Sudden weight loss and behaviour change

My 12 year old boy has all of a sudden started to drop a lot of weight, he has always been a big cat and it’s very noticeable over the last 2 weeks he is ravenous all the time but the last few days I have noticed his coat is almost hard and sticky in places and he has gotten extremely clingy and has started to cry loudly today for no reason, he has already lost some vision in one eye some years ago totally unrelated but the last 24 hours he is acting totally out of sorts and wanting to be sat on top of me in bed and covered with “our special blanket” and reaching out to touch me all the time and even shouting really loudly as if he can’t locate me if he has returned to the room and I don’t immediately acknowledge I’m there waiting for him, he has been acting like ge doesn’t get fed for a few weeks now, we do have other cats including his brother from the same litter who is fine and not showing any signs of any problems, Boots has always been the extra spoiled kitty in the house with me ( the others are spoiled by other house members equally) but his behaviour has totally changed and is very worrying and has been noticed by everyone how he is extra clingy/hungry and now seems quite disorientated at times, it is a Sunday tomorrow and this behaviour has become much more apparent the past 24 hours, I was going to book an appointment Monday anyway because of his weight loss but I’m becoming more concerned by the hours, as I type he is nudging the phone away from me and trying to push his head into me as if for reassurance I’m here when he is lying on top of me with our blanket over him and I have my other hand on his back to comfort him but he is reaching out checking im still here, I’m not sure what I’m writing this for, I know he needs to see a vet asap, I think I just would like any reassurance he will be ok or similar situations with good outcomes as I’m not ready for bad news he is my ginger baby bit and is almost 13 years old i was there when he was born and he’s like a big brother to my human children

Hello - I afraid I cant tell you that your cat is going to be ok, but the possibilities do include hyperthyroidism and diabetes (which can be helped if caught early enough) as well as others, including some diseases that can't be treated.   If he has taken a sudden turn over the last few days, you may need to see the vet as soon as possible so do please call the vets for triage.   Triage is where they listen to the symptoms and decide how urgently your cat needs to be seen.  Untreated diabetes can be an emergency.  Do please let us know how things go.

Participant: Sounds very familiar to our elderly cat as well. He was diagnosed w/ hyperthyroidism and most likely also has cognitive decline disorder (or kitty dementia). Look up "sundowner syndrome" symptoms and see if that sounds like your baby. I ordered  a powder from Amazon that we sprinkle in his food each day and it has seemed to have helped reduce him from being so vocal at night time/bedtime. Also placing him on the thyroid meds helped the most.  Good luck with him. It can become exhausting!