17 yr. old male siamese cat

Question
He is getting very thin, but eating as much as before.  He was constipated  for a while but has had a large bowl movement in the last few days.  He throws up at least once a day.  What can I give him to put some weight on him?

Answer
Hi Mary!  I am a big fan of siamese...they are beautiful!  I have a little girl of my own.
I would highly recommend a vet visit for some bloodwork.  He is showing symptoms of a few common "old age" ailments like Renal Disease, Hyperthyroidism, Mega Colon, ect.  A CBC/Chemistry blood panel done by your veterinarian will be able to pin point the problem and treat accordingly.
A lot of older cats develop kidney/renal problems. This will show as the Creatinine level in the blood work. Although this can not be reversed, some fluids and a special diet can help maintain kidney values at a functioning level, and many cats continue to live normally throughout the rest of their lives.
Hyperthyroidism is where the thyroid gland is under-active.  Soloxine, Thyrone, ect., medications are used to control this with a great success rate.  Your veterinarian may perform a T-4 blood panel, which will test the values of the thyroid gland.
Mega Colon hits almost 75% of all elderly cats.  Some cases are so mild they are unnoticed, while others are so prominent they could be life threatening.  Mega colon is where the colon literally can not function properly anymore.  The nerve endings become damaged and the muscle is no longer able to push feces out as normal.  Cefulac Syrup and Cisapride are two medications that are used to treat this ailment.
These are at the top of the list of things I would consider your kitty having, given the symptoms, but the only way to have this ailment diagnosed and treated would be by your veterinarian.  I hope this has been helpful and please, if you have time, let me know how he does.  I will be thinking of that blue-eyed boy!
Give him some get well kisses for me,
Hilary