What Causes Hair Loss? Understanding the Most Common Reasons for Hair Thinning
What You'll Learn
Hair loss is one of the most common concerns I see in clinical practice, yet it's also one of the most misunderstood.
Many people assume hair loss is simply a normal part of aging or that it only affects men. Others believe there is little that can be done once thinning begins.
The reality is much more encouraging.
Hair loss is often the result of one or more underlying factors affecting the normal hair growth cycle. While genetics certainly play an important role, hormones, nutritional deficiencies, thyroid disorders, stress, medications, inflammation, and certain medical conditions can all contribute to hair thinning.
The first step toward successful hair restoration is understanding why your hair is falling out.
In this article, we'll review the most common causes of hair loss, explain how different conditions affect the hair follicle, and discuss why identifying the underlying cause is essential for developing an effective treatment plan.
Why This Matters
Hair loss is not a diagnosis.
It is a symptom.
Just as a fever can have many different causes, hair loss can result from a wide variety of medical, hormonal, nutritional, and genetic factors.
Treating every patient with the same shampoo, supplement, or medication without understanding why the hair loss occurred often leads to disappointing results.
For example, a patient with iron deficiency may not respond optimally to scalp treatments until their iron stores are corrected. Someone with untreated thyroid disease may continue to experience hair thinning despite using topical therapies. Another patient may have androgenetic alopecia, where preserving existing follicles early can significantly improve long-term outcomes.
Identifying the underlying cause allows treatment to be individualized rather than relying on a one-size-fits-all approach.
The earlier the cause is recognized, the greater the opportunity to slow progression, preserve existing hair, and support healthier long-term hair growth.
Hair Loss Is Often Multifactorial
One of the biggest misconceptions about hair loss is that there is usually one single cause.
In reality, multiple factors frequently contribute at the same time.
For example, a woman may have:
Female pattern hair loss
Low ferritin
Vitamin D deficiency
Hashimoto's thyroiditis
Chronic stress
A man may have:
Male pattern baldness
Elevated DHT sensitivity
Insulin resistance
Poor sleep
Chronic inflammation
Each factor places additional stress on the hair follicle.
When multiple contributors are addressed together, patients often experience better outcomes than treating only one issue.
The Most Common Causes of Hair Loss
Let's look at the most common reasons hair begins to thin.
Genetics (Androgenetic Alopecia)
The most common cause of hair loss in both men and women is androgenetic alopecia.
This condition is influenced by genetics and the hair follicle's sensitivity to dihydrotestosterone (DHT), a naturally occurring hormone derived from testosterone.
Over time:
The growth phase becomes shorter.
Hair follicles gradually shrink.
Hair shafts become thinner.
Hair density decreases.
Some follicles eventually stop producing visible hair.
Men typically notice a receding hairline and thinning at the crown, while women more commonly experience diffuse thinning over the top of the scalp or widening of the part.
Because this process is gradual, many people do not recognize significant hair loss until 30 to 50 percent of hair density has already been lost.
Early intervention offers the best opportunity to preserve existing follicles and slow progression.
Hormonal Changes
Hormones play a major role in regulating the hair growth cycle.
Changes in hormone levels may contribute to increased shedding or progressive thinning.
Examples include:
Menopause
Pregnancy
Postpartum hormone shifts
Polycystic ovary syndrome (PCOS)
Testosterone imbalance
Elevated androgen levels
Changes in estrogen or progesterone
Hormonal hair loss often develops gradually and may be accompanied by changes in menstrual cycles, acne, unwanted facial hair, decreased energy, or other symptoms depending on the underlying cause.
At Rejuvenate Indy, hormone optimization is often an important component of a comprehensive hair restoration plan when clinically appropriate.
Thyroid Disorders
The thyroid gland influences nearly every cell in the body, including hair follicles.
Both hypothyroidism and hyperthyroidism can disrupt the normal hair growth cycle, leading to increased shedding, slower regrowth, and changes in hair texture.
Patients with thyroid dysfunction often experience additional symptoms such as:
Fatigue
Weight changes
Dry skin
Cold intolerance
Constipation
Brain fog
Because thyroid disorders are relatively common and frequently overlooked, laboratory evaluation is often an important part of a comprehensive hair loss assessment.
Iron Deficiency and Low Ferritin
Iron plays a critical role in healthy hair growth. Hair follicles are among the most metabolically active tissues in the body, requiring an adequate supply of oxygen and nutrients to produce strong, healthy hair.
When iron stores become depleted, the body prioritizes delivering oxygen to essential organs rather than supporting hair growth. As a result, hair follicles may enter the resting phase prematurely, leading to increased shedding and slower regrowth.
Many patients are surprised to learn they can have low iron stores even if they are not anemic.
Ferritin, which reflects the body's stored iron, is often a more useful marker when evaluating chronic hair shedding. While laboratory reference ranges may consider lower ferritin levels "normal," many hair restoration specialists prefer ferritin levels substantially higher when optimizing hair growth.
Common risk factors for iron deficiency include:
Heavy menstrual cycles
Pregnancy
Vegetarian or vegan diets
Frequent blood donation
Gastrointestinal disorders affecting absorption
Previous bariatric surgery
Identifying and correcting iron deficiency is often one of the most important steps in restoring healthy hair growth.
Nutritional Deficiencies
Healthy hair requires an adequate supply of nutrients to support the continuous production of new hair fibers.
Deficiencies in protein, vitamins, and minerals can disrupt the hair growth cycle and contribute to increased shedding or poor-quality hair.
Important nutrients include:
Protein
Iron
Zinc
Vitamin D
Vitamin B12
Folate
Biotin (only when deficiency exists)
Essential fatty acids
While supplements are widely marketed for hair growth, they are not a substitute for identifying and correcting true deficiencies. More is not always better, and excessive supplementation may even be harmful in some cases.
Whenever possible, nutritional deficiencies should be confirmed through a comprehensive history, dietary assessment, and appropriate laboratory testing.
Physical and Emotional Stress
Stress affects nearly every system in the body, including the hair follicles.
Major physical or emotional stress can trigger a condition known as telogen effluvium, in which a larger-than-normal number of hair follicles enter the resting phase simultaneously.
Because follicles remain in the resting phase for several months before shedding, patients often notice increased hair loss two to four months after the stressful event rather than immediately.
Common triggers include:
Serious illness
Surgery
High fever
Childbirth
Divorce
Job loss
Significant emotional trauma
Rapid weight loss
Major life changes
The encouraging news is that stress-related hair loss is often temporary once the underlying trigger has resolved and the body has recovered.
Medications
Certain medications may contribute to increased hair shedding or hair thinning.
Examples include:
Blood thinners
Some blood pressure medications
Certain antidepressants
Chemotherapy agents
Retinoids
Anticonvulsants
Some hormonal medications
GLP-1 medications when associated with rapid weight loss
Excessive vitamin A supplementation
In many cases, the medication itself is not directly damaging the follicle. Instead, changes in metabolism, nutrition, or rapid weight loss may alter the normal hair cycle.
Patients should never discontinue a prescribed medication without first discussing it with their healthcare provider.
Autoimmune Conditions
Some forms of hair loss occur because the immune system mistakenly attacks the hair follicle.
The most common example is alopecia areata, which often presents as smooth, round patches of sudden hair loss.
Other autoimmune conditions that may contribute to hair thinning include:
Hashimoto's thyroiditis
Lupus
Lichen planopilaris
Frontal fibrosing alopecia
Some autoimmune forms of hair loss are reversible, while others can cause permanent damage if not recognized and treated early.
This is one reason why persistent or unusual hair loss should always be evaluated by an experienced healthcare professional.
Aging
As we age, changes in hair density are common.
Hair follicles gradually spend less time in the active growth phase, and individual hairs often become finer over time.
While aging itself cannot be prevented, many age-related contributors to hair loss—including hormone changes, nutritional deficiencies, chronic inflammation, and metabolic health—can often be optimized.
The goal is not necessarily to restore the hair you had at age twenty, but to preserve and maximize the health of the follicles you have today.
Lifestyle Factors
Daily habits also influence long-term hair health.
Factors such as poor sleep, smoking, chronic inflammation, insulin resistance, obesity, crash dieting, and inadequate protein intake can all negatively affect the hair growth cycle.
Supporting overall health often supports healthier hair.
This is why successful hair restoration extends beyond topical treatments or procedures. Hair follicles respond to what is happening throughout the entire body.
When Should You Seek Evaluation?
Some hair shedding is completely normal.
However, persistent thinning should not simply be accepted as "part of getting older."
Consider scheduling an evaluation if you notice:
Increased hair shedding lasting longer than three months
A widening part
Progressive thinning on the top of the scalp
A receding hairline
Patchy areas of hair loss
Thinning eyebrows or eyelashes
Scalp itching, pain, redness, or scaling
Hair loss accompanied by fatigue, weight changes, or hormonal symptoms
The earlier the underlying cause is identified, the more treatment options are typically available and the better the opportunity to preserve existing hair.
Clinical Pearl
One of the biggest mistakes I see is treating every patient with the same hair loss solution.
Hair loss is a symptom, not a diagnosis. Two patients may have nearly identical thinning patterns but completely different underlying causes. One may have iron deficiency, another may have thyroid dysfunction, while another has androgenetic alopecia or a combination of several factors.
The best treatment plan begins with identifying why the hair loss is occurring rather than simply trying to stimulate hair growth.
Functional Medicine Perspective
At Rejuvenate Indy, we believe successful hair restoration starts with looking beyond the scalp.
Hair follicles respond to your overall health. Hormone balance, thyroid function, iron stores, vitamin and mineral status, metabolic health, inflammation, stress, sleep, and nutrition all influence the health of the hair follicle and its ability to produce strong, healthy hair.
While treatments such as microneedling, exosomes, red light therapy, and medications can play an important role, they often produce the best results when combined with identifying and addressing the underlying contributors to hair loss.
Our goal is not simply to help you grow hair today, but to create the healthiest environment possible for long-term hair growth.
Myth vs. Fact
Myth: Hair loss is caused by only one thing.
Fact: Hair loss is frequently multifactorial. Genetics, hormones, thyroid dysfunction, iron deficiency, nutritional deficiencies, stress, medications, inflammation, and lifestyle factors can all contribute simultaneously. Identifying each contributor allows for a more personalized and effective treatment plan.
What I Tell My Patients
"Hair loss is rarely just a scalp problem. Your hair is often one of the first places your body tells us something isn't quite right. Instead of chasing the newest product or supplement, we focus on understanding why your hair is thinning so we can treat the cause—not just the symptom."
Optimize This
You can support healthier hair growth by:
Eating adequate protein every day.
Correcting iron, vitamin D, and other nutrient deficiencies when appropriate.
Optimizing thyroid and hormone health.
Managing chronic stress.
Prioritizing quality sleep.
Avoiding smoking and nicotine products.
Maintaining stable blood sugar and metabolic health.
Avoiding crash dieting and rapid weight loss whenever possible.
Seeking evaluation early if you notice persistent hair thinning or excessive shedding.
Frequently Asked Questions
Is it normal to lose hair every day?
Yes. Most healthy adults naturally shed approximately 50 to 100 hairs each day as part of the normal hair growth cycle.
What is the most common cause of hair loss?
The most common cause is androgenetic alopecia, also known as male pattern baldness or female pattern hair loss. However, many people have multiple contributing factors, including nutritional deficiencies, thyroid dysfunction, hormonal changes, or chronic stress.
Can stress really cause hair loss?
Yes. Significant physical or emotional stress can trigger telogen effluvium, causing increased hair shedding several months after the stressful event.
Can low iron cause hair loss?
Yes. Low iron stores, particularly low ferritin levels, are a common contributor to chronic hair shedding and diffuse hair thinning, especially in women.
Can thyroid disease cause hair thinning?
Yes. Both hypothyroidism and hyperthyroidism can disrupt the normal hair growth cycle and contribute to increased shedding or slower regrowth.
Will my hair grow back?
That depends on the underlying cause. Many forms of hair loss can improve when identified early and treated appropriately. However, some conditions may result in permanent follicle damage if left untreated for too long.
When should I seek evaluation?
If you notice persistent hair shedding, progressive thinning, widening of your part, a receding hairline, patchy hair loss, or scalp symptoms, it's a good idea to schedule a comprehensive evaluation. Early diagnosis often provides the greatest opportunity for successful treatment.
References
American Academy of Dermatology Association. Hair Loss: Overview. https://www.aad.org
Messenger AG, Sinclair RD. Follicular Disorders and Hair Loss. Journal of the American Academy of Dermatology.
Olsen EA. Female Pattern Hair Loss. Journal of the American Academy of Dermatology.
Almohanna HM, et al. The Role of Vitamins and Minerals in Hair Loss: A Review. Dermatology and Therapy. 2019.
Trüeb RM. Molecular Mechanisms of Hair Growth and Hair Loss. Experimental Gerontology.
International Society of Hair Restoration Surgery (ISHRS). Patient Education Resources.
StatPearls. Androgenetic Alopecia. Updated regularly.
Medical Disclaimer: The information provided in this article is for educational purposes only and is not intended to replace individualized medical advice, diagnosis, or treatment. Hair loss has many potential causes, and treatment should always be based on a comprehensive medical evaluation. Do not begin, discontinue, or modify medications or supplements without consulting a qualified healthcare professional. If you are experiencing persistent hair loss or scalp concerns, schedule an evaluation with a licensed healthcare provider.
Last Reviewed: August 2026
Reviewed By: Jessica Reynolds, MSN, APRN, FNP-C, PMHNP-BC
Founder, Rejuvenate Indy
