Why Is My Hair Thinning? The Most Common Causes, Explained

Updated 2026-10-08 · Green Elixir Editorial Team

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Hair thinning rarely has a single obvious cause, and the right next step depends entirely on which cause applies to you. A scalp oil will not fix low iron, and an iron pill will not change hereditary thinning. This guide explains the most common reasons hair thins, how they usually look, and when a doctor visit makes more sense than any product.

The hair growth cycle in brief

Each hair follicle cycles through phases. In the growth phase (anagen), a scalp hair grows for several years. A short transition phase (catagen) follows, then a resting phase (telogen) of a few months, after which the hair falls out and a new one starts. At any moment most of your hairs are growing, which is why the American Academy of Dermatology considers shedding of roughly 50 to 100 hairs a day normal.

Thinning happens when more hairs leave the growth phase early, when new hairs grow back finer, or when follicles are damaged.

What is the most common cause of thinning hair?

Hereditary pattern hair loss, also called androgenetic alopecia, is the most common cause in both men and women. In men it usually starts with a receding hairline or thinning crown. In women it more often shows as a widening part and reduced volume on top, while the front hairline stays in place. MedlinePlus explains that in female pattern hair loss, follicles gradually produce shorter, finer hairs.

Pattern hair loss is gradual and progressive. The options with the strongest evidence are topical minoxidil and, for men, prescription finasteride. See hair loss treatment options.

Why am I suddenly shedding so much hair?

Sudden, diffuse shedding is often telogen effluvium. A shock to the body, such as a high fever, surgery, childbirth, crash dieting, a severe illness or major emotional stress, pushes many hairs into the resting phase at once. DermNet notes that shedding typically starts about two to three months after the trigger, which is why people often do not connect the two.

Telogen effluvium usually settles once the trigger has passed, although it can take months for volume to return. If shedding goes on for more than about six months, a doctor should look for ongoing causes.

Can low iron cause hair loss?

Yes, iron deficiency is a recognized contributor to hair shedding, especially in women with heavy periods, people who eat little meat, and frequent blood donors. The NIH Office of Dietary Supplements lists tiredness, weakness and trouble concentrating among the signs of iron deficiency. A blood test can check iron stores. Do not take iron supplements without testing, because too much iron is harmful.

Can thyroid problems cause thinning hair?

Both an underactive and an overactive thyroid can lead to diffuse hair thinning. MedlinePlus lists thyroid disease among the causes of hair loss. Other clues include unexplained weight change, feeling cold or hot, changes in heart rate, and fatigue. Thyroid problems are diagnosed with a simple blood test, and hair often improves when the thyroid is properly managed.

What causes round bald patches?

Round, smooth bald patches often point to alopecia areata, an autoimmune condition in which the immune system attacks hair follicles. MedlinePlus Genetics describes it as patchy hair loss that can affect the scalp and other areas. Hair may regrow on its own, but a dermatologist can confirm the diagnosis and discuss treatment. Hair growth supplements and oils are not a substitute for that assessment.

Can hairstyles and hair care cause thinning?

Yes. Tight ponytails, braids, buns, extensions and weaves can pull on follicles over time and cause traction alopecia, often at the hairline and temples. Frequent bleaching, chemical straightening and high heat damage the hair shaft so it breaks, which looks like thinning even though the follicles are fine. Loosening styles early matters, because long-standing traction can cause permanent loss.

Can medications or hormones cause hair loss?

Many medicines list hair loss as a possible side effect, and hormonal changes play a large role. Starting or stopping birth control, pregnancy and the postpartum period, menopause and conditions such as polycystic ovary syndrome can all affect hair. Never stop a prescribed medication because of hair concerns without speaking to your doctor; there may be an alternative.

Supplements that can cause hair loss

Surprisingly, yes. The NIH Office of Dietary Supplements notes that very high intakes of vitamin A and selenium can lead to hair loss. Taking several hair or beauty supplements at once can push totals higher than you realize. Our vitamins for hair loss guide explains safe limits in plain terms.

How can I tell shedding from breakage?

Look closely at the hairs you lose. A hair shed from the root usually has a tiny white or clear bulb at one end and is full length. A broken hair has no bulb and is often shorter, with a blunt or frayed end. Shedding points toward the hair cycle, hormones, nutrition or stress. Breakage points toward hair care: heat, bleach, chemical treatments, rough brushing or mineral buildup. The fixes are very different, so this two-minute check is worth doing before you buy anything.

When should I see a doctor about thinning hair?

See a doctor or dermatologist if:

• Hair loss is sudden, patchy, or comes out in clumps.
• Your scalp is red, scaly, painful, itchy or has sores.
• You have other symptoms such as fatigue, weight change or irregular periods.
• Thinning started after a new medication.
• You are under 25 and thinning quickly, or you are worried about it.

Mayo Clinic also advises seeing a doctor if hair loss is distressing or if you want to pursue treatment. Early care tends to give better results for most types.

Where do hair growth products fit in?

Once medical causes are ruled out, some people want to add a scalp product or supplement. Use them as extras, not replacements for diagnosis. Our reviews of FoliPrime and HairVigor Vitality list what each official website states and what the research says about the ingredients, and the home page comparison shows them side by side.

Sources consulted: aad.org, medlineplus.gov, dermnetnz.org, ods.od.nih.gov, mayoclinic.org.