Your cat might be thirsty for a good reason. Perhaps she has just eaten dry cat food which should make a domestic cat thirsty because it’s so dry! It’s a good thing that she is drinking after eating it. There are obviously normal, healthy reasons to be thirsty so you have to decide initially whether your cat is abnormally thirsty. You will do that through observation by comparing how much she is drinking now and how much she drank in the past. If in doubt contact your veterinarian or, if you like, you can read a page I have on the subject of how much domestic cats drink [link].
But I think the question in the title is from a concerned person because their cat is abnormally thirsty. And the number one reason will be chronic kidney disease. With increased thirst there will be frequent urination. It is as if the water is passing straight through them without being processed by their kidneys.
Increased thirst is called polydipsia and increased urination is called polyuria. These are also signs of diabetes and perhaps less rarely hyperthyroidism. Polydipsia and polyuria are more common in older cats. Most very old cats develop some kind of kidney failure as it seems that the kidneys of domestic cats wear out faster than the other organs. Also, domestic cats are living longer and therefore there is more chance for their organs to wear out during their lifetime.
The reason why kidney failure causes polydipsia
A study entitled: High Water Intake and Progression of Chronic Kidney Diseases published on the US National Library of Medicine, National Institutes Of Health website, tells us why failing kidneys results in polydipsia and polyuria.
The kidneys are unable to concentrate the urine “maximally” (to a maximum level) which results in the kidneys excreting more water in order to eliminate the “solutes” (the waste products dissolved in the water) acquired in the food that a cat has eaten. Because of this, cats are losing more water and therefore they need to ingest more water to make up for that loss. As a consequence they are forced to drink more water which is presented to their owner as a very thirsty cat. Of course, you will also see your cat going to the cat litter far more often to pee.
For the sake of completeness, in addition to chronic kidney disease (CKD), sugar diabetes and hyperthyroidism, liver disease and urinary tract disease can also cause increased thirst and urination.
Like many cat guardians, I have experienced, first hand, my cat dying of chronic kidney disease. This was many years ago but it was very noticeable that she was drinking and urinating a lot more. There shouldn’t be any doubt about it. Also, the litter box will be much wetter than normal and you will notice your cat drinking all the time if it is particularly bad.