Weight-loss risk in cats

A friend of mine thinks my 10-year-old domestic male neutered cat has lost weight.

- 

He is still eating and is a bit slower now, which I have assumed to be due to his age. What can cause weight loss, how can I measure this, and should I be concerned?Many diseases causing weight loss in cats show obvious signs and should prompt veterinary attention. Underlying diseases include tooth or mouth pain, liver or kidney disease, gastrointestinal diseases, infectious diseases, pancreatitis and abdominal tumours. Weight loss in these circumsta