Some of this information you will notice when you examine the scalp. Dandruff may be a sign of an allergic reaction or an excessively dry scalp.
Redness, bumps, raised areas or itching may be a sign of an infection (bacterial or fungal) as well as insect bites or more serious problems such as melanoma. Any raised rash, scaly areas (raised white scaly areas may be psoriasis) or open cuts/wounds should be referred to their physician or a dermatologist. Patchy areas on the scalp, eyebrow, facial hair (men), may be alopecia areata. This is usually a sudden hair loss that may progress to totalis (all the scalp hair) or universalis (all body hair). It may only be an allergic reaction but the cause is not really understood. Hair loss may be a symptom of a disease such as lupus and I usually recommend that people with alopecia areata see their doctor to rule out serious autoimmune disease.
Another cause of patchy loss would be scarring alopecia. This is usually seen as a red area that may have some drainage. When it heals, it is very smooth with no hair. A diagnosis of scaring alopecia is usually done by biopsy of the area by a dermatologist. There are no follicles in the scarred areas and regrowth is not possible. The hairs around these areas may be strengthened with treatment and may make these areas easier to hide with other hair.
There may also be chemicals (perms, strong chemicals) or physical scaring (heat, accident) where regrowth may not be possible.
Occasionally a person with Trichotillomania will come for treatment. These people pull out their hair, eyebrows, eyelashes and may benefit from counseling to find out the cause of this abusive type of behavior. If they continue to pull out their hair, the loss may become permanent.