Chances are your cat does not have Cushing’s disease. As of 2002, fewer than 100 cats had been reported having Cushings in the history of veterinary literature.
The disease serves to illustrate its counterpart, Addison’s disease; both of these ailments affect the adrenal gland, most specifically its over or underproduction of cortisol.
In Cushing’s disease, it is the overproduction of cortisol which is the case. This is usually caused by tumors, or the (sustained) effects of steroid drugs.
Around 15% of the cats diagnosed with tumors were found to have adrenal gland tumors. Conversely, the other 85% had tumors in the pituitary gland which caused the overproduction of precursor hormones to those in the adrenal gland.
If it revealed that the Cushings' disease was caused by medication, then that medication should be stopped immediately. In the case of tumors, it is possible to surgically remove them in many instances.