Itchy brown bumps on back of thighs what could it be?

Keratosis pilaris. It might be keratosis pilaris, a condition in which the tiny hair follicles are plugged. It happens on the backs of people's upper arms, the backs of their thighs, and their buttocks. If it's kp, you can try over the counter cerave sa lotion twice daily to improve it, although you will never cure it. Kp is just the way your skin grows.