Specific gravity dropping

Spot is my hyperthyroid baby. I don't know his age ("older than 10") since I found him abandoned last year. He had radioiodine treatment in June, but his thyroid went up after treatment, so he's been back on the methimazole (Tapazole). At the end of July, his specific gravity of his urine was 1.034, well within the normal range. By the beginning of September, it had dropped to 1.024. It has been dropping ever since, and it is currently at 1.017 as of today.

His kidney values have always come back as normal, with his creatinine tending toward the low side. His behavior lately has been kind of antsy. He has been chasing my other cat around the house and picking on her a bit. He has been very cuddley with me most of the time. It's not his thyroid, which was actually low when we tested it two weeks ago (we lowered his dose and tested him again today to be sure it doesn't shoot up again).

Does anyone have any ideas as to what might be going on with him? I haven't been able to find much information about low specific gravity levels other than in connection to kidney disease. Since his kidney levels are normal, we don't think he has CRF. I'd love to hear suggestions from anyone with similar experiences or stories.

Is he on anything other than methimazole? A drug that has a diuretic effect (which methimazole does not normally do) could cause lower urine specific gravity. Diabetes would be another possibility.

He's on three things: methimazole (currently 7.25mg per day, 5 in the morning, 2.5 at night), Interferon (1mL of the solution once per day), and glucosamine chondrotin. He has only been on the glucosamine for about a month though, and his urinary problems started in July. I initially noticed because he peed outside the litterbox (something my other cat had done for a while) and it was pretty dilute, especially compared to Willow's urine. He had one incident in September where he had glucose in his urine, but his blood sugar was normal, so the vet thought it was a fluke. When his latest test comes back, I'll post his glucose results. Every other urinalysis he has had has been negative for sugar, so I'm leaning away from diabetes mellitus, though diabetes insipidus is a possibility. My vet says it's very rare in cats, but she and I both know that Spot doesn't follow the path of a normal cat.

Everything came back normal. We did a modified water deprivation test (gave him a small measured amount of water and tested the specific gravity in the morning). It came up some (1.019). Since all his other lab values are normal, the vet suggested that we just leave him be for a while. I'm going to try to keep him home for a whole month (if I can!) and then take him in for a weight check.

Any further updates on it? My cat is in a similar situation with the USG 1.034 in feb and now 1.024. He isn't hyperthyroid. 10yr old. Blood work being done Monday

Spot's primary issues were hyperthyroidism and heart disease.  He never ended up showing any signs of kidney problems between when I posted and when he died of heart failure, despite the low specific gravity numbers.  Those numbers could have been due to excessive drinking, in his case due to the hyperthyroidism.  I hope the bloodwork is able to provide some answers.

Sorry to hear of Spot's passing.  It gives me some hope that the low USG numbers may not be kidney disease 100%.  Both cats tested 1.024 and the other was 1.029.  Just need to wait on bloodwork results.  Also testing for thyroid issues as well.