Clinically alopecia areata is characterized by the sudden appearance of a round or oval patch of
non-scarring and painless hair loss with spontaneous remissions and exacerbations.  I’ve seen the patches to be from a cm in diameter in some cases on clients and they have not really known what has caused it other than stress.  Also, fungal infections on the scalp can cause patchy hair loss but the hair grows back when the fungal infection is treated.  If hair is being lost from around the hairline of the face - forehead and thinning out on the top of the head, then it could be linked to elevated androgens and dht.  In the case of fungal infection there may be itchiness and redness around the affected area, so if that isn’t a symptom then I would think more about the hromone levels.  Stress can also causes hairloss, but then I often wonder now if it is because all the hormones elevate--not just cortisol, but dht producion and that it is that that causes hair to thin out.  Patchy hair loss can also occur without fungal infection and that is generally attributed to stress also.

Some info here:

http://www.geocities.com/hair_loss_qa/hair_loss_types.htm