What's Actually Causing Your Hair Loss: The Hormonal Picture

If you've searched "why am I losing my hair," you've probably been handed a product before you were handed an explanation. That's backwards. "Male pattern" or "female pattern" baldness names where the hair is thinning — it doesn't tell you why, and the why matters, because some causes are permanent-but-treatable and others are reversible if you catch them.

This page is the honest version: what actually drives hair loss at the hormonal level, which of it is worth testing for, and — just as important — when a blood test won't tell you anything you don't already know.

"Pattern Baldness" Is a Description, Not a Diagnosis

Androgenetic alopecia (AGA) — the medical name for pattern hair loss — is the most common cause of thinning in both men and women. When a hairline recedes at the temples, or a part slowly widens, that pattern is what a clinician recognizes AGA by. It's diagnosed by how it looks and progresses, not by a number on a lab report.

That's the first thing worth understanding, because it explains why "get your hormones tested" is oversold. AGA is not usually a sign that your hormones are out of range. It's a sign that certain follicles are genetically sensitive to a normal hormone. Tests are useful for finding the other things that thin hair — and for ruling causes out — not for confirming the most common cause.

DHT and 5-Alpha-Reductase: The Well-Evidenced Core

The mechanism behind pattern hair loss is one of the best-established facts in the field. The enzyme 5-alpha-reductase converts testosterone into dihydrotestosterone (DHT), a more potent androgen. In follicles that are genetically susceptible, DHT gradually shrinks the hair over successive growth cycles — a process called miniaturization — until the strands become finer, shorter, and eventually stop covering the scalp (hormonal background of non-scarring alopecia review).

The strongest proof that this pathway is causal comes from blocking it. Finasteride inhibits 5-alpha-reductase, and in a two-year controlled trial it halted loss in 83% of men and produced regrowth in 66% (Kaufman 1998, PMID 9777765). That's the logic behind the standard treatments — and it's why the useful question is rarely "are my hormones high?" but "are my follicles sensitive, and do I want to intervene?"

Thyroid Dysfunction: A Reversible Contributor

The thyroid gland sets your metabolic pace, and hair follicles are metabolically demanding, so thyroid disease — both under- and over-active — can push hair into shedding. It typically shows up as diffuse thinning across the whole scalp (telogen effluvium) rather than a receding pattern, and it's often accompanied by other clues: fatigue, cold sensitivity, weight change, or dry skin.

Thyroid dysfunction is one of the causes worth actively excluding in a diffuse-shedding workup — a retrospective study found hypothyroidism associated with telogen effluvium and with more severe shedding, and recommended thyroid testing as part of the evaluation (thyroid & telogen effluvium, 2024). The reason this matters: unlike pattern hair loss, thyroid-driven shedding often reverses once the thyroid is treated. Telogen effluvium in general is a signal to look for an underlying trigger — endocrine, nutritional, or otherwise — rather than to start a pattern-hair-loss drug (telogen effluvium review).

Testosterone, SHBG, and Free-T: What the Numbers Mean

This is where the most testing money gets wasted. It feels intuitive that pattern hair loss means "too much testosterone" — but that's usually not the case. The evidence points to follicle sensitivity to DHT, not elevated circulating androgens, as the driver: many people with AGA have entirely normal testosterone, and studies comparing androgen levels in affected versus unaffected people have not found a reliable difference (hormonal background review).

Sex hormone-binding globulin (SHBG) matters mainly because it determines how much of your testosterone is "free" and biologically active — so a free-testosterone figure is more informative than a total. But for a typical man with a receding hairline and no other symptoms, a testosterone panel usually comes back normal and changes nothing. It becomes worthwhile when there are other signs — low libido, fatigue, infertility — or in women with signs of androgen excess (see below), not as a routine hair test.

Women: Postpartum and Perimenopausal Shifts

Hormonal hair loss looks different in women, and the triggers are more often about change than about baseline levels.

• Postpartum. During pregnancy, high oestrogen keeps hair in its growth phase; after delivery, oestrogen drops and a large cohort of hairs shifts into shedding at once. The result — heavy shedding two to four months after birth — is dramatic but usually temporary. It's covered in depth in our postpartum hair loss guide.
• Perimenopause and menopause. As oestrogen falls and its balance with androgens shifts, many women notice diffuse thinning or a widening part. This can overlap with female-pattern loss, which makes the picture harder to read from symptoms alone — and is exactly the situation where testing to rule out iron, thyroid, and vitamin D issues earns its keep.

Because finasteride is contraindicated in women who may become pregnant, and because reversible causes are more common in women, the diagnostic step matters more here than almost anywhere else.

Iron and Vitamin D: Common, and Commonly Overstated

Two deficiencies come up constantly, and both deserve an honest framing rather than the usual hype.

Iron (ferritin). Ferritin reflects your iron stores, and it can be low well before a standard blood count shows anemia. Low iron stores are more common in women with excessive hair loss (low iron stores, 2007), and a meta-analysis found women with non-scarring alopecia tend to have lower ferritin (systematic review, 2022). But the evidence is genuinely mixed — not every controlled study finds a difference — so the honest position is: correcting a real iron deficiency is worthwhile, especially in women, but a normal ferritin doesn't rule out pattern hair loss, and iron is not a regrowth treatment for people who aren't deficient.

Vitamin D. Vitamin D deficiency is highly prevalent in people with several kinds of alopecia (meta-analysis, 2024), and there's an inverse association between vitamin D levels and non-scarring hair loss. The catch, again, is honesty about direction: the evidence that supplementing vitamin D corrects hair loss is lacking (review, 2021). Treat a diagnosed deficiency because it's worth treating for your health — not as a proven hair cure.

Which Markers to Actually Ask For

If your situation warrants testing (see the next section), a focused panel beats a scattershot one. The markers with the best signal-to-noise for hair loss are ferritin, thyroid (TSH and free T4), vitamin D, and — in the right context — testosterone with SHBG. We break down each one, what it rules in or out, and at-home versus in-lab options, in our companion guide: which blood tests to get before treating hair loss.

When Testing Is Worth It — and When It Isn't

This is the part most affiliate sites skip, because "get tested" is easier to monetize than "you probably don't need to." Here's the honest line.

Testing is worth it when:

• The shedding is diffuse (all over) rather than a classic receding or crown pattern.
• It came on suddenly or is unusually heavy.
• You're a woman, where reversible causes are more common and finasteride isn't an option.
• You have other symptoms — fatigue, cold sensitivity, weight change, heavy periods, low libido — that point to thyroid, iron, or hormonal issues.

Testing usually isn't worth it when:

• You're a man with a slow, classic pattern (receding temples or thinning crown), no other symptoms, and a family history of the same. In that situation a hormone panel almost always comes back normal and doesn't change what you'd do next. Spending money to confirm "normal" is not the same as getting useful information.

A normal panel is information when a cause was plausible — it rules things out. It's just not information worth paying for when nothing pointed to a cause in the first place.

What To Do Next

If your loss is the classic slow pattern and nothing above raises a flag, the evidence-based path is treatment, not testing — start with our best minoxidil guide. If anything here did raise a flag — diffuse shedding, sudden onset, you're a woman, or you have systemic symptoms — the right first step is finding the cause, and some of those causes need a clinician, not a supplement.

Deciding between a test-first service and a prescribe-first one? We compare the two approaches in Hone Health vs Hims: diagnostics or just a prescription?

Sources

1. Kaufman KD, Olsen EA, Whiting D, et al. Finasteride in the treatment of men with androgenetic alopecia. Journal of the American Academy of Dermatology. 1998;39(4):578-589. PMID: 9777765.
2. Grymowicz M, Rudnicka E, Podfigurna A, et al. The Hormonal Background of Hair Loss in Non-Scarring Alopecias. Biomedicines. 2024;12(3):513. PMID: 38540126.
3. Thyroid dysfunction as a cause of telogen effluvium: a retrospective study. Medicine (Baltimore). 2024;103(1):e36803. PMID: 38181279.
4. Malkud S. Telogen effluvium: a review. Indian Journal of Dermatology, Venereology and Leprology. 2013;79(5):591-603. PMID: 23974577.
5. Rushton DH, Norris MJ, Van Neste D. Low iron stores: a risk factor for excessive hair loss in non-menopausal women. European Journal of Dermatology. 2007;17(6):507-511. PMID: 17951130.
6. Iron Deficiency and Nonscarring Alopecia in Women: Systematic Review and Meta-Analysis. Skin Appendage Disorders. 2022;8(2):83-92. PMID: 35415182.
7. Vitamin D deficiency in non-scarring and scarring alopecias: a systematic review and meta-analysis. Frontiers in Nutrition. 2024;11. PMID: 39416654.
8. Gerkowicz A, et al. Role of vitamin D in hair loss: a short review. Journal of Cosmetic Dermatology. 2021;20(11):3345-3351. PMID: 34553483.