Like the other poster said, when it comes to cardio, find something that you enjoy doing that you could do for 20 to 30 minutes at a time. Go for a walk. Just start walking at a steady pace and as time goes by, increase your speed. See how far you can go in those 20 to 30 minutes.
Have you talked with your doctor about exercise? Since you have problems with your thyroid, it may be a good idea to talk to your doctor to see if it would be better for you to do exercises that aren't as high in intensity but still provide a great workout (rowing, swimming, walking to walk/jog intervals, hiking, water aerobics (or some other aquatic activity)).
Also, make sure that you are eating appropriately too. I know this is the fitness section but what you eat will make a difference (even if you make small changes/swaps - i.e., wheat instead of white, etc.). I hope this helps.
Maybe this link can help: www.livestrong.com/article/477000-what-is-
Another site: chriskresser.com/why-you-may-need-to-exerc