November 7, 2026)
  10. NIH Office of Dietary Supplements. Vitamin D: fact sheet for health professionals. ods.od.nih.gov (accessed November 7, 2026)
  11. MedlinePlus Lab Tests. Testosterone levels test; DHEA sulfate test. medlineplus.gov, medlineplus.gov (accessed November 7, 2026)
  12. American Academy of Dermatology. Hair loss in new moms. aad.org (accessed November 7, 2026)

This article is general health information. It does not replace a medical examination, diagnosis or treatment. Do not start iron or other supplements, or stop a medicine, without talking to your doctor.