Dihydrotestosterone, commonly called DHT, is the primary hormone responsible for hair loss. It’s a potent form of testosterone that binds to receptors in hair follicles, gradually shrinking them until they can no longer produce visible hair. But DHT isn’t the only hormonal player. Cortisol, thyroid hormones, estrogen, prolactin, and even vitamin D all influence whether your hair stays in its growth phase or falls out prematurely.
How DHT Shrinks Hair Follicles
Your body converts testosterone into DHT using an enzyme called 5-alpha reductase. Everyone produces some DHT, but in people with a genetic predisposition to pattern baldness, the hormone latches onto receptors in scalp follicles and triggers a process called miniaturization. The follicle physically shrinks, producing thinner, finer strands with each growth cycle. Over months and years, affected follicles eventually stop producing visible hair altogether.
This is the mechanism behind male pattern baldness and the most common form of female pattern hair loss. The genetics determine which follicles are sensitive to DHT, which is why hair loss typically follows predictable patterns: a receding hairline and thinning crown in men, and a widening part in women. Follicles on the sides and back of the head are generally resistant to DHT, which is why those areas tend to keep their hair even in advanced baldness.
Cortisol and Stress-Related Shedding
Chronic stress raises levels of cortisol, the body’s main stress hormone. Research from Harvard’s Stem Cell Institute revealed how this works at the cellular level: cortisol acts on a cluster of cells beneath the hair follicle called the dermal papilla. Under high cortisol, these cells stop releasing a signaling molecule that normally activates hair follicle stem cells. Without that signal, stem cells can’t enter the growth phase to regenerate new hair.
The result is telogen effluvium, a condition where large numbers of hairs shift from active growth into a resting phase and then fall out. You typically notice increased shedding two to three months after a stressful event because that’s how long it takes for resting hairs to loosen and drop. The good news is that stress-related shedding is usually reversible once cortisol levels return to normal, though full regrowth can take six months or longer.
Thyroid Hormones: Too Much or Too Little
Your thyroid gland produces two key hormones, T3 and T4, that directly regulate the hair growth cycle. These hormones stimulate the proliferation of cells in the hair matrix, increase the production of growth-promoting factors like IGF-1, and prolong anagen, the active growth phase. When thyroid levels are balanced, your hair spends more time growing and less time resting.
Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can disrupt this balance. Low thyroid hormone levels cause hairs to prematurely exit the growth phase and enter the resting phase, leading to diffuse thinning across the entire scalp rather than in a localized pattern. The hair often feels dry and brittle as well. Thyroid-related hair loss typically reverses once levels are corrected with medication, though it can take several months to see the difference.
Estrogen, Menopause, and Postpartum Changes
Estrogen has a protective effect on hair follicles. It helps keep follicles in their growth phase and counterbalances the influence of androgens like DHT. When estrogen levels drop, as they do during menopause, androgens become relatively more active in the hair follicles even though their absolute levels haven’t changed. This shift can trigger thinning that looks similar to pattern hair loss, with hair becoming finer and less dense, particularly along the top of the scalp.
Pregnancy offers a vivid demonstration of estrogen’s role. During pregnancy, elevated estrogen keeps more hairs in the growth phase than usual, which is why many women notice thicker hair. After delivery, estrogen plummets, and all those extra hairs that were held in the growth phase shift to resting and then fall out at once. Postpartum hair loss typically starts about three months after giving birth and resolves on its own within six to twelve months.
PCOS, Insulin, and Excess Androgens
Polycystic ovary syndrome, or PCOS, connects hair loss to a cascade of hormonal imbalances. Many women with PCOS have insulin resistance, where the body’s cells don’t respond efficiently to insulin. The ovaries and frequently the adrenal glands overproduce androgens in response. These excess androgens circulate in the blood and act on hair follicles in the scalp, causing hairs to stop growing and eventually leading to thinning or balding patterns.
The same excess androgens can also stimulate coarse hair growth on the face and body, a condition called hirsutism. So PCOS can paradoxically cause hair to thin on your head while growing more aggressively elsewhere. Addressing insulin resistance through lifestyle changes or medication often helps reduce androgen levels, which can slow or partially reverse the scalp hair loss.
Prolactin’s Underrecognized Role
Prolactin, a hormone best known for its role in milk production, also influences hair growth. Elevated prolactin (hyperprolactinemia) inhibits hair shaft elongation and pushes follicles prematurely out of the growth phase. After thyroid disorders, hyperprolactinemia is the next most common hormonal trigger of telogen effluvium.
Prolactin also acts as a modifier of androgen metabolism. High prolactin levels increase free testosterone while decreasing the binding proteins that normally keep testosterone in check. This means elevated prolactin can amplify the effects of androgens on hair follicles, compounding the problem. Prolactin levels can rise due to certain medications, pituitary gland issues, or chronic stress.
Vitamin D and the Hair Growth Cycle
Vitamin D functions as a hormone in the body, and its receptor plays a direct role in initiating new hair growth cycles. Research in both mice and humans shows that inactivating the vitamin D receptor leads to alopecia. In mouse studies, animals lacking this receptor completely failed to restart hair growth after shedding, while those with a functioning receptor followed normal growth patterns.
Low vitamin D levels are common, particularly in people who live in northern climates or spend limited time outdoors. While severe deficiency is linked to hair loss, it’s worth noting that supplementing vitamin D when you’re already at normal levels won’t boost hair growth. A simple blood test can determine whether deficiency might be contributing to your thinning.
Targeting DHT for Treatment
Because DHT is the dominant hormone in pattern hair loss, most pharmaceutical treatments focus on reducing it. These medications work by blocking 5-alpha reductase, the enzyme that converts testosterone into DHT. Clinical data shows that a topical formulation can reduce scalp DHT levels by roughly 70%, while oral versions achieve around a 50% reduction. This slows or stops miniaturization in many people, and some see partial regrowth of thinned hair.
For hair loss driven by other hormones, treatment looks different. Thyroid-related shedding requires correcting the underlying thyroid imbalance. PCOS-related thinning often improves with strategies that lower androgen levels or address insulin resistance. Stress-induced shedding may not need medical treatment at all if the source of elevated cortisol resolves. Identifying which hormone is driving your hair loss is the critical first step, because the treatments are as different as the causes.