Yes, more info is needed, especially if you are hypo. With Hypo the physical activity diminishes in many cases and buttocks, legs or thighs might lose some of the tone that you used to enjoy.
I was wondering when you had your last checkup and blood work done.
Good luck to you
M
What dosage of synthroid are you on and how long have you been on the same dosage? Do you have copies of your current labs, so you can post them for us to better assess your situation? Be sure to include reference ranges, which vary lab to lab and have to come from your own report.
Have you gained weight all over? If so, that's a hypo symptom. If not, maybe you just need to exercise more/differently, to tone your thighs.