What Causes Severe Hair Loss?

Significant hair loss—often defined as shedding more than 100 hairs per day, noticeable thinning, or visible scalp exposure—can stem from a wide range of underlying causes. These include hormonal imbalances such as androgenetic alopecia (the most common form in both men and women), thyroid dysfunction (both hypothyroidism and hyperthyroidism), and postpartum or menopausal shifts in estrogen and testosterone levels.

Autoimmune conditions like alopecia areata can trigger sudden, patchy hair loss when the immune system mistakenly attacks hair follicles. Nutritional deficiencies—particularly iron, ferritin, vitamin D, zinc, and biotin—may also impair the hair growth cycle, especially in individuals with restrictive diets, gastrointestinal malabsorption disorders, or chronic blood loss.

Medications represent another frequent contributor: chemotherapy agents, anticoagulants, beta-blockers, retinoids, and certain antidepressants have well-documented associations with telogen effluvium, a reversible condition characterized by diffuse shedding several months after a triggering event. Severe physical or emotional stress—including major surgery, acute illness, or psychological trauma—can similarly disrupt the hair cycle, pushing large numbers of follicles prematurely into the resting (telogen) phase.

Chronic inflammatory scalp disorders—such as lichen planopilaris, frontal fibrosing alopecia, and discoid lupus erythematosus—may cause scarring alopecia, where hair follicles are permanently destroyed and replaced by fibrous tissue. Early diagnosis and intervention are critical to prevent irreversible damage.

A thorough clinical evaluation—including detailed history, scalp examination, trichoscopy, and targeted laboratory testing—is essential to distinguish between reversible and permanent forms of hair loss and to guide evidence-based treatment strategies.