Hyperthyroidism is pretty common in older kitties. There is a Yahoo! support group (Feline HyperT) that is pretty helpful. Watch for signs of itchy ears and lethargy--they can be symptoms of too high of a dose, and 2 (5mg) tabs per days is a high starting dose. Do you know what his T4 was? If you haven't already, ask the vet for his records. It can be very helpful to keep copies of the records yourself so you can watch for changes. Hyperthyroidism is very much a treatable condition.