The cat does need to see the vet, and have some blood work done. Older cats are prone to hyperthyroid, but it can be other things, as Karen pointed out. The diagnostic tools include the blood work.

If it is hyperthyroid, then left untreated, things deteriorate quickly. The heart and kidneys are unable to keep up. The liver also becomes quite stressed. Cats get the same treatment for this as humans: one pill daily to rebalance the hormones. The sooner it is diagnosed and treated, the better.

My first cat developed this. I'd never owned a cat before, and things had been 'wrong' for quite some time before I got him in to the vet. He developed a calcium build up which fused several of the vertebrae in his spine, making it almost impossible for him to jump - on my bed, the sofa, etc. So I bought some steps for him, and started the medication to halt things in place.