Kitty has diarrhea

My cat Omar is 15 years old, and for the last few weeks he's been having diarrhea. The vet did blood work on him and tested his urine and stool. So far the only thing they could find is a slightly elevated thyroid and a rapid heartbeat. We already knew he had a heart murmur.
So he's on thyroid meds (low dosage) and we've put him on w/d prescription food (dry). But still the diarrhea keeps coming. He's lost nearly a pound (and he's only 9 lbs. normally.)
Any thoughts? I just want him to be more comfortable and stop losing weight. Thanks so much!

Did your cat have the diarrhea before the vet ran these tests? There are a few things you could try to help. You can withhold food for a day just to give the stomach a chance to heal itself and then give small meals every few hours when you begin feeding again to make sure you don't overwhelm the stomach. But you should consult with your vet about it. Maybe call them and ask if this would be okay. Also, there are fibre diets out there like Medi-Cal Fibre that can help cats if they need to add bulk to their stools. It also helps if they have the opposite problem of needing softer stools. Not sure where you live but this is a Canadian diet. Again, a quick call to the vet would help. You could also ask them about pumpkin.