Hair Loss: Which Blood Tests Should You Get?

Hair loss has a reputation as a cosmetic concern, but dermatologists and endocrinologists treat it as a diagnostic clue. The hair follicle is one of the most metabolically active structures in the body — it needs steady nutrients, balanced hormones, and a functioning thyroid to cycle through growth, rest, and regeneration. When any of those systems falter, hair shedding (telogen effluvium) or thinning is often the earliest visible warning sign, sometimes appearing weeks or months before bloodwork would routinely flag the underlying issue.

Three categories explain the overwhelming majority of medically-driven hair loss:

• Nutritional deficiencies — iron, vitamin D, B12, zinc, and protein deficiencies are individually capable of causing significant shedding. Iron deficiency (low ferritin) is the most frequently identified cause in premenopausal women.
• Thyroid dysfunction — both hypothyroidism (underactive) and hyperthyroidism (overactive) cause hair loss. Hypothyroidism causes diffuse thinning across the scalp; hyperthyroidism causes finer, more fragile hair. Even subclinical thyroid dysfunction (mildly abnormal TSH with normal Free T4) is enough to trigger noticeable shedding.
• Hormonal imbalances — in men, declining testosterone with rising DHT contributes to androgenic alopecia (male-pattern baldness). In women, elevated androgens (from PCOS, adrenal hyperplasia, or perimenopausal hormone shifts) can cause female-pattern thinning, particularly along the part line and crown.

This is why a comprehensive blood workup before turning to topical treatments or supplements makes sense: many "permanent" cases of hair loss respond completely once an underlying ferritin, thyroid, or hormonal issue is identified and corrected. Treatment without diagnosis often means months of expensive products that don't address the actual cause.

Ferritin (iron stores). The most commonly identified cause of hair loss in women. Ferritin measures total body iron stores, and it depletes well before standard iron studies become abnormal. The standard "normal" lab range starts at 11-15 ng/mL, but dermatology research consistently shows that ferritin below 30 ng/mL is associated with active hair shedding, and ferritin below 50 ng/mL with delayed regrowth. The optimal range for hair health is generally 50-100 ng/mL. If your ferritin is in the teens or twenties and you're shedding, that's almost certainly contributing — and it's correctable in 3-6 months with oral iron supplementation.

TSH + Free T4 (thyroid function). A complete thyroid screen for hair-loss workup needs both TSH (the pituitary signal) AND Free T4 (the actual circulating hormone). TSH alone misses cases where TSH is "normal-ish" but Free T4 is at the low end of the range — a pattern that still causes hair shedding. Anything above TSH 2.5 mIU/L combined with Free T4 in the lower third of the reference range is worth investigating further with an endocrinologist or functional medicine practitioner. Hashimoto's thyroiditis (autoimmune hypothyroidism) is the most common cause of hypothyroidism in the US; if TSH is elevated, adding TPO antibodies confirms the diagnosis.

Vitamin D, 25-Hydroxy. Vitamin D receptors are present on hair follicles, and deficiency disrupts the hair growth cycle (specifically the transition from telogen/rest to anagen/growth phase). Studies show 40% of US adults are clinically deficient (below 20 ng/mL) and another 30% are insufficient (20-30 ng/mL). The optimal range for hair growth appears to be 40-80 ng/mL. Correcting vitamin D deficiency takes 8-12 weeks of supplementation (typically 5,000 IU daily of D3) to see a meaningful change in hair density.

Vitamin B12. B12 deficiency causes diffuse thinning, premature graying, and slow regrowth. It's especially common in vegans, vegetarians, people on long-term metformin or proton pump inhibitors, and adults over 60 (reduced absorption with age). The standard "normal" range starts at 200 pg/mL, but levels below 400 pg/mL are increasingly recognized as functionally deficient. Optimal range is generally 500-900 pg/mL.

Zinc. Zinc is a critical cofactor for keratin synthesis (the protein that makes up hair) and androgen metabolism. Zinc deficiency causes hair shedding, brittle hair, and slow regrowth. It's often overlooked because routine screening doesn't include it. If you're a vegetarian, eat a low-protein diet, or take long-term PPI medications, zinc deficiency is worth checking.

If the nutritional and thyroid workup comes back clean and hair loss persists, the next step is hormonal evaluation. The pattern of testing differs significantly by sex.

For men experiencing diffuse thinning or recession:

• Total + Free Testosterone with SHBG — low testosterone (especially low free testosterone after accounting for SHBG binding) causes diffuse thinning and reduced hair density. This is separate from androgenic alopecia (male-pattern baldness, driven by DHT genetic sensitivity); both can coexist.
• DHEA-S — adrenal androgen production; low levels can contribute to overall androgen deficiency picture.
• The Men's Hormone Panel ($159.99) covers Testosterone (Free & Total), SHBG, Estradiol, DHEA-S, Cortisol, Prolactin, and IGF-1 — the complete male hormonal picture in one draw.

For women experiencing hair thinning, especially with widening part or crown thinning:

• Total Testosterone + DHEA-S — elevated androgens cause female-pattern hair thinning. The most common cause is PCOS (polycystic ovary syndrome), which affects 8-13% of reproductive-age women and is dramatically underdiagnosed. PCOS-pattern hair loss often comes with acne, irregular periods, and metabolic issues — but can also present as hair thinning alone.
• Estradiol + Progesterone — particularly in perimenopause (typically late 30s through 40s), declining and erratic estrogen causes hair shedding. Progesterone decline starts even earlier and contributes to texture and density changes.
• The Women's Hormone Panel ($159.99) covers Estradiol, Progesterone, Testosterone (Free & Total), SHBG, DHEA-S, FSH, LH, Prolactin — the complete female hormonal picture.

Postpartum hair loss is a special case: it's nearly universal (occurring in 40-50% of women) and starts 2-4 months after delivery as estrogen levels drop dramatically. It typically resolves on its own within 6-12 months, but if it persists beyond a year or coincides with other symptoms (fatigue, weight changes), a full thyroid + hormone workup is warranted. Postpartum thyroiditis is also common (5-10% of women) and presents in the same window.

When to order blood tests: If you've been noticing more hair in the shower drain, on your pillow, or in your brush for more than 6-8 weeks — that's beyond the normal shedding range and worth investigating. Diffuse shedding across the entire scalp typically points to nutritional or thyroid causes (telogen effluvium). Patterned thinning (widening part, crown thinning, hairline recession) points more toward hormonal causes (androgenic alopecia). Either way, the blood workup is the same starting point.

Preparation: Morning draws (7-10 AM) are best for thyroid and hormone testing — TSH, testosterone, and cortisol all follow circadian rhythms and read more reliably early in the day. Fasting for 8-12 hours improves ferritin and iron accuracy. If you take biotin supplements (common in hair growth supplements), stop them 72 hours before the draw — biotin interferes with several hormone and thyroid assays and can produce false readings.

What to expect from your results: Many people are surprised to find one (or more) clear contributing factor on their first comprehensive workup. The most common findings in our experience are: ferritin below 30, vitamin D below 30, and TSH between 2.5-4.5 with Free T4 in the lower half of the range. Any of these alone is enough to cause active shedding; together, they multiply the effect.

Treatment timeline: Even with prompt correction of the underlying cause, hair regrowth is slow because of the hair growth cycle. Existing shedding from a current telogen effluvium episode will continue for 2-3 months after the trigger is corrected (hair already in the resting phase will still fall out). New growth takes another 3-6 months to become visible, and full restoration to baseline density takes 9-18 months. Patience is essential — and consistency with the corrective treatment (iron supplementation, vitamin D replacement, thyroid medication) for the full duration.

When to retest: 3 months after starting treatment for any deficiency, recheck the relevant markers to confirm levels are improving. Final retest at 6-12 months to confirm correction. If hair loss continues despite normal labs, dermatology referral is appropriate — there are causes beyond bloodwork (scalp conditions, autoimmune alopecia areata, scarring alopecia) that need direct evaluation.