What Hormones Stimulate Hair Growth?

Hair growth is driven by a surprisingly long roster of hormones, and no single one acts alone. Androgens like testosterone and dihydrotestosterone (DHT) are the most powerful players, converting fine body hairs into thick, dark terminal hairs on the face, chest, and limbs. But the relationship between hormones and hair is full of contradictions: the same androgen that thickens a beard can thin the scalp. Thyroid hormones, estrogens, insulin-like growth factor 1, melatonin, and even vitamin D all influence how long a hair follicle stays in its active growth phase, while stress hormones and prolactin can push follicles into dormancy. Understanding which hormones do what, and where on the body they do it, is the key to making sense of hair growth patterns.

Androgens and the Hair Growth Paradox

Androgens, including testosterone, DHT, and their precursors like DHEAS and androstenedione, are the dominant hormonal drivers of hair growth on most of the body. They work by binding to receptors inside the dermal papilla cells of the hair follicle. In areas like the face, chest, and pubic region, androgens transform the small, pale, barely visible vellus hairs into the thicker, darker terminal hairs that appear during and after puberty. Most follicles also need an enzyme called 5-alpha reductase to convert testosterone into the more potent DHT before this transformation happens.¹

Here is the paradox researchers have spent decades trying to untangle: while androgens stimulate robust hair growth on the body and face, they can shrink and ultimately kill hair follicles on the scalp. This is the core mechanism behind androgenetic alopecia, the most common form of hair loss in both men and women. The difference lies in how individual follicles respond to the same hormonal signal. A follicle on the chin interprets DHT as a cue to grow bigger and darker; a genetically susceptible follicle on the crown of the head interprets the same molecule as a signal to miniaturize and eventually stop producing visible hair.²

Research has shown that in the balding scalp, androgens disrupt a critical growth-signaling pathway inside dermal papilla cells, effectively suppressing the signals that hair follicle stem cells need to differentiate and produce new hair. This explains why drugs that block androgen activity or inhibit 5-alpha reductase can slow or partially reverse scalp hair loss without stopping body hair from growing normally.³

Thyroid Hormones Keep Hair in Its Growth Phase

Thyroid hormones, specifically T3 (triiodothyronine) and T4 (thyroxine), play an underappreciated role in keeping hair follicles actively growing. Research on human scalp hair follicles cultured outside the body found that T4 increases the rate at which hair matrix cells divide, while both T3 and T4 reduce the rate at which those cells die off. T4 also extends the duration of anagen, the active growth phase, possibly by dampening production of a protein called TGF-beta2 that normally signals the follicle to stop growing.⁴

More recently, researchers tested whether thyroid hormones could stimulate hair growth when applied directly to the skin surface. When low doses of T3 were applied topically to organ-cultured human scalp skin, the percentage of hair follicles remaining in their active growth phase increased significantly compared to controls.⁵ This is consistent with the well-known clinical observation that people with hypothyroidism often experience diffuse hair thinning and that restoring normal thyroid levels usually allows hair to recover. The hair follicle appears to be genuinely dependent on adequate thyroid hormone supply to maintain its growth cycle at a normal pace.

Estrogens and the Menopause Connection

Estrogens generally support scalp hair retention, which is part of why many women notice fuller hair during pregnancy, when estrogen levels are exceptionally high. The hair follicle is an estrogen-sensitive tissue, and the presence of estrogen helps keep follicles in their active growth phase for longer than they would otherwise stay. This is not the same as directly stimulating growth the way androgens stimulate beard hair, but it effectively reduces shedding and maintains hair density.

The protective role of estrogen becomes most visible when it declines. During menopause, ovarian estrogen production drops while the relative proportion of androgens rises. This hormonal shift directly affects the hair follicle, leading to decreased hair density, thinner individual hair shafts, and changes in texture.⁶ The effect is a double hit: less estrogen protection combined with relatively more androgen influence on follicles that may already be genetically sensitive. This is why female-pattern hair loss often becomes noticeable around or after menopause even in women with no prior history of thinning.

Conditions involving excess androgens complicate things further. In polycystic ovarian syndrome, for example, elevated androgen levels can cause both unwanted facial and body hair growth (hirsutism) and thinning of scalp hair at the same time. Treatment can be tricky because therapies designed to promote scalp hair growth can sometimes worsen unwanted hair elsewhere on the body.⁷

IGF-1 and the Follicle’s Own Growth Signal

Insulin-like growth factor 1 (IGF-1) is not a classic hormone in the way most people picture one, but it functions as a powerful local growth signal within the hair follicle itself. Produced mainly by dermal papilla cells in hair-bearing skin, IGF-1 directly stimulates follicle cell division and encourages follicles to enter and stay in their active growth phase. It works partly by blocking the programmed cell death that would otherwise shorten the growth cycle, and it also boosts blood supply to the follicle by increasing production of a vascular growth factor called VEGF.⁸

IGF-1 sits at a crossroads between several hormonal systems. Growth hormone, produced by the pituitary gland, stimulates the liver and other tissues to produce IGF-1, which is why growth hormone levels influence hair. Interestingly, the relationship between growth hormone and scalp hair is not straightforward. In conditions of growth hormone excess such as acromegaly, people develop excessive body hair growth, yet recombinant growth hormone actually inhibited the growth of female human scalp hair follicles in laboratory studies, likely by triggering the same catagen-inducing protein (TGF-beta2) that thyroid hormones suppress.⁹ On the other end of the spectrum, people with dysfunctional growth hormone receptors (a condition called Laron syndrome) develop notable hair follicle defects and alopecia. So the growth hormone axis matters, but its effects on hair depend heavily on the dose and which follicles you are looking at.

Stress Hormones Suppress Hair Growth

If you have ever noticed hair thinning during or after a period of intense stress, there is good biology behind it. The stress hormone cortisol (and its equivalent in rodents, corticosterone) directly suppresses the activity of hair follicle stem cells. A landmark study in mice found that elevated corticosterone prolongs the resting phase of the hair cycle, keeping follicles dormant for far longer than normal. The mechanism involves the dermal papilla, the cluster of signaling cells at the base of each follicle: high stress hormones suppress the expression of a molecule called GAS6, which stem cells depend on as a signal to wake up and begin a new growth cycle.¹⁰

The encouraging finding from the same research was that restoring GAS6 expression could overcome the stress-induced block on hair growth, even when stress hormone levels remained high. This suggests the damage from chronic stress is not permanent but rather a suppression that can potentially be reversed if the right growth signals are restored. It also means that cortisol’s hair-inhibiting effect is not about killing follicles outright but about keeping them locked in a dormant state. Remove the stress signal, or bypass the downstream block, and the follicle can resume normal cycling.

Melatonin as a Hair Growth Promoter

Melatonin is best known for regulating sleep-wake cycles, but it has a separate and direct effect on hair follicles. Human hair follicles grown in culture show significantly faster growth when exposed to melatonin, and this effect appears to work through specific melatonin receptors on the follicle. Researchers confirmed the receptor-mediated mechanism by showing that a melatonin-blocking drug suppressed the growth-stimulating effect.¹¹

A small double-blind, placebo-controlled trial tested a topical melatonin solution on women with androgenetic alopecia and diffuse hair thinning. After six months of daily application, women in the melatonin group showed a significant increase in the proportion of actively growing (anagen) hairs in both the front and back of the scalp compared to placebo.¹¹ Melatonin’s role in seasonal coat changes in other mammals is well established, and some researchers think its effect on human hair may be a vestige of the same seasonal machinery. In many mammals, seasonal prolactin rhythms (which are themselves regulated by melatonin) control when the coat thickens and thins with changing day length.¹²

Prolactin Pushes Follicles Toward Rest

Prolactin, the hormone best known for stimulating milk production, has a hair-inhibiting effect that has been documented in both mice and humans. When high doses of prolactin were applied to actively growing mouse hair follicles in culture, the follicles prematurely exited their growth phase and entered the regression phase known as catagen, along with a drop in the number of dividing hair bulb cells.¹³ The same effect occurs in human scalp follicles: treatment with prolactin significantly slowed hair shaft growth and triggered premature catagen, along with increased cell death in the hair bulb.¹⁴

What makes prolactin especially interesting is that hair follicles both respond to it and produce it locally. The follicle does not just passively receive a hormonal signal from the pituitary gland; it generates its own prolactin and uses it in an autocrine loop that influences the timing of its growth cycle. Studies in mice lacking the prolactin receptor found that these animals molted earlier than normal mice, suggesting that prolactin normally acts as a brake on the transition from the resting phase to the next growth phase.¹⁵ For most people this is not clinically relevant, but it matters for anyone with abnormally elevated prolactin levels (hyperprolactinemia), which can contribute to hair thinning.

Prostaglandins and Vitamin D

Not every molecule that stimulates hair growth circulates through the bloodstream. Prostaglandins, which are lipid-based signaling molecules produced locally in tissues, have become a focus of hair research because different prostaglandins push hair follicles in different directions. Prostaglandin E2 (PGE2) has shown a clear growth-promoting effect: in experimental models, PGE2 increased the proportion of follicles entering the early growth phase and boosted the number of follicle stem cells.¹⁶ This is the same class of molecule behind the well-known side effect of eyelash growth with certain glaucoma eye drops. Prostaglandin D2, by contrast, has been implicated in suppressing hair growth on the balding scalp, making the prostaglandin balance a topic of active drug-development interest.

Vitamin D, specifically its active hormonal form calcitriol (1,25-dihydroxyvitamin D3), also promotes hair follicle activity. In mouse studies, calcitriol prolonged the active growth phase of hair follicles and enhanced the ability of dermal papilla cells and outer root sheath cells to divide and migrate, all through the vitamin D receptor.¹⁷ Severe vitamin D deficiency is associated with alopecia in some cases, and the vitamin D receptor itself is essential for normal hair cycling regardless of circulating vitamin D levels. The practical takeaway is that correcting a deficiency might help, but megadosing vitamin D beyond normal levels has not been shown to produce extra hair growth.

Clock Genes and Why Timing Matters

Hair follicles contain their own set of circadian clock genes, and these internal timers are influenced by hormonal signals from elsewhere in the body. Thyroid hormones and light exposure, for instance, can modulate the activity of clock genes within the follicle, and these genes in turn participate in regulating both hair growth and pigmentation.¹⁸ This creates a layered system in which the overall hormonal environment sets the broad conditions for growth, but local molecular clocks fine-tune the timing of when individual follicles enter or exit each phase of their cycle. Disrupted circadian rhythms from shift work or chronic sleep deprivation, for example, could theoretically interfere with the normal hormonal signaling that maintains healthy hair cycling, though clinical evidence of this is still limited.

What Gender-Affirming Hormone Therapy Reveals

One of the clearest real-world demonstrations of how hormones control hair growth comes from gender-affirming hormone therapy (GAHT). Transgender women taking estradiol and antiandrogens often see a reduction in facial and body hair and an improvement in androgenetic alopecia on the scalp, since the treatment reduces androgen activity. Transgender men taking testosterone typically develop thicker facial and body hair, and some also experience the onset or acceleration of male-pattern scalp hair loss.¹⁹

These outcomes are essentially a controlled experiment in hormonal influence. Shifting the androgen-to-estrogen ratio in one direction promotes facial hair and can thin the scalp; shifting it the other direction reverses both patterns. Despite these consistent observations, research into hair-specific outcomes for transgender and gender-diverse populations remains surprisingly thin.²⁰ The individual variability in response is substantial, with some people seeing dramatic changes in hair growth within months and others seeing much less effect after years of therapy. Genetics, age at the start of treatment, and pre-existing follicle sensitivity all modify the outcome.

Why Location on the Body Changes Everything

A question that comes up once people understand the basics is: why does the same hormone cause growth in one spot and loss in another? The answer lies in the dermal papilla, the tiny cluster of specialized cells at the base of each follicle that acts as the local interpreter of hormonal signals. Dermal papilla cells on the chin, chest, or limbs respond to androgens by ramping up signals that enlarge the follicle and thicken the hair shaft. Dermal papilla cells on the top of the scalp, in genetically susceptible people, respond to the same androgens by releasing growth-inhibiting signals that miniaturize the follicle over successive cycles.²

This location-specific programming is set by the genetics of the dermal papilla cells themselves, not by anything circulating in the blood. It is why two people with identical testosterone levels can have very different patterns of body and scalp hair. It also explains why hair transplants work: a follicle taken from the back of the scalp, where the dermal papilla cells are not programmed to respond negatively to androgens, will continue growing normally when moved to a balding area. The hormones in the blood remain the same, but the follicle brings its own instructions for how to read them.

The same principle applies across all the hormones discussed here. Thyroid hormones, estrogens, prolactin, and cortisol do not affect every follicle equally. Each follicle’s response depends on which receptors its dermal papilla cells express, how much of the relevant enzymes they contain, and what genetic programs are active in that specific skin region. The hormonal environment is shared; the follicle’s interpretation of it is not.