T4 or Creatinine first?

I've got an 11-year-old female who is draining her water bowl twice a day. Her urine is getting lighter, which worries me. I was looking at her increased thirst and thinking about testing. Should I start with a T4 level or a creatinine check? I know Smokey has glaucoma and costs enough, but I want to be smart about this before the next vet bill hits.

If the cat is losing weight, T4. If not, creatinine. Jasper is 7 and has hyperthyroidism, so I see that side often. Managing thyroid levels is tricky but usually the first thing we screen for in polydipsia.

No weight loss yet. She's actually a bit chunky, like Mittens. Does that change things? I recall price of blood panels being steep, so I don't want to run both if one will tell me enough.

Wait, does the cat have arthritis? My friend's cat drinks more when in pain? Whiskers and the hip dysplasia thread mentioned something about pain causing weird behaviors? Or is that just me being paranoid because Ginger won't stop jumping on counters?

@valm81 No arthritis that I know of. She moves fine. I'm looking at this strictly as a kidney vs thyroid issue. Lady's kidney numbers showed that creatinine can climb fast in 9-year-olds, but she was already diagnosed.

i deal with hairballs so i know about weird digestion stuff. Smokey's hairball schedule is a nightmare but not related to thirst. just saying. if the urine is light its dilute so kidneys might be failing to concentrate. that sounds like creatinine to me?

I found a thread where someone asked a similar question and the answer was to check the medical history first. My cat's history actually links to her last year's workup which showed mild protein in urine. Does that tip the scale toward kidney or thyroid?