Before taking Cytomel, please RESEARCH,RESEARCH, and RESEARCH some more. When you think you have a good grasp of the subject, do some more reading. There are tons of good threads out there on the subject. There are also tons of good threads out there on joint problems, tendinitis, etc.
As far as cardio, use the recumbent stationary bike, or the elliptical machine. I am very prone to tendinitis can't use a stair stepper or treadmill. What is your specific problem with your knee?