Is this an unusually excessive amount of weight loss?

Not quite sure what a stone is, but 40 pounds in 11 months is not exsessive. Thats less than a pound a week. People who try to lose weight usually aim fro 2-3 lbs a week. Obviously, your body had it to lose and naturally got rid of your extra pounds. You wont continue losing weight forever, your body will eventually get to a place where it feels comfortable. Having elevated testosterone definetly triggered your weighloss...ever notice how guys have naturally lower body fat percentage that women?? Its from their different chemical consistancies. You probably had a low testosterone level before, and just didn't know it, since it'snot exactly something the average person gets tested for. Hope that helps.