From the information that you wrote in your question "12 year old, rapid weight loss, Prior to the weight loss, he was diagnosed with Hypo-thyroidism because of a rapid weight gain, very lethargic, just lays around, doesn't respond to any stimulation, very sensitive near his back end, and lower abdomen, now back to just under his normal weight...but with all of the other symptoms...he is not himself.almost like he is slowly dying" I deduce that your dog either has anal sacs that require expressing or else he has feces in his rectum that needs to be helped to be eliminated.
Constipation and impactions can cause an elderly dog to become toxic and apathetic because of the absorption of waste products from the dog's impacted rectum into his blood stream. Occasionally the stool builds up inside of the dog's colon and causes pressure on the nerves to the other organs including the dog's heart. So that impacted feces can cause a dog's heart to stop. Lack of water intake or the reluctance to defecate on a regular basis due to weakness, lack of exercise ("very lethargic, just lays around" or due to painful anorectal problems e.g. anal sacs that need expressing or possibly an enlarged prostate can cause hard, dry feces. If your dog is on any drugs then they may be causing constipation. An X-ray of your dog's abdomen can show that he has fecal retention.
Constipation is a common problem in geriatric dogs. Usually once the dog's dry feces are softened and removed then the dog will be back to his active and playful self with a normal appetite. Feces can be removed either by oral milk of magnesia, 1/2 of a pediatric glycerine suppository, a warm tap water enema, or by manual removal. The longer that feces remain in a dog's rectum then the more toxic, apathetic, and lethargic they will become. The last part of the colon is where water is re-absorbed from the feces so that the longer the feces remain in the dog's rectum then the drier the feces becomes.
Your dog's anal sacs might need expressing. When a dog's anal sacs are full, then the dog sometimes will also not evacuate his bowels leading to an obviously toxic-looking elderly dog. So by removing the waste contents of your dog's anal sacs first, then he might have a good bowel movement. Be very gentle. Your vet can also remove the contents of the anal sacs. I advise that you observe the vet or whomever does this procedure to be sure that they treat your dog with loving care as most people don't especially enjoy performing this task. This is why I advise dog owners to learn how to express their dog's anal sacs. Usually if the dog is in the hands of its loving owner then it is more likely to have a better outcome from the procedure. You can learn how to express your dog's anal sacs. Always remember that "less is more" when you are treating your geriatric dog. It's like chipping away at a rock a little bit at a time will eventually get the job done. Place a paper towel against your dog's anal glands (they're located at the 4:00 and 8:00 position of your dog's anus as if it were a clock and as you look at him from behind. His anal sac will feel like a grape under his skin near his rectum. Be careful of his prostate gland. Very gently press them to express their contents. Or, put some K Y jelly on your gloved index finger and place it inside your dog's rectum and then gently squeeze his anal gland and express its contents. Usually this will be sufficient for the dog to have a bowel movement also. You might not want to view this link because it is graphically explicit regarding anal sac removal surgery but it will at least show you where the glands are located. http://www.thepetcenter.com/sur/as.html
Check your dog's teeth because sometimes elderly dogs have loose teeth that are painful so that they don't eat because they can't chew.
Offer your dog Hill's prescription diet a/d anorexic diet mixed with slightly boiled (for a few seconds) calve's liver or calve's kidney that's been chopped so your geriatric dog can gum his food if his teeth are painful. The fresh liver or kidney will stimulate his appetite. Also you can mix in 1-4 Tbsp of coarse wheat bran so that he has fiber. Increase the amount of fiber in your dog's diet. Dogs like vegetables. Give your dog a fresh carrot to chew on.
Give your dog Pedialyte because he is dehydrated. Dehydration occurs rapidly in elderly dogs that have lost their appetite. Some signs of dehydration are depression and loss of appetite. Place some Pedialyte in a bowl next to his regular water bowl. Sometimes a dog will not drink anything if they are offered a new source of hydration so it's wise to offer him both.
Common safe laxatives for a constipated dog are: milk of magnesia (magnesiuim hydroxide) is an osmotic laxative. Give your geriatric dog 1 teaspoonful of Milk of magnesia every day by mouth or Mineral Oil 1 teaspoonful every day by mouth which is a lubricant to help his bowels move.
You can give him 1/2 of a pediatric Glycerin suppository if you can't get him to take the milk of magnesia or the mineral oil. If he doesn't have a bowel movement after the 1/2 suppository then give him another 1/2 pediatric glycerin suppository in 8 hours. If still no bowel movement then you can give him an enema to moisten and soften feces making them easier to pass. Warm isotonic saline or tap water (5-10 mL/kg) will help to loosen the feces. If the impaction isn't relieved then you or the vet will have to use a gloved index finger to very gently remove any dry feces from his rectum. Apply KY jelly to your gloved index finger and be very gentle. Complete removal of all feces might take 2-3 days. Remember that a little at a time is much better than removing too much and causing your dog any pain in his rectum. Also, most of the time when you remove a little bit then the dog is able to eat better and he is stronger and can usually move the rest of the feces from his bowels on his own.
Sometimes pets tend to lie around waiting for you to motivate them and join them in some activity that is appropriate. Your Sheltie might be just bored. I know this sounds overly simplistic, however, Shelties are bred to work. They need to feel useful by participating in an appropriate stimulating activity (within their capability of course) so they will have an appetite for life and an appetite to eat. Spend some time appropriately exercising your sheltie and massaging his body (especially his abdominal area) to help him move his bowels.
Since he is a working breed he might feel like eating if he is encouraged to spend at least a few minutes outside everyday to stimulate his appetite for life and food. It might be enough just to sit outside with him in the fresh air for a few minutes every day to perk up his bored geriatric canine appetite. Maybe just play an easy gentle game of fetching a soft tennis ball inside the house if he is not able to go outside. Since your dog is geriatric that doesn't mean that he should be relegated to his doggie bed. Take his doggie bed outside if the weather is permitting and allow him to smell the stimulating fresh air and hear the sounds of the birds. If he can't go outside because of the weather being too cold then put him in the car with the heater on and then both of you go for a ride so that he can look out of the window.
Keep your dog warm so that he doesn't have to use up his energy maintaining his body heat. We keep our home at a constant temperature of 80 degrees because of our older pets.
Older dogs don't require much food to maintain their health like when they were younger. It's normal for an older dog's metabolism and appetite to gradually slow down because their heart muscle is not as strong and can't pump as efficiently as when they were younger but that doesn't mean they're going to die soon. Older dogs require more sleep to restore the myelin sheaths on their nerves and to restore their heart muscle fibers. Provide your dog with a quiet and dark place that he can go to get out of the glare of lights. Glaring lights are painful for older dogs.
Well, I hope this information is helpful for your dog. If you have additional questions I will gladly answer them, otherwise please click"ACCEPT". "POSITIVE FEEDBACK" and a "BONUS" would be greatly appreciated.