Losing 50–100 hairs per day is normal — that's the standard hair cycle. When shedding accelerates beyond that, or when hair grows back thinner and finer each cycle, something is disrupting the biology of the hair follicle. The tricky part is that hair loss is almost never monocausal. It's usually a convergence of several factors amplifying each other.
DHT Sensitivity (Androgenetic Alopecia)
The most common cause of progressive hair thinning in both men and women. Dihydrotestosterone (DHT) — a potent metabolite of testosterone — binds to receptors in genetically susceptible hair follicles, gradually miniaturizing them over repeated cycles until they no longer produce visible hair. The key word is "sensitivity" — testosterone levels matter less than follicle receptor sensitivity, which is genetic.
Iron Deficiency (Without Anemia)
This is the most commonly missed cause of hair loss in women. Iron is required for DNA synthesis in the rapidly dividing cells of the hair matrix. The hair follicle is highly sensitive to iron depletion — and ferritin (stored iron) can be insufficient for optimal hair growth even when hemoglobin is in the normal range. Many doctors only test hemoglobin, not ferritin.
Chronic Stress and Telogen Effluvium
Prolonged physical or emotional stress pushes a large proportion of hair follicles simultaneously into the telogen (resting/shedding) phase. This produces a dramatic diffuse shedding that typically appears 2–4 months after the stressor — which is why it's so confusing (the shedding doesn't match the current stress level, it matches what happened months ago). It's technically reversible if the stressor is addressed.
Thyroid Dysfunction
Both hypothyroidism and hyperthyroidism cause diffuse hair loss. Thyroid hormones (T3, T4) regulate the hair growth cycle directly — thyroid receptor alpha1 is expressed in hair follicle cells. Hypothyroidism is particularly common in women and is frequently subclinical (normal TSH but low free T3/T4), making it easy to miss on standard testing.
Protein and Biotin Deficiency
Hair is approximately 95% keratin protein. Insufficient dietary protein — common in restrictive diets, veganism without adequate planning, or very low-calorie diets — directly impairs hair shaft synthesis. Biotin, while overhyped as a hair growth supplement in people who are replete, is genuinely important in deficiency states. Low biotin produces characteristic thin, brittle hair with diffuse shedding.
Scalp Inflammation and Microbiome Disruption
A growing body of research implicates scalp microbiome dysbiosis in both dandruff and hair loss. Malassezia yeast overgrowth produces inflammatory byproducts that disrupt follicle function. Seborrheic dermatitis — a common scalp condition — creates an inflammatory microenvironment that accelerates hair miniaturization even without overt itching or flaking visible to the sufferer.
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