Cat lost 1/3rd of weight

This is a bit long.  Sorry. Some background: 11 years ago we took delivery of three kittens at 8 weeks, one male and two female, not from the same litters (we’ve owed five cats previously and have always multi cat household).  These three all lived in perfect harmony and there was a particularly strong bond with one of the females in that it tended to want to be in the vicinity of the male cat – they would go out and come in together and frequently the female would groom the male (all three were neutered at 6 months old).  All are wormed regularly and have their annual jabs. Recently we lost the male (11 years old) and the behaviour of the female changed immediately.  She stayed outside all the time, become more dominant and rarely came in and she lost weight dramatically, within a few months she went from 4.0kg to 2.9kg.  Observing her outside it is clear she has diarrhoea, she is hungry all the time and eats constantly but the weight doesn’t improve, presumably due to the diarrhoea.  Her outside hours have settled down and she is now back to normal indoor / outdoor time but she is still very thin although her general mood is back to how it was.  There is no sign of parasites or worms that we can see. She has been to our vet and they can find nothing wrong, in fact they say there is no sign of diarrhoea but watching her in the garden, there clearly is.  We considered that it may be stress due to the change in circumstance but it is nearly a year since we lost the male.  It is also clear that she has taken over defending the territory, something the male used to do. So we’re at a loss to know what to do to get her weight back up.  Any advice greatly appreciated.

Hello!   I'm a bit foxed that you tell me that your vet has 'found nothing wrong' - if a cat has dropped from 4.0kg to 2.9kg, this is a huge adverse finding in itself (more than a quarter of body weight lost) and worthy of investigation.  Did they propose a work-up (e.g. blood tests etc?)  If blood and urine tests came back clear, what did they suggest next?  Unexplained loss of a quarter of body-weight would normally lead to further investigation.  I don't know what stage has been reached in proceedings, so it is very difficult for me to suggest what you should do - further, you already have a vet working on this.  I do strongly urge you to visit or phone them (whatever your surgery does), in order that you understand 1) What the vet thinks is the cause of the weight-loss, or what possibilities they still have on their list  2) How to rule out each of the causes still on that list e.g. diabetes can often be ruled in or out with various tests / history  and 3) What your vet proposes is done next and on what time scale, as this may well be urgent.   Please do let us know how you get on.

Usually, when a cat eats and eats but loses or doesn't put on weight, it is because the energy gained from the food is being used too quickly elsewhere.  Possibilities for that particular symptom include diabetes (where the energy gained can't be used in the cells, so is pushed out through the kidneys - lots of pee), hyperthyroidism (which increases the metabolic rate several-fold), cancer (which also increases the metabolic rate several-fold), and liver disease.   None of these is minor and we would strongly advise speaking to your vet ASAP.

PS To answer your query, worms, contrary to common belief, almost never present like this.  I would be concerned that there may be more going on.

Thank you for the response.  I should have mentioned our vet did already do a full blood workup and it came back showing nothing.  She is actually booked in again this week already.

The term 'full work up' has no set definition, but here we use it to mean investigating the clinical signs systematically until every possibility has been ruled out.  Helpfully, polyphagia (eating excessively) with weight-loss is a common (but serious), presentation in older cats. There are many set protocols / patterns for finding the cause. If they have more tests booked, then it sounds as though the process simply isn't complete yet.  When it is complete ( as far as the GP vet can complete it), if they still haven't found the cause of the clinical signs, there are people they can refer to depending on what they are most expecting to find, eg for MRI scans in case of difficult to detect tumours, or blood tests specific to more obscure diseases.   Sometimes the process can seem everlasting and overwhelming, so questions that are always useful include:  'what is still on your list of possibilities?' and 'how many of those could we do something about?' I can't make suggestions because I don't know what has been ruled out already, so can only say: 1) Hang on in there and 2) ask your vet to explain where they're up to, a lot, as it makes the whole process seem like less of a black box. All the best and we hope that they have some useful news for you soon.  

Thank you for the useful assistance.  Once we find finally what it is, I will of course post up for other members who may encounter the same.  I will ask the vet about the workup though.

Vet just called with new blood test results and has diagnosed hyper thyroid.  Bit perplexed as to why the 'full' bloods they did in July didn't pick that up but anyway at least we now know what it is.

Hello!  There could be a couple of reasons for this, but the most likely may be that your cat didn't have hyperthyroidism back in July; how long have the signs been going on for?  You are right in saying that sometimes a routine full blood profile doesn't include T4, so this may also be possible; I wonder if your cat was underweight with a racing heart back then?  6 months is a long time in the metabolic life of a cat.  There is, of course, no such thing as a blood profile that picks up everything that it is possible to pick up; even some full in-house profiles may not include a T4 test for hyperthyroidism;  rather the word 'full' implies using the machine to its maximum breadth of tests, including testing electrolytes, for example, which are often optional.  However, even if a T4 is not tested, a profile run in a hyperthyroid cat will often include clues, for example a raised ALT in isolation, which tends to prompt the vet to look further. I hope that this helps - but for a full explanation of your particular case, you'd need to speak to your vet.