Thinning hair has dozens of possible causes, ranging from genetics and hormonal shifts to stress, nutrition, and even the way you style your hair. Losing between 50 and 150 hairs a day is normal. The concern starts when you notice more hair in the drain than usual, a widening part, or patches where hair once grew thick.
Most thinning falls into one of a few broad categories, and understanding which one applies to you is the first step toward doing something about it.
Genetics and Pattern Hair Loss
The single most common cause of thinning hair is androgenetic alopecia, better known as male or female pattern hair loss. It affects roughly half of all men by age 50 and a significant number of women as well, particularly after menopause. The process is driven by a hormone called dihydrotestosterone, or DHT. DHT binds to receptors on hair follicles and gradually shrinks them, a process called miniaturization. Over time, each affected follicle produces thinner, shorter, lighter hairs until it eventually stops producing visible hair altogether.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it’s more diffuse, usually appearing as a widening part line while the frontal hairline stays intact. The speed and severity depend on how sensitive your follicles are to DHT, which is largely inherited. Observations in men with a genetic inability to convert testosterone into DHT confirm that DHT is the key hormone behind pattern hair loss in men.
Hormonal and Thyroid Changes
Your thyroid gland plays a direct role in keeping hair in its active growth phase. Thyroid hormones stimulate energy production inside hair follicle cells and promote the release of growth factors that keep hairs growing longer before they shed. When thyroid levels drop too low (hypothyroidism) or climb too high (hyperthyroidism), hair can shift prematurely into its resting phase, leading to diffuse thinning across the scalp rather than in one specific area.
Other hormonal shifts can trigger thinning too. Pregnancy, childbirth, stopping or switching birth control pills, and menopause all change the balance of estrogen, progesterone, and androgens in ways that affect the hair cycle. Postpartum shedding is one of the most dramatic examples: hormone levels plummet after delivery, and many women notice significant hair loss two to four months later. This type of shedding is almost always temporary.
Conditions like polycystic ovary syndrome (PCOS) can raise androgen levels in women, producing a pattern similar to androgenetic alopecia. A blood test measuring androgen levels can help identify whether excess androgens are contributing to your thinning.
Stress and Telogen Effluvium
A major physical or emotional stressor can push a large percentage of your hair into its resting (telogen) phase all at once. Normally, only about 5 to 10 percent of your hair is in this phase at any given time. With telogen effluvium, roughly 30 percent of hairs stop growing and enter the resting phase before falling out. Common triggers include surgery, severe illness, high fever, rapid weight loss, a death in the family, or job loss.
The tricky part is timing. Because resting hairs sit in place for two to four months before falling out, you won’t notice the shedding until well after the triggering event. That delay often makes it harder to connect the hair loss to its cause. The good news is that telogen effluvium is typically self-limiting. Once the stressor resolves, most people see regrowth within six to nine months.
Nutritional Deficiencies
Hair follicles are among the most metabolically active cells in your body, and they need a steady supply of nutrients to function. Iron deficiency is the best-studied nutritional cause of thinning hair. In one case-control study of women aged 15 to 45, those with telogen effluvium had an average ferritin level (a measure of stored iron) of just 16.3 ng/mL, compared to 60.3 ng/mL in women without hair loss. You don’t need to be fully anemic for low iron to affect your hair. Ferritin can drop well before your red blood cell counts show a problem.
Other nutrients linked to hair thinning include vitamin D, zinc, and B vitamins, particularly biotin. Crash diets, restrictive eating patterns, and bariatric surgery are common triggers because they can deplete multiple nutrients at once. If your thinning is diffuse and you’ve recently changed your diet or lost weight rapidly, nutritional deficiency is worth investigating.
Medications That Cause Thinning
A surprisingly long list of medications can trigger hair shedding as a side effect. Chemotherapy drugs cause the most severe form, pushing hairs out of the active growth phase almost immediately. But many everyday prescriptions cause a subtler, slower thinning through telogen effluvium. Common culprits include:
- Blood thinners like heparin and warfarin
- Blood pressure medications including beta-blockers and ACE inhibitors
- Hormonal medications such as birth control pills and hormone replacement therapy
- Anticonvulsants like valproic acid (which causes hair loss in 12 to 28 percent of users, depending on dose) and carbamazepine
- Mood stabilizers and antidepressants, with lithium causing thinning in 12 to 19 percent of users
- Retinoids used for acne or psoriasis
- Cholesterol-lowering drugs, NSAIDs, and some antipsychotics
If your thinning started within a few months of beginning a new medication, that timing is a strong clue. Don’t stop a prescription on your own, but the connection is worth raising with your prescriber.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition in which the immune system attacks hair follicles. It looks different from most other causes of thinning. Instead of diffuse shedding, it typically produces discrete, smooth, round patches of hair loss on the scalp. Some people also lose eyebrow, eyelash, or beard hair. In severe cases, the condition can progress to complete loss of scalp hair or even all body hair.
Alopecia areata can start at any age and often comes and goes unpredictably. It’s distinct from pattern hair loss in that the follicles aren’t miniaturized by hormones. They’re suppressed by inflammation. Many people experience spontaneous regrowth, though the condition can recur.
Hairstyles and Mechanical Damage
The way you wear your hair can cause thinning if it puts constant tension on the follicles. This is called traction alopecia, and it’s caused by hairstyles that pull. Cornrows, tightly braided hair, tight buns and ponytails, hair extensions or weaves (especially on chemically relaxed hair), and even rollers worn to bed regularly can all contribute. The constant rubbing of a hat or headscarf over tightly pulled-back hair adds to the risk.
Early traction alopecia is reversible. You’ll notice thinning along the hairline or wherever the tension is greatest. But if the pulling continues long enough, the follicles scar over and the skin becomes smooth and shiny. At that stage, the hair loss is permanent. The American Academy of Dermatology recommends loosening any style that causes pain, bumps along the hairline, or visible thinning at the edges.
Aging Itself
Even without a genetic predisposition to pattern hair loss, hair naturally thins with age. Senescent alopecia, as it’s called clinically, is a separate process from hormone-driven thinning. It involves the gradual loss of stem cells from the hair follicle rather than DHT-driven miniaturization. The result is finer, slower-growing hair across the entire scalp rather than the patterned recession seen in androgenetic alopecia. This process accelerates after age 60 and affects nearly everyone to some degree.
How the Cause Gets Identified
Because so many conditions overlap in how they look, pinning down the cause of thinning hair often requires a physical exam and bloodwork. Doctors typically examine your scalp for the pattern and distribution of loss, which alone can distinguish pattern baldness from telogen effluvium or alopecia areata. A gentle pull test can gauge how actively you’re shedding.
If an underlying condition is suspected, common blood tests include ferritin (to check iron stores), thyroid-stimulating hormone (TSH), and androgen levels. If there’s scaling or broken hairs, a fungal culture may be ordered to rule out scalp ringworm. In ambiguous cases, a small scalp biopsy can reveal whether follicles are miniaturizing, inflamed, or scarred.
Knowing the cause matters because treatments vary widely. Hormone-driven thinning responds to medications that block DHT or stimulate growth. Nutritional deficiencies resolve with supplementation. Telogen effluvium from stress usually resolves on its own. Traction alopecia requires a change in styling habits. Getting the right answer early gives you the most options, particularly for causes that can become permanent if left unaddressed.