My 15yo seems to have a few patches of thinning fur, and i wondered how normal this is in aging cats - she is only the second of mine to get to 15 (the 17yo came as a 17yo), so I am in new territories here!! She had full bloods including thyroid in Jan, and everything was perfect for her age. She isn't pulling clumps out, and the skin is fine, not red or anything. They are all on her underneath - armpits, side of leg and under tail. She is eating well and playing etc, so fine in herself - although she is getting a bit more vocal this days.