How do doctors measure the loss of disc height?

Xray/MRI/CT scans. As one gets older--i would say after 50, the discs begin to lose their strength/shape and flatten out, thus one slowly loses height over the years. This can be measured, typically a decrease of your total height means the discs are flattening out. Xray/mri/ct scans can show the loss of this height, espcially if you have older ones to compare! if u are getting shorter, it is likely so. Good luck.