What Causes Thinning Of Eyebrows?

My son s eyebrow s are thinning, about a year ago my husband took him to a dermatologist and the Dr. gave him a cream called Desonide, 0.05%. I ve been putting the cream on him ever night. This has not resolve the problem at all. Is there anything else that will help him get his eyebrow back. He was diagnose with Psoriasis.