For example, our HCM cat Myrna at "Cats with HCM" has been on iron supplements since April to improve kidney function because her diuretic needs to be increased due to congestion and her BUN and creatinine were creeping up. Her kidney function has improved dramatically as shown in the kidney values so I highly recommend that you give your cat iron if it is on high levels of diuretic and/or the BUN and creatinine are creeping up. We give Nature Made and use the 65mg tabs and give three 1/2 tabs a day. Start with one 1/2 tab and increase to two 1/2 tabs and up to three a day if needed. It may lead to dark stool which is normal or diarrhea. If the diarrhea isn't severe (more than once or twice a day) and doesn't prevent the cat from feeling well, then continue the iron. But if you are concerned, cut back to one or two 1/2 tabs. Why? Because iron makes red blood cells which the kidneys need to better process the diuretic to help the med get rid of congestion. The body needs it to make oxygen which keeps the cat breathing well, keeps up the blood pressure (to normal levels), which keeps muscles strong and nerves in the body (heart, kidney, etc.) functioning better. Even if the cat has "normal" iron blood levels, this is essential and we have seen great success with Myrna.
Don't forget to give a potassium supplement if the cat's on a diuretic even if the potassium blood level is "normal". If it's below 5 on the blood test, give it. We use Sundown 595mg and break them in half. How much depends on how low is the potassium. Myrna from the other page, began with two tabs a day but is now taking 3000mg of potassium a day (per the label not the actual amount.)