Hair loss has dozens of possible causes, but most cases come down to a handful of common triggers: genetics, hormonal changes, stress, nutritional gaps, medical conditions, or physical damage to the follicle. Two-thirds of American men have noticeable hair loss by age 35, and by 50, roughly 85% have significant thinning. Women experience hair loss too, though the pattern and causes often differ. Understanding which type you’re dealing with is the first step toward knowing whether it’s reversible.
Genetics and Hormones: The Most Common Cause
Androgenetic alopecia, commonly called male or female pattern baldness, accounts for the vast majority of hair loss. It’s driven by a hormone called dihydrotestosterone, or DHT, which is converted from testosterone in your body. DHT binds to receptors on hair follicles and gradually shrinks them, a process called miniaturization. Over time, affected follicles produce thinner, shorter, lighter hairs until they stop producing visible hair altogether.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it tends to appear as a widening part or overall thinning across the top of the scalp while the hairline stays intact. The condition is inherited, and having close relatives with pattern baldness on either side of the family increases your risk. It can begin as early as the late teens or early twenties, though it progresses slowly over years or decades.
Stress-Related Shedding
A condition called telogen effluvium is responsible for the dramatic hair shedding many people notice after a major physical or emotional stressor. Normally, only a small percentage of your hair is in its resting phase at any given time. After a trigger like surgery, severe illness, rapid weight loss, or intense emotional stress, up to 70% of actively growing hairs can prematurely shift into the resting phase. Those hairs then fall out two to three months later, which is why many people don’t connect the shedding to its original cause.
The good news is that acute telogen effluvium typically resolves on its own within six months. The follicles aren’t damaged, so once the trigger passes, new growth resumes. The delay between the stressor and the shedding, and then the additional months before regrowth is visible, can make the whole process feel alarmingly long. But full recovery is the norm.
Pregnancy and Postpartum Changes
During pregnancy, elevated estrogen levels extend the hair’s growth phase, which is why many pregnant women notice thicker, fuller hair. After delivery, estrogen drops back to normal and all the hair that was “held” in the growth phase enters the shedding phase at once. This postpartum hair loss usually begins about three months after giving birth and resolves within 6 to 12 months. It can feel dramatic, with large clumps coming out in the shower or brush, but it’s temporary and doesn’t indicate any underlying problem.
Thyroid Problems
Both an underactive and overactive thyroid can cause hair loss. Thyroid hormones play a direct role in the hair growth cycle, and when levels are too high or too low, your body may shed hair excessively and even stop growing new hair altogether. The texture of your hair often changes too, becoming dry, coarse, and brittle. This type of shedding is typically diffuse, meaning it happens evenly across the scalp rather than in patches. Once thyroid levels are brought back into a normal range with treatment, hair growth generally resumes.
Iron and Nutritional Deficiencies
Low iron is one of the most overlooked causes of hair thinning, especially in women. You don’t need to be anemic for iron levels to affect your hair. Research suggests that when ferritin (the protein that stores iron in your body) drops below about 70 ng/mL, it may be too low to support a normal hair growth cycle, even if your standard blood counts look fine. This is sometimes called nonanemic iron deficiency, and it’s common in women with heavy periods, vegetarians, and frequent blood donors.
Other nutritional gaps that contribute to hair loss include deficiencies in zinc, biotin, vitamin D, and protein. Crash diets and extreme calorie restriction are particularly notorious for triggering shedding because they can deplete multiple nutrients at once while also putting the body under physical stress.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition where the body’s immune system mistakenly attacks hair follicles. Normally, hair follicles have a kind of immune protection that shields them from attack. In alopecia areata, that protection breaks down, and immune cells swarm the follicle in such numbers that pathologists describe the pattern under a microscope as a “swarm of bees.” The result is nonscarring hair loss that typically appears as smooth, round bald patches on the scalp, though it can affect the face and body as well.
The condition is unpredictable. Some people experience a single episode with full regrowth, while others have recurring patches or more extensive loss. Nail changes like small pits or rough texture often accompany the condition. Newer treatments targeting the specific immune pathways involved have expanded options significantly in recent years.
Medications That Trigger Shedding
Several classes of medication list hair loss as a known side effect. High-dose retinoids (often prescribed for severe acne or psoriasis) are among the most common culprits. In one study, over 63% of patients taking higher doses of a retinoid for psoriasis experienced hair loss. Certain antifungal medications can also cause shedding, with one survey finding that 79% of patients on long-term voriconazole treatment developed scalp hair loss.
Mood stabilizers and seizure medications, particularly valproic acid, carry a roughly 11% incidence of hair loss. Lithium, blood thinners, beta-blockers, and some antidepressants can also contribute. The shedding from medications is usually reversible once the drug is stopped or the dose is adjusted, though it can take several months for regrowth to become visible.
Tight Hairstyles and Physical Damage
Traction alopecia is hair loss caused by repeated pulling or tension on the hair follicle. Cornrows, tight braids, locs, extensions, weaves, and tightly pulled ponytails or buns are the most common causes. Rollers worn to bed regularly and even hats or head scarves worn over tightly pulled hair can contribute.
Early warning signs include pain or stinging on the scalp, crusting, and a phenomenon called “tenting,” where sections of the scalp visibly lift up like a tent when hair is pulled. At this stage, the damage is still reversible if you switch to a looser style. But when pulling continues over months or years, the follicles scar over and you’re left with smooth, shiny bald patches where hair can no longer grow. This is one of the few types of hair loss that’s entirely preventable but permanently damaging if ignored.
Scalp Conditions and Infections
Seborrheic dermatitis, the condition behind persistent dandruff, can contribute to temporary hair shedding. A yeast called Malassezia lives on everyone’s skin, but when it overgrows in oily areas like the scalp, it breaks down skin oils into fatty acids that irritate the skin. This triggers itching, flaking, and inflammation. Over time, the irritation weakens the skin’s outer barrier, making it easier for the yeast to keep thriving. The hair loss associated with seborrheic dermatitis comes primarily from scratching and inflammation in affected areas, not from direct follicle damage, so it’s not permanent.
Fungal infections like ringworm of the scalp can also cause patchy hair loss, particularly in children. These infections typically produce scaly, red patches and broken hair shafts, and they respond to antifungal treatment with full regrowth once the infection clears.