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What causes hair loss?

15 October 2021 | 9 mins read | What causes hair loss?

Did you know that it is normal to lose 50 to 100 strands of hair every day? When you consider that at any given moment we have between 100,000 and 150,000 strands of hair on our heads, you can imagine that 50-100 strands might not be missed.

But sometimes we lose more hair than that — perhaps 200 or more strands a day. That amount of hair loss can be noticeable and can even be a sign of an underlying medical condition.

What causes hair loss, or “alopecia”? What can you do about it?

How does hair grow?

To understand what can go wrong, it helps to first understand how hair grows under normal circumstances.

Each of our scalp hairs develops underneath the skin in its own tiny structure called a follicle. The follicle holds on to the base of the hair for several years as the hair grows longer and eventually falls out. Growing and falling out happen in phases. The two phases to know are:

Growth: At any given time, around 90% of our scalp hairs are growing. The growth phase lasts from around two to six years.

Rest: At any given time, around 10% of our scalp hairs have stopped growing. They will fall out within two to three months, ending the rest phase. Another hair will then develop from that same follicle in a new growth phase.

What can go wrong?

Many factors can lead to hair loss. For example:  

Hormone imbalances:

Hormones affect both the timing of the follicular phases and the health of the follicles. These can affect hair loss by:

  • Delaying or preventing the resting follicles from beginning a new growth phase. Hair gradually becomes sparser as individual hairs fall out and take longer to be replaced. Eventually follicles might stop producing new hair altogether.
  • Abruptly shifting a high percentage of follicles into the rest phase, leading to a lot of hair being shed some months later. This is a common occurrence in pregnancy. When hormones begin to normalize some weeks after delivery, the unusually high number of follicles that were resting suddenly shed their hairs in order to re-enter the growth phase.
  • Hair typically returns to normal within a year or so.
  • Interacting with genetics in a way that results in female pattern hair loss (FPHL). During and after menopause, around 30% of women will develop FPHL as a result of normal changes to the balance of androgen and estrogen. Hair loss is first noticeable at the part and at the temples, but eventually hair coverage on the rest of the scalp can also become sparse.
  • Triggering premenopausal FPHL along with excess body hair growth. Women with Polycystic Ovary Syndrome (PCOS) often have a high level of androgenic hormones. Androgenic hormones can cause hair thinning or loss in women prior to menopause. For more information, see Halza’s article about PCOS (add link here)

Nutrient deficiencies:

Too little intake of certain nutrients including essential fatty acids, iron zinc, selenium, and protein are likely to result in hair loss within a few months. Nutrient deficiencies can result from many things, including:

  • Long-term illness
  • Chronic disease
  • A highly restrictive or crash diet
  • Limited access to healthful food
  • Certain medications

When the nutrient deficiency is corrected, hair should re-grow.

Permanent damage or scar tissue:

If a follicle becomes permanently damaged or blocked by scar tissue, no new hair production or growth will occur from that follicle. Some examples of the cause of scar tissue include:

  • Untreated infection or inflammation
  • Trauma or surgery involving the scalp
  • High doses of radiation therapy to the head

Prompt treatment of inflammation or infection can sometimes prevent permanent damage.

Cancer treatment: 

Chemotherapy can send the follicles that are currently in the growing phase directly into the resting phase. This causes large clumps of hair to fall out within a few weeks of exposure to the drugs. Hair typically begins to grow back within 6 months of completion of treatment.


Alopecia areata is an autoimmune disease in which the body’s own immune system attacks the hair follicles. Hair loss can be unpredictable but typically begins with one or two coin-sized patches which sometimes enlarge, affect different areas of the scalp and body, and/or grow back multiple times. In some cases, the patches of hair loss are permanent, and in rare cases, a person can completely lose all scalp and body hair.

Alopecia areata can be caused by certain thyroid or collagen diseases, or by diabetes. Treating the underlying condition may help.


Quite a few medications are known to cause some hair loss, although each medicine and brand of medicine has its own side effects. Someone experiencing hair loss who takes any of the following types of medications should speak with the prescriber to see if a change might be possible.

  • Anticonvulsant
  • Anticoagulants
  • Antihypertensives
  • Anti-arrhythmic


Stress can cause hair follicles to go from a growth phase into a resting phase. Hair growth from those follicles stops. Therefore, when hairs fall out, they are not immediately replaced. Thinning is not usually noticed until at least several weeks after the onset of stress.

Hair will regrow when stress levels are lower.

Traction alopecia:

This form of hair loss occurs when tight ponytails, buns, cornrows, or other hairstyles consistently tug at the base of the hair. The tugging can cause temporary damage to the follicles, followed by permanent damage if the tugging continues.

Trichotillomania/Hair pulling disorder:

Trichotillomania is a mental health disorder in which a person has repeated, uncontrolled urges to pull out their hair for reasons unrelated to a general medical condition. The disorder and the resulting bald patches can cause significant distress and affect a person’s ability to function socially or at work.

The bottom line

Despite the fact that hair loss is extremely common, it can cause significant psychological distress. A full head of hair is typically associated with youth and virility, while sparse or no hair is sometimes associated with old age, frailty, and even the loss of one’s place in society. At the very least, hair loss alters one’s appearance, and can even affect one’s sense of self-identity.

There are many options for treatment of hair loss. However, with the exception of resolving an underlying medical condition which also restores hair growth, none are perfect. Some treatments work only for as long as they are used; once discontinued, hair loss can resume. Other treatments are relatively new and have not been extensively studied. Various at-home treatments can be effective, but it is important to choose high-quality brands and follow directions exactly. Hair transplants can look natural and are long-lasting, but are a form of surgery, and are expensive.

If medical treatments don’t work, there are techniques that can help your hair to look fuller and to prevent fragile hair from breaking. But if a full, thick head of hair is important to you, don’t try to self-diagnose or to disguise your hair loss. See a physician so that you have the best chance of having a healthy body and the head of hair with which you are most comfortable.

How Halza can help?

Manage your family’s health easily with the Halza app. Store, track & share all of your child’s medical records to have them with you, wherever you go. Monitor your child’s growth and vaccination schedule as well as upload reports and doctor’s notes all with the Halza app. QuickShare a complete overview of you or your child’s health with any attending doctor in seconds, during emergencies or whenever you need.

Simplify your health journey with Halza.

Jill Garner
Jill GarnerRegistered Nurse and Diabetes Educator