Why Is My Hair Falling Out? A Diagnostic Checklist

Hair loss has more than 30 recognized causes. Some are hormonal, some nutritional, some medication-related, and some driven by physical stress on the follicle itself. The challenge is not that information about hair loss is scarce. The challenge is that too many possible explanations exist, and without a structured way to evaluate them, most people either fixate on the wrong cause or feel too overwhelmed to investigate at all. This checklist organizes the most common triggers into categories, lists the key symptoms that distinguish each one, and tells you what tests or observations to bring to your dermatology appointment. It is not a diagnosis. It is a way to think clearly before you sit down with a specialist.

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How to use this checklist

Read through each category. For any cause that matches your symptoms, note it. Most people will find 1-3 plausible explanations. That is normal. Hair loss can have overlapping causes, and a dermatologist may identify a primary driver alongside contributing factors. The goal here is to narrow the field so your first appointment is focused and productive.

For each category, you will find the typical symptoms, the usual timeline, and the specific tests worth requesting. Bring this list with your notes to your appointment. Dermatologists work faster when patients arrive organized.

Category 1: Androgenetic alopecia (pattern hair loss)

This is the most common form of hair loss, affecting roughly 50% of men over 50 and 40% of women by age 70, according to Ho et al. (2015) in the Journal of the American Academy of Dermatology. It is driven by dihydrotestosterone (DHT) acting on genetically sensitive follicles.

Key symptoms: In men, a receding hairline at the temples and thinning at the crown. In women, a widening part line with preserved frontal hairline. Hair does not fall out suddenly. It miniaturizes gradually, becoming finer and shorter with each growth cycle.

Timeline: Slow and progressive over years. Most people notice visible thinning 2-5 years after miniaturization begins. Onset can start as early as the late teens.

Tests to request: Usually diagnosed clinically. Dermoscopy (magnified scalp exam) reveals hair diameter diversity. Blood work for DHT, free testosterone, and DHEA-S can support the diagnosis, especially in women.

Category 2: Thyroid disorders

Both hypothyroidism and hyperthyroidism disrupt the hair growth cycle. The American Thyroid Association estimates that 20 million Americans have some form of thyroid disease, and hair loss is among the most reported symptoms. Vincent and Bhagavan (2008) reported in the Journal of Clinical Endocrinology and Metabolism that thyroid-related hair loss is typically diffuse, affecting the entire scalp rather than specific zones.

Key symptoms: Diffuse thinning across the whole scalp. Hair feels dry and brittle. In hypothyroidism, you may also notice fatigue, weight gain, cold sensitivity, and thinning of the outer third of the eyebrows. In hyperthyroidism, hair may become extremely fine and soft.

Timeline: Shedding usually starts 2-4 months after thyroid levels become abnormal. It can persist until levels are stabilized for several months.

Tests to request: TSH, free T3, and free T4. TSH alone is not always sufficient. Subclinical thyroid dysfunction (normal TSH but abnormal T3/T4) can still cause shedding. Thyroid antibodies (TPO, TgAb) help rule out Hashimoto's thyroiditis.

Category 3: Iron deficiency and low ferritin

Iron is essential for hair follicle cell division. Even without full-blown anemia, low ferritin (the body's iron storage protein) is associated with increased hair shedding. A 2006 study by Rushton in the European Journal of Dermatology found that women with ferritin levels below 40 ng/mL had significantly higher rates of diffuse hair loss compared to those above 70 ng/mL.

Key symptoms: Diffuse shedding. Hair may feel thinner overall but without a specific pattern. Fatigue, pale skin, shortness of breath with exertion, and brittle nails are common accompanying signs.

Timeline: Shedding begins 2-3 months after iron stores drop below critical levels. Recovery takes 6-12 months after ferritin normalizes above 50-70 ng/mL.

Tests to request: Serum ferritin, serum iron, TIBC (total iron-binding capacity), and CBC. Ferritin is the most important marker. Many labs flag ferritin as "normal" at 12 ng/mL, but hair-specific research suggests 40-70 ng/mL as the minimum for optimal follicle function.

Category 4: Vitamin D deficiency

Vitamin D receptors are present on hair follicles, and research suggests the vitamin plays a role in follicle cycling. Banihashemi et al. (2013) reported in Skin Pharmacology and Physiology that women with female pattern hair loss had significantly lower vitamin D levels than controls. Deficiency is extremely common: the Endocrine Society estimates that over 40% of US adults have levels below 20 ng/mL.

Key symptoms: Diffuse thinning, often alongside fatigue, bone pain, and muscle weakness. Not a standalone cause in most cases but frequently a contributing factor.

Tests to request: 25-hydroxyvitamin D. Optimal levels for hair are generally considered 40-60 ng/mL by dermatologists specializing in hair loss, though the official sufficiency threshold is 30 ng/mL.

Category 5: Stress-related hair loss (telogen effluvium)

Telogen effluvium (TE) is the body's response to a significant physiological or psychological stressor. Grover and Khurana (2016) reported in the Journal of Clinical and Diagnostic Research that TE accounts for roughly 30% of all hair loss cases seen in dermatology clinics. The mechanism is straightforward: stress pushes a large percentage of follicles into the resting (telogen) phase simultaneously. Two to three months later, those follicles release their hairs at once.

Key symptoms: Sudden, diffuse shedding. Hair comes out in handfuls during washing or brushing. No specific pattern. Scalp is not inflamed or itchy. The connection to a stressful event 2-3 months prior is the defining clue.

Common triggers: Surgery, high fever (including COVID-19), childbirth, crash dieting, severe emotional stress, starting or stopping medications, and major illness.

Timeline: Shedding peaks 2-4 months after the trigger. Most cases resolve within 6 months if the trigger is removed. Chronic TE (lasting beyond 6 months) may indicate an ongoing trigger that has not been identified.

Tests to request: TE is primarily a clinical diagnosis. Blood work helps exclude overlapping causes: CBC, ferritin, thyroid panel, and zinc.

Category 6: Medication-related hair loss

Dozens of medications list hair loss as a side effect. The mechanism is usually either anagen effluvium (direct toxic damage to the hair matrix, as in chemotherapy) or telogen effluvium (follicles shifted into the resting phase). Suchonwanit et al. (2018) cataloged over 100 medications associated with hair loss in the American Journal of Clinical Dermatology.

Commonly implicated medications:

• SSRIs and antidepressants (sertraline, fluoxetine, paroxetine): hair shedding typically starts 2-4 months after starting or changing dose
• Blood thinners (heparin, warfarin): telogen effluvium pattern, usually reversible after discontinuation
• Retinoids (isotretinoin, acitretin): dose-dependent thinning, typically reversible
• Hormonal contraceptives: certain progestins with androgenic activity can trigger or accelerate pattern loss, especially in women genetically predisposed
• Beta-blockers (metoprolol, propranolol): diffuse shedding is reported in 1-5% of users
• Thyroid medications (if dosed incorrectly): both over- and under-treatment can cause shedding

What to track: The date you started or changed any medication. Compare this to when shedding began. A 2-4 month gap between medication change and shedding onset is the classic telogen effluvium pattern.

Category 7: Hormonal changes (PCOS, postpartum, menopause)

Hormonal shifts are among the most common hair loss triggers in women. Polycystic ovary syndrome (PCOS) affects 6-12% of women of reproductive age, and elevated androgens are a hallmark. Postpartum shedding affects roughly 40-50% of new mothers. Perimenopause and menopause bring declining estrogen, which can unmask androgen-driven thinning.

PCOS symptoms: Thinning at the crown and temples, often accompanied by acne, irregular periods, and excess facial or body hair. Blood work may show elevated free testosterone, DHEA-S, or androstenedione.

Postpartum timeline: Shedding typically begins 2-4 months after delivery and peaks around month 4-6. Most women recover fully by 12 months postpartum without treatment.

Menopause pattern: Gradual diffuse thinning, often noticed as a wider part line or visible scalp through the hair. Onset correlates with declining estrogen levels, typically in the late 40s to early 50s.

Tests to request: Free testosterone, DHEA-S, androstenedione, estradiol, progesterone, FSH, LH. For PCOS, a pelvic ultrasound may also be recommended.

Category 8: Autoimmune hair loss (alopecia areata)

Alopecia areata is an autoimmune condition where the immune system attacks hair follicles, causing round, smooth patches of complete hair loss. It affects approximately 2% of the population at some point in their lifetime, according to the National Alopecia Areata Foundation. The follicles are not destroyed, which means regrowth is possible, but the course is unpredictable.

Key symptoms: One or more smooth, round bald patches appearing suddenly. No scarring, no redness (typically). "Exclamation mark" hairs at the patch border: short hairs that taper at the base. Nail pitting in some cases.

Timeline: Patches can appear within days to weeks. Roughly 50% of patients see spontaneous regrowth within 12 months, according to Pratt et al. (2020) in the Journal of the American Academy of Dermatology. Recurrence is common.

Tests to request: Usually diagnosed clinically. ANA (antinuclear antibody) and thyroid panel recommended to screen for associated autoimmune conditions.

Category 9: Traction alopecia (mechanical hair loss)

Traction alopecia is caused by repeated pulling force on the hair follicle. Tight ponytails, braids, extensions, weaves, and tight headwear are the primary culprits. Khumalo et al. (2016) found in the Journal of the American Academy of Dermatology that one-third of African American women showed clinical signs of traction alopecia, making it one of the most common forms of hair loss in this population.

Key symptoms: Hair loss along the hairline, temples, or wherever the pulling force is greatest. Small bumps or pustules around the follicle in early stages. The pattern directly maps to the hairstyle.

Timeline: Gradual. Early traction alopecia is reversible if the pulling stops. Prolonged traction (years) can cause permanent follicle damage and scarring.

Tests to request: Clinical diagnosis. No blood work needed. A scalp biopsy may be ordered if scarring is suspected.

Category 10: Zinc and protein deficiency

Hair is 95% keratin protein, and protein deficiency directly impairs follicle output. Zinc is a cofactor in over 300 enzymes, including those involved in hair follicle cell division. Kil et al. (2013) reported in Annals of Dermatology that serum zinc levels were significantly lower in patients with all types of hair loss compared to healthy controls.

Key symptoms: Diffuse thinning. Hair feels weak and breaks easily. In severe protein deficiency, hair may lose pigment and become reddish or lighter. Zinc deficiency may also present with skin lesions and slow wound healing.

At-risk groups: Vegetarians and vegans (if protein intake is not carefully managed), people on very low-calorie diets, post-bariatric surgery patients, and those with malabsorption disorders.

Tests to request: Serum zinc, total protein, albumin, and prealbumin. A dietary history review with your doctor or a registered dietitian is often more revealing than blood work alone.

What to do with your checklist results

Once you have read through each category, write down the 1-3 causes that best match your symptoms, timeline, and risk factors. For each, note the specific tests you want to discuss with your dermatologist. This transforms a vague "I'm losing my hair" appointment into a focused diagnostic conversation.

Before your visit, start tracking your shedding and density with structured photos. Even 2-4 weeks of consistent documentation gives your dermatologist a much clearer picture than verbal descriptions alone. Log the date shedding started, any triggers you can identify, medications you take, and any other symptoms you have noticed.

Hair loss is rarely a mystery that cannot be solved. It is almost always a puzzle with identifiable pieces. The difference between people who find answers quickly and those who spend months in uncertainty is usually not the complexity of their condition. It is whether they arrived at the first dermatology conversation with organized observations or scattered worries. This checklist is designed to put you in the first group.