Hair growth stops when follicles shift from their active growing phase into a resting or dormant state. This transition can be triggered by hormones, stress, nutritional gaps, medical conditions, medications, and even physical damage to the scalp. Some of these causes are temporary and fully reversible, while others lead to progressive or permanent thinning.
How the Hair Growth Cycle Works
Every hair follicle cycles through three phases: an active growth phase (anagen) lasting two to seven years, a brief transition phase (catagen), and a resting phase (telogen) lasting about three months. At any given time, roughly 85 to 90 percent of your scalp hairs are in the growth phase. When something disrupts this balance, more follicles get pushed into the resting phase prematurely, and fewer hairs are actively growing. The result is thinning, shedding, or both.
A wide range of factors promote this shift from growth to rest: inflammation, hormonal changes, stress, nutritional deficiency, poor sleep, and certain medications. Understanding which factor is at play matters because the fix depends entirely on the cause.
Hormones and DHT
The most common reason hair growth slows or stops is androgenetic alopecia, commonly called pattern baldness. It affects both men and women and is driven by a hormone called DHT (dihydrotestosterone). An enzyme called 5-alpha reductase converts testosterone into DHT, which then binds to receptors in genetically susceptible hair follicles on the scalp.
What happens next is a slow, cyclical shrinking process. DHT speeds up the growth phase so the follicle can’t reach its full size, then pushes it into rest sooner. With each cycle, the follicle gets a little smaller, producing thinner, shorter hairs until eventually only fine, nearly invisible “peach fuzz” remains. This is called follicle miniaturization, and it’s why thinning from pattern baldness is gradual rather than sudden.
The same hormone behaves completely differently depending on the body site. In areas like the beard, chest, and underarms, DHT enlarges follicles and promotes thicker hair. On the scalp, it does the opposite. This paradox explains why people with conditions like PCOS (polycystic ovary syndrome) can experience both excess facial hair growth and scalp hair thinning at the same time. In PCOS, elevated androgen levels accelerate follicle miniaturization on the scalp while stimulating hair growth elsewhere.
Stress and Telogen Effluvium
A major physical or emotional stressor can push a large percentage of your hair follicles into the resting phase all at once. This condition, called telogen effluvium, causes diffuse shedding across the entire scalp rather than in a specific pattern. Common triggers include high fever, serious illness, major surgery, significant blood loss, crash dieting, childbirth, and severe emotional distress. In one clinical study, fever accounted for 33% of cases, psychological stress for 30%, and systemic illness for 23%.
The shedding doesn’t happen immediately. There’s a delay of two to three months between the triggering event and noticeable hair loss, which is why people often don’t connect the two. The good news is that telogen effluvium is usually self-limiting. Hair shedding typically stops within three to six months after the trigger is removed, though cosmetically significant regrowth can take 12 to 18 months.
At the biological level, stress raises cortisol, which damages the protective structures surrounding the hair follicle by reducing their production and accelerating their breakdown. Stress also triggers the release of a signaling molecule called substance P, which activates inflammatory cells around the follicle and pushes it out of the growth phase early.
Nutritional Deficiencies
Hair cells are among the fastest-dividing cells in the body, which makes them highly sensitive to nutritional shortfalls. Iron deficiency is one of the most well-documented nutritional causes of slowed hair growth. Standard lab reference ranges flag iron stores (ferritin) as low only below about 20 ng/mL, but research suggests that optimal hair growth requires ferritin levels around 70 ng/mL. Treatment outcomes for hair loss improve significantly when ferritin is above 40 ng/mL, meaning you can be technically “normal” on a blood test and still not have enough iron to support healthy hair cycling.
Vitamin B12 also plays a role, with levels between 300 and 1,000 ng/L associated with better hair growth. Severe calorie restriction or crash dieting can trigger telogen effluvium even without a single nutrient deficiency, simply because the body deprioritizes hair production when energy is scarce.
Thyroid Disorders
Both an underactive and overactive thyroid can disrupt hair growth. Thyroid hormones directly influence hair follicle function: they extend the growth phase, stimulate the division of cells in the hair bulb, and even help maintain hair pigmentation. When thyroid hormone levels drop (hypothyroidism), cell division in the follicle slows, the growth phase shortens, and resting hairs fail to re-enter the growth phase on schedule. The result is diffuse thinning that affects the entire scalp rather than one specific area.
Because thyroid-related hair loss develops gradually and looks similar to other forms of diffuse thinning, thyroid function tests are a routine part of any clinical hair loss evaluation.
Autoimmune Hair Loss
In alopecia areata, the immune system attacks hair follicles directly. The body’s own immune cells, particularly a type of white blood cell called CD8+ T cells, infiltrate the follicle and release inflammatory signals that shut down hair production. This creates the characteristic smooth, round patches of hair loss that can appear suddenly on the scalp, beard, eyebrows, or elsewhere.
In the early stages, immune cells swarm around the follicle. In chronic cases, the follicles shrink (miniaturize), but the attacking immune cells remain. The follicles themselves are not destroyed, which is why regrowth is possible even after months or years of loss. Alopecia areata is unpredictable: some people experience a single episode with full regrowth, while others have recurring or expanding patches.
Medications That Halt Hair Growth
Certain drugs interfere with hair growth through two distinct mechanisms. Some cause an abrupt shutdown of cell division in the hair bulb, leading to rapid, widespread shedding within weeks. Chemotherapy drugs are the most well-known example, and this type of loss (anagen effluvium) can cause near-total hair loss because it targets follicles in their active phase.
Other medications trigger a gentler but still noticeable shift from growth to rest, causing gradual thinning two to three months after starting the drug. Drug classes associated with this pattern include blood thinners (anticoagulants), vitamin A derivatives (retinoids), interferons, and cholesterol-lowering medications. In most cases, hair regrows after the medication is stopped or adjusted.
Physical Damage to Follicles
Repeated tension on the hair, from tight ponytails, braids, weaves, extensions, or headwear, can cause a specific type of loss called traction alopecia. This condition follows a two-stage pattern. In the early stage, follicles become inflamed and hair density decreases, but the damage is nonscarring and fully reversible if the tension stops. Early warning signs include small bumps around the hairline, hair casts (white sleeves around the shaft), and broken hairs.
If the pulling continues over months or years, the inflammation leads to permanent scarring around the follicle. Once the stem cells at the base of the follicle are destroyed by this scarring process, no regrowth is possible. The transition from reversible to permanent is gradual, which is why early intervention makes a significant difference.
How Hair Loss Gets Diagnosed
Because so many different factors can stop hair growth, a clinical evaluation typically starts with the pattern and timeline of the loss. Gradual thinning that begins after puberty with a family history of baldness points toward androgenetic alopecia. Sudden diffuse shedding two to three months after a stressful event suggests telogen effluvium. Smooth, well-defined patches suggest alopecia areata.
Dermatologists use a handheld magnifying tool called a dermoscope to look for miniaturized hairs (a hallmark of pattern baldness) or fractured hairs with a tapered “exclamation point” shape (characteristic of alopecia areata). Blood work often includes thyroid function, a complete blood count, and iron studies, with ferritin being particularly important. A review of current medications helps rule out drug-induced causes. Biopsy is rarely necessary unless the diagnosis is unclear after these steps.
In women, pattern thinning is often first noticed during a bout of telogen effluvium, when a stressor like illness or hormonal change unmasks an underlying genetic tendency that might have gone unnoticed otherwise. This overlap means that sometimes more than one factor is stopping hair growth at the same time.