it is a simple blood test, & then a tiny pill to the dose she needs to re balance her Thyroid hormones, usually they pick up massively within a few days
Does she show other symptoms??, weight loss, jumpy, looking a bit bedraggled etc?? if not, it may have just started, or it something else causing it, if she's "hissy" if you feel around her abdomen & any sickness or loose stools then that would indicate Kidneys.....that also is a simple blood test
Your Vet will know best, especially as she has other health issues, it may be related to that, I'm not a vet & don't know your lovely girls history, & can only reply based on my experience of the Cats I've owned over the years
Maybe ring your Vet & speak with them, see what they have to say, I'm sure they won't have you stress her for no good reason, & if its Kidneys it really is important she's seen quickly, I lost my old Tom Cat as he seemed to pick up, so I didn't take him & put it down to something he'd eaten....he hadn't, just the weather cooled down so his Kidneys were under a bit less stress, he had to be PTS:(