There exist lots of causes of scalp hair loss. Generally, most hair loss is not connected with periodic or internal disease, nor is poor diet a frequent factor. Thyroid disease may also cause hair loss, but thyroid tests on people who have average hair loss are usually normal. Although many medications list "hair loss" among their potential side effects, drugs are also not overall common causes of thinning or lost hair. On the other hand, with cancer treatments and immune suppression medications, hair loss is not uncommon.

A very useful way to classify hair loss is by whether the loss is localized and patchy, or whether it affects large areas or the whole scalp itself

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