Why is my dog shaking and weakness in leg and increase thirst?

The vet has take blood and came back fine and did a body scan came back fine. He did do something to his back after jumping off the bed, He has nooo previoulsy health problem,

Shaking is most always associated with epilepsy. I suggest you better take a second opinion and get some neurological tests done for your dog to make sure of his condition.Here's a link that can give you helpful information on the early signs of epilepsy and its causes. hope this helpshttp://elfurl.com/3kp58

epilepsy, kidney failure, hip dysplasia...all cause the symptoms you are describing. i would be getting a second opinion.

It's possible the shaking is due to pain from a pinched nerve. Dogs can slip a disc just like people, especially with his history of back injury. I'm not sure where the increased thirst is from unless the pain is causing stress, which in turn is making him thirsty. I'd have his back x-rayed and maybe check into a canine chiropractor. They can work wonders on animals. I've used chiropractors on my horses and my dogs and they do make a difference. I'd also see if your vet could perhaps provide a mild pain reliever such as Rimadyl for short term use to see if it helps.