What Can Cause Hair Fall?

Hair fall is normal. Most people lose between 50 and 100 hairs a day, and those hairs are replaced as part of the hair growth cycle. Noticeable thinning or shedding happens when something disrupts that cycle — the most common causes are genetics, hormonal changes, illness or surgery, certain medications, nutritional deficiencies, and stress. In many cases the trigger can be identified and addressed.

What surprises many people is how often the cause is temporary. A stressful event, a fever, or a crash diet can push large numbers of hairs into a resting phase at once. The shedding shows up months later, which makes the connection easy to miss.

What Can Cause Hair Fall?

Hair loss has more than one mechanism, and the mechanism usually points to the cause. Hair follicles cycle through growth (anagen), transition, rest (telogen), and shedding. When this cycle is disturbed, hair falls out faster than it regrows. The main drivers fall into a few groups.

• Genetics. Hereditary hair loss, called androgenetic alopecia, is the most common cause of progressive thinning in both men and women. It is driven by a sensitivity to the hormone dihydrotestosterone (DHT) in genetically prone follicles.
• Hormonal changes. Thyroid problems, pregnancy, childbirth, menopause, and polycystic ovary syndrome can all shift the hair cycle.
• Illness, surgery, and fever. A significant physical stressor can trigger widespread shedding weeks to months later.
• Medications. Some drugs used for cancer, depression, high blood pressure, arthritis, and acne can contribute to hair loss.
• Nutritional deficiencies. Low iron stores, low vitamin D, and inadequate protein intake are commonly assessed causes.
• Stress and scalp conditions. Emotional stress can contribute to shedding, and conditions like alopecia areata or scalp infection cause distinct patterns of loss.

The pattern matters. Diffuse shedding across the whole scalp points toward a systemic trigger. Loss concentrated at the temples and crown points toward genetics. Patchy, smooth bald spots suggest an autoimmune process. A doctor can usually narrow the cause from the pattern, history, and a few basic tests.

How Does the Normal Hair Cycle Work?

Understanding the cycle explains why hair loss often lags behind its trigger. Each follicle runs on its own clock, so hairs are not all in the same phase at once.

Most scalp hairs — roughly 85 to 90 percent — are in the growth phase, which lasts several years. A small percentage are in a short transition phase, and about 10 to 15 percent are resting. At the end of rest, the hair is shed and a new one begins growing in its place. Because hairs cycle independently, losing some every day is expected.

When a trigger pushes a large share of follicles into the resting phase at the same time — a condition called telogen effluvium — they shed together a few months later. This is why someone who had a high fever in spring may notice heavy shedding in summer. The hair was already committed to falling out before the person noticed anything.

Can Stress Cause Hair Loss?

Yes, but the relationship is more specific than most people assume. Emotional or physical stress can push hair follicles into the resting phase, leading to increased shedding. This is the telogen effluvium pattern, and it typically appears weeks to a few months after the stressful event.

The shedding is usually temporary. As stress resolves and follicles re-enter the growth phase, hair generally returns. The key word is generally — recovery depends on the stressor being removed and no other cause being present.

There is a separate and less common condition called trichotillomania, an urge to pull out one’s own hair. It is a recognized psychological condition, not a cosmetic habit, and it warrants clinical support rather than willpower.

Which Hormonal and Medical Conditions Cause Hair Loss?

Hormones and underlying disease are among the most treatable causes, which is why they are worth ruling out early. Several conditions reliably affect hair.

Thyroid disorders. Both an underactive and an overactive thyroid can cause diffuse hair thinning. The hair loss usually improves once thyroid levels are corrected, though it can take months.

Pregnancy and childbirth. High estrogen during pregnancy keeps more hairs in the growth phase. After delivery, estrogen drops and many hairs shift into rest at once, causing noticeable shedding in the months after birth. This is common and usually self-limiting.

Polycystic ovary syndrome (PCOS). PCOS involves elevated androgens and can cause thinning at the crown in women, often alongside irregular periods and acne.

Alopecia areata. This is an autoimmune condition in which the immune system attacks hair follicles, producing smooth, round patches of loss. It can affect the scalp, beard, or eyebrows.

Scalp infection. Fungal infection of the scalp can cause patchy loss with scaling or broken hairs. It is more common in children and requires medical treatment.

Can Medications and Treatments Cause Hair Loss?

Yes. Medication-related hair loss is a well-recognized effect of several drug classes. It is usually diffuse and reversible after the drug is stopped, but not always, and stopping a medication should only be done with a prescribing clinician.

Drugs commonly associated with hair loss include certain chemotherapy agents, some antidepressants, a number of blood pressure medications, some arthritis drugs, and certain acne treatments. Hormonal therapies and some anticonvulsants are also on the list. Not everyone who takes these drugs loses hair, and the risk varies by drug and dose.

If you suspect a medication is involved, the right move is to raise it with the prescriber. Do not stop a prescribed medicine on your own, especially one for blood pressure, seizures, or mental health. The prescriber can weigh alternatives.

Can Poor Nutrition Cause Hair Loss?

Nutritional shortfalls are a genuine and commonly checked cause of hair shedding. Hair is a fast-growing tissue and is sensitive to shortages of the building blocks it needs.

Iron. Low iron stores, even without anemia, are frequently linked to shedding in women. Ferritin, the storage form of iron, is the usual test.

Protein. Hair is made largely of protein. Severe or prolonged protein restriction can push follicles into rest.

Vitamin D and zinc. Low levels of both have been associated with hair loss in some studies, though the evidence is not as strong as for iron. Testing is reasonable when deficiency is suspected.

There is an important caution here. Taking supplements when you are not deficient does not reliably regrow hair, and some supplements carry risks in excess. The sensible approach is to test first and treat an actual deficiency, ideally with clinical guidance.

What About Crash Diets and Rapid Weight Loss?

Rapid weight loss is a well-documented trigger for shedding. Crash diets and very low-calorie plans can shortchange the body of protein, iron, zinc, and other nutrients at the same time as they impose physical stress. The combination commonly produces telogen effluvium.

The shedding usually shows up a few months into or after the diet. Gradual, nutritionally adequate weight loss is less likely to trigger it than extreme restriction. This is one area where the fix is straightforward in principle — restore adequate nutrition — though hair recovery still follows its own timeline.

When Should You See a Doctor About Hair Loss?

See a clinician if the loss is sudden, patchy, accompanied by scalp itching or scaling, or comes with other symptoms like fatigue, weight change, or irregular periods. Rapid, widespread shedding or loss that continues for months also warrants evaluation.

Signs that point toward a specific cause rather than ordinary shedding include smooth bald patches, broken hairs, redness or scaling of the scalp, and loss of eyebrows or body hair. These are not typical of simple telogen effluvium and deserve a proper look.

A clinician can often narrow the cause with history, a scalp exam, and basic blood tests such as thyroid function, ferritin, and vitamin D. Not every case needs every test, and treatment depends entirely on what is found. There is no single remedy that addresses all causes, which is why guessing at treatments tends to disappoint.