There are several factors that can affect hair growth such as hormone levels and products that can slow down or stop baldness such as Rogaine. However, the eyebrows are one of the few places on the body where hair growth is not determined by levels of circulating hormones. Also, medicines such as Rogaine only work on male pattern type baldness and will not help speed up your eyebrow growth rate.

Another factor that could make your eyebrows grow slowly and thin are hormonal.  For example, hypothyroidism will lead to slow eyebrow hair growth and in fact, one of the early hallmark signs of an underactive thyroid are thin sparse outer eyebrows.  So the tails of your eyebrows will be thinner and patchy compared to the rest of your brows.  This is an indicator that you will need to get your thyroid hormones TSH, T3, and T4 levels checked.  If you are hypothyroid, then medication such as Synthroid, Armour, or raw thyroid supplements will restore your hormone levels to the optimum level.  It will still take several months though for your eyebrow hairs to start regrowing again.