First, I do want to note that you have taken the right course of action so far in trying to determine the root of your lass's weight loss despite an excessive appetite (polyphagia). I agree that it is quite abnormal and frustratingly our common issues that would be revealed by blood sample (diabetes, hyperthyroidism, liver/kidney disease) appear not to be the basis for her signs.
With those aside, we need to consider other potential causes. When doing so, we can divide into 3 categories: (1) conditions that cause weight loss due to decreased nutrition intake (obviously not the case with her good appetite), (2) those due to increased output (again less likely if she isn't having diarrhoea but we cannot rule out kidney/urinary based protein loss with her signs), and (3) those internal issues that cause weight loss by siphoning nutrition away from the body (this is where those organ and metabolic diseases would fit but other issues would include worms, cancer, etc). So, in your lass's case, the main areas fo focus for us would be to consider whether weight is being lost due to losses from urinary protein or if there is something internal stealing nutrition from the body.
In regards ***** ***** to the bottom if her signs, to start if you haven't already, you should consider worming her. Severe worm burdens can cause increased appetite and weight loss. This tends to be most common in young cats but it would be a good idea to worm her in case. Now there are a range of worming products available over the counter but you want to use a good quality wormer. In this situation, it would be ideal for you to treat her with Drontal, Panacur, or Milbemax as they will cover all the worms in question. Do make sure to have an idea of her weight before purchase wormer to make sure you get the correct dose for her size.
Further to worming, you could also consider having a urine sample checked. This can be a free catch from an empty litter box to check for abnormally low specific gravity (an early sign of kidney disease even before the bloods show this) and high levels of protein. That said, I would just note that if her kidney values were even high normal on the bloods, then you may want to speak to her vet about a urinalysis with a urea:creatnine ratio (which requires the urine be taken straight from the bladder via needle/syringe). The benefit of this is that it can help us determine if we do have abnormal protein losses in the urine and can detect very early stage kidney disease. And while kidney troubles don't often cause elevated appetite, you will find a few cats that do so alongside the weight loss induced by the condition.
Finally, since cancer is something we'd have to consider with her age and signs, you may want to speak to your vet about the potential for this. If they didn't feel any sinister lumps, then you may want to consider having her scanned via ultrasound. This can be helpful for picking up early stage tumors and those that diffusely spread through the organs instead of causing palpable lumps. This can be a very good non-invasive means of looking inside her to determine if there is something sinister present siphoning away her nutrition. Depending on their findings on scan, they may be able to sample any mass via ultrasound guided biopsy or may be able to see if this is something that can be removed with surgery.
Overall, you have taken the right approach thus far and ruled out a lot of our common causes for these signs. Therefore, at this stage, we do have to consider those other more subtle and potentially more sinister issues. Therefore, at this point do worm her if you have not already and consider a urine analysis and/or ultrasound scan (which would be done at the same time). These will hopefully rule out these other concerns but may shed light on which of these other potential causes are present and at the root of her signs.
I hope this information is helpful.
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All the best,
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