Another update. I'm up stupidly late because my sleep schedule is complete borked (partly due to depression, partly due to playing Civ 6 very late recently).So, the verdict? Hyperthyroidism. He also lost FOUR POUNDS since his last visit a month ago.
But at least now I know what the problem was and it's treatable. I don't know how much longer I'll have my little buddy, but I want to give him the best life even if it might not be for many more years.
Anyway, I'm worried about Diomedes again. Since getting him home and giving him medications for hyperthyroidism, he doesn't seem to be improving. Some decent bowel movements but infrequent. No constipation, as far as I know.
But..he's not eating as well as he used to, even canned food he liked in the past. And since starting on the thyroid meds, he's drinking a LOT more water. Like, he didn't drink water all that often, but now I'm refilling his bowl almost every day. Even with a full bowl of food, he'll go to his water instead.
Concerned, I called the vet, who are thankfully open 24/7. After discussing the matter, she thinks it might be related to kidney disease or kidney failure. She said hyperthyroidism can mask the bigger problems with kidney failure (something to do with blood pressure). He was due to have his thyroid levels checked soon, so I booked to bring him in this Thursday.