Hormonal Causes of Hair Loss

Hair follicles are sensitive to several hormones, and shifts in those hormones can produce visible hair loss in different patterns depending on which hormone is changing and how. This page covers the main hormonal drivers of hair loss in adults: dihydrotestosterone (DHT) in pattern hair loss, estrogen shifts around pregnancy and menopause, thyroid disease, and androgen excess in conditions like polycystic ovary syndrome (PCOS). The categories overlap and sometimes coexist, but each has a recognizable signature.

DHT and pattern hair loss

The most common hormonal cause of hair loss in adults is sensitivity of scalp follicles to dihydrotestosterone (DHT), the more potent androgen produced from testosterone by the enzyme 5-alpha-reductase. Pattern hair loss (androgenetic alopecia) develops when genetically susceptible follicles, concentrated on the top, front, and crown of the scalp, are exposed to DHT repeatedly over years. The mechanism shortens the active growth phase of the hair cycle and progressively shrinks the follicle, eventually to the point that it produces only fine vellus hair or stops producing visible hair entirely.

What is important to understand about pattern hair loss as a hormonal cause is that the absolute level of DHT in the bloodstream is not the variable that matters. Two adults with identical DHT levels can have very different hair-loss outcomes depending on the receptor density and 5-alpha-reductase activity at their scalp follicles, which is genetically determined. Lowering DHT (the mechanism of finasteride and dutasteride) slows or halts the process in genetically susceptible scalps because it reduces the substrate driving the miniaturization, not because the original DHT level was abnormal.

The deeper biology lives on the DHT page. The genetic causes child page covers why follicle sensitivity is the variable.

Estrogen shifts in pregnancy and postpartum

Estrogen has a direct effect on the hair growth cycle: elevated estrogen lengthens the anagen (active growth) phase, so a smaller fraction of follicles rests at any given time. During pregnancy, estrogen levels rise substantially, and many pregnant people notice their hair feels thicker and fuller in the second and third trimesters as a result.

After delivery, estrogen drops sharply. The cohort of follicles that was being held in anagen by elevated hormone levels now exits together. About three months later, they shed in synchrony, producing the dramatic-looking shedding pattern known as postpartum telogen effluvium. The follicles are not damaged; they have been pushed off schedule, and they re-enter anagen on their own. Most postpartum hair loss recovers within 12 months.

The postpartum hair loss child page in the women cluster covers the timeline in detail, and the companion guide postpartum hair loss timeline covers the practical month-by-month picture.

Perimenopause and menopause

Estrogen levels also decline at menopause, and the decline can produce a slower, more sustained version of the hormonal shift that drives postpartum shedding. The mechanism interacts with the relative shift in the testosterone-to-estrogen ratio: as estrogen falls, the unopposed androgen exposure rises, which can drive female-pattern hair loss in genetically susceptible women.

The visible result is typically a gradual thinning at the central scalp and a widening part, sometimes with a degree of frontal recession (less prominent than the male temple recession but present). The onset is usually in the late forties or fifties and progresses over years.

Hormone replacement therapy (HRT) has variable effects on hair: in some women it preserves density by maintaining the estrogen side of the ratio, while in others it does not appreciably change hair-loss progression. Decisions about HRT are made for the broader symptom picture rather than for hair loss specifically. The menopause child page covers this in more depth.

Thyroid disease

The thyroid gland affects the hair cycle through a different mechanism than the androgen pathway. Both hypothyroidism (low thyroid function) and hyperthyroidism (high thyroid function) can produce diffuse hair shedding, distinct from pattern hair loss.

In hypothyroidism, the hair cycle slows. More follicles are in telogen for longer, and the new anagen hair grows in more slowly. The shedding pattern is diffuse, often with brittle, dry hair. Other features (fatigue, cold intolerance, weight gain, dry skin) usually accompany the hair changes.

In hyperthyroidism, the hair cycle accelerates abnormally. The same follicles may not have enough time to produce normal-length anagen hair before they cycle into telogen. The result is diffuse shedding, often with fine, brittle hair. Other features include heat intolerance, weight loss, palpitations, and anxiety.

Both forms respond to treatment of the underlying thyroid condition. The shedding usually improves within several months of restoring euthyroid status. The relevant labs are TSH, free T4, and sometimes free T3. The hair loss in women cluster covers the thyroid-and-hair-loss context in more depth, since thyroid disease is meaningfully more common in women.

Androgen excess (PCOS and adrenal causes)

Polycystic ovary syndrome (PCOS) produces elevated circulating androgens (including testosterone and DHT precursors), which can drive female-pattern hair loss alongside the other features of the syndrome: irregular menstrual cycles, hirsutism (unwanted facial and body hair), acne, and weight changes. The hair-loss pattern usually looks like classic female-pattern thinning at the central scalp and a widening part, though the temple area can also be affected.

PCOS is the most common cause of pathologic androgen excess in women of reproductive age. Other less common causes include congenital adrenal hyperplasia and androgen-secreting tumors of the ovary or adrenal gland; these are rare and present with more dramatic features.

Treatment of PCOS-related hair loss involves both managing the syndrome (often with hormonal contraceptives, anti-androgens like spironolactone, or insulin sensitizers like metformin) and treating the hair-loss component directly with topical or low-dose oral minoxidil. Our PCOS page covers the syndrome in more depth.

Low testosterone in men

Counterintuitive but important: low testosterone in men is generally not a cause of scalp pattern hair loss. The variable driving pattern loss is follicle sensitivity to DHT, not absolute androgen levels, so the small amount of androgen present at low-normal testosterone is more than enough to drive miniaturization in a sensitive follicle.

What low testosterone does affect is hair in androgen-dependent areas: body and facial hair tend to be thinner and grow more slowly in hypogonadism, the opposite of what people often expect for "low T." Diffuse scalp shedding is sometimes seen in low testosterone when the underlying picture also involves significant systemic illness, but the mechanism is the broader stress rather than the testosterone level itself.

Testosterone replacement therapy (TRT) in genetically susceptible men can paradoxically accelerate pattern hair loss by increasing the substrate available for DHT conversion. The Endocrine Society guideline lists hair loss among the recognized adverse effects of testosterone therapy in susceptible men (Bhasin et al., J Clin Endocrinol Metab, 2018). The companion guide does low testosterone cause hair loss covers the full picture.

Birth control and hair loss

Hormonal contraceptives can affect hair in either direction depending on the formulation. Pills containing progestins with high androgenic activity (older formulations, some norethindrone-based pills) can drive female-pattern hair loss in susceptible women. Pills with anti-androgenic progestins (drospirenone, dienogest) or those that increase sex hormone-binding globulin (which lowers free androgen) are usually neutral or mildly protective against pattern loss.

Hair shedding sometimes occurs when starting or stopping any hormonal contraceptive, mediated by the abrupt hormonal shift triggering telogen effluvium. It usually resolves within months.

The drug-induced hair loss cluster covers birth-control-related hair changes in more detail.

Putting the hormonal causes together

The hormonal causes of hair loss share a common feature: they shift the balance of factors that determine how follicles cycle. Pattern hair loss shifts the balance through follicle sensitivity to DHT. Postpartum and menopausal shedding shift it through estrogen decline. Thyroid disease shifts it through cycle-timing changes. PCOS shifts it through androgen excess. Each has its own diagnostic features and its own evidence-based treatment.

The most common practical mistake is to assume that hair loss is one hormonal problem with one hormonal answer. The correct first step is usually a focused medical history that includes timing, pattern, family history, and other symptoms. A telehealth or in-person assessment is the right place to sort the differential. Curekey's hair assessment is one way to start.