Can just thinning eyebrow in 5 yr old be symptom of alopecia areata?common clinical manifestation? what is the outlook for progression to scalp etc?

It can be a sign of : Alopecia Areata, which can also occur in the scalp. Unilateral eyebrow thinning can also be from Trichitillomania, pulling out eyebrow hair, or Tinea Capitis, a fungal infection. Less likely causes in kids are Telogen Effluvium, a response to physical or emotional stress, or hypothyroidism. His pediatrician can examine him & either provide treatment or refer him to a dermatologist, if needed.

Thinning of eyebrows: It could be a symptom of alopecia but to know for sure you should take the child to be examined as there are other causes.