Thank you Jane.
I have to say I am a wee bit concerned about Molly. If she is still using the litterbox for feces most of the time, this tells us that the problem is not the litterbox or litter. Instead, to see inappropriate urination in an elderly cat passing a large volume of urine, with lack of concentration changes, and her increased thirst all suggest we have another issue triggering her lapse in litter box manners.
Issues that can manifest this way at her age will include conditions like hyperthyroidism, diabetes, liver disease, and kidney troubles. As well, while it isn't nice to think about, we must keep in mind that some cancers in cats her age can manifest these types of signs with little other hint of their presence.
In regards XXXXX XXXXX which is the cause here, it would be ideal to have her checked by her vet (if she is due for a vaccination soon, you could move it up a wee bit early and have her checked out at that time). The vet will be able to have a feel of her and just make sure there are no sinister lumps and bumps to blame for these signs. If she is clear of obvious masses, then you may want your vet to check a blood sample to assess her organ health and rule out the above issues. This is the most straight forward means to diagnose which is to blame and therefore guide you on how to approach the underlying condition (which address this often settles the behavioral signs too).
Alternatively, if you want to take things a bit slower or aren't keen to see the vet at this stage, then you can consider first submitting a urine sample to your vet. Often we can obtain a ‘donation’ if the kitty is left overnight in a non-carpeted room with an empty litter box. The vet will be able to analyze it and determine if there is anything abnormal. They will be able to appreciate changes to the urine's white blood cell content (a marker of infection and common with diabetes), the presence of glucose/ketones (markers of diabetes) and bilirubin (a sign of liver disease). Furthermore, the vet will be able to check the urine's specific gravity, which tells us if the kidneys are concentrating the urine properly (since dilute urine is seen with kidney disease). Overall, this is quite a cheap and non-invasive means of checking if there are any of these differentials could be affecting her. And depending on what is found, this can tell you if one of these issues are affecting her or whether we can rule them out right off the bat. (Though do note that with hyperthyroidism, we can only test via blood sample since the hormone doesn't enter the urine).
Overall, I am quite concerned about Molly. While her behavior is the most obvious issue, it is very likely to be a side effect of one of these underlying conditions. Therefore, it would be prudent to have her checked out and at least have a urine sample (or blood sample) checked. Because if you are able to identify which of these issues are triggering her signs sooner rather then later, it will give you the best chance to address and treat them as effectively as possible. And if we find it is one of our more treatable conditions, then you will be able to halt these signs, get her back on track, and possibly get her using her litterbox as she should.
I hope this information is helpful.
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All the best,
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