The average human head has about 100,000 hair follicles. Each follicle can grow about 20 individual hairs in a person's lifetime.[1] The typicla rate of hair loss and hair sheddng is about 50-100 strands a day. The incidence of pattern baldness varies greatly.

TGhe male sex hormone DHT initiates follicular miniaturization. Here hair shaft diameter progressively decreases to form vellus hair or "peach fuzz". Eventually, it may completely disappear. Hair loss and balding may start in the teens. The tendency for pattern hair loss is mostly genetically-set. Male pattern baldness is classified on the Hamilton-Norwood scale.

While there is some basis for this belief, both parents contribute to their offspring's likelihood of hair loss. Most likely, inheritance is technically "autosomal dominant with mixed penetrance". Also, severe stress may cause a hair loss.

Fungus infections can cause hair loss. LIkewise, alopecia areata can result in hair loss ranging from local to the entire body (Alopecia areata universalis). Other causes of hair loss include cicatricial alopecia.

Tumours also may cause localized hair loss and hair thinning. Examples include sebaceous nevus, basal cell carcinoma, and squamous cell carcinoma. Hypothyroidism can cause hair loss.