Hi,
In addition to hyperthyroidism, diabetes also carries symptoms of increased drinking and urination (of course, as you said, they do go hand in hand).
Per the breathing, it's considered panting if he's opening his mouth.
In any case, I believe at this time, bloodwork should be done to rule out hyperthyroidism and/or diabetes or any other illness to which older cats are usually prone. The changes in his eating, weight loss, increased drinking and deeper/faster breathing definitely bear a vet evaluation at this time. The blood tests shouldn't be too expensive; I've had them done for my cat who is diabetic and almost 13.
Please keep me posted on the blood results. Thanks!
Cher : )
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