I looked up something about a dog that drinks a lot of water & is skinny, this is what a response was (from http://www.justanswer.com/pet-dog/0mle0-dog-drinks-lot-water-really-skinny.html)
Diabetes, Cushing’s disease, or kidney problems. These are the Big Three of those signs, of which I’d vote for diabetes or kidney problems first.
Diabetes causes it because the high sugar in the blood makes the kidney produce urine to attempt to dilute it. Drinking is in response. Weight loss is because the patient has trouble utilizing glucose for energy and is metabolizing his body fats for it.
Kidney problems are because the kidney is losing it’s ability to concentrate urine, thus saving body water, and is passing more water through into urine, instead. Drinking is an attempt to keep up with the losses.The weight loss is because, along with the loss of water, kidney problems also lose a lot of protein through the urine, and the body is breaking muscle down to maintain the blood levels of proteins.
Cushings causes several other problems. It is a condition of the adrenal glands and results in excesses of cortisone-like products in the blood. Cortisone stimulates urination, and drinking follows to make up for it.
Also, hyperthyroid will cause a weight loss, simply because the patient’s metabolism is turned way up. Excess water intake is not as common a finding. Rather, they usually have very healthy appetites.
I hope this helps. The suggestion is have your vet check it out before it turns into a bigger problem Best wishes.
& then my guess would be any chance it could be worms, sometimes they’re not diagnosed?
Piddles had a combo of roundworms & coccidia… but the roundworms weren’t visible at my vet’s microscope, or at the lab they sent it off too. She was horribly sick & skinny & my vet said it was just the coccidia, they didn’t think it was worms. Took her to a different vet & they saw the roundworms, I got the meds & it was like an overnight improvement (but it took quite some time for her to get fattened up). ALSO my cats had tapeworm (I saw it) when I first got them & they did a fecal test (this is at a different vet) & they said they had no worms. Umm no, I saw it & I know what tapeworm is!
Try a natrual de-wormer called…(see below)
found at this website: http://wolfcreekranch1.tripod.com/diatomaceous_earth.html
WORMS AFFECTED BY DIATOMACEOUS EARTH:
Food grade DE eliminates roundworms, whipworms, pinworms, and hookworms within 7 days of being fed daily. Studies that advise this are on our DE FAQ page and elsewhere throughout the internet.
We always suggest feeding DE for a minimum of 30 days for roundworms, whipworms, or hookworms, due to their life cycle and needing to catch any hatching eggs, as DE does not kill eggs.
Lungworms, including hookworms that have migrated to the lungs for which upper respiratory symptoms would be seen – sneezing, coughing, nasal discharge, runny eyes, etc., take double the recommended daily DE dose for at least 90 days.
ALSO: Here is a website that says how to help a skinny dog gain weight: http://www.ehow.com/how_4869623_do-skinny-dog-gain-weight.html