Overactive Adrenal glands (Cushing's disease = hyperadrenocorticism) or corticosteroid medications often cause low specific gravity urine in dogs. (2ndchance.info)
For example a cat with normal kidney function will have a high specific gravity (as in very concentrated urine) whereas a cat suffering from kidney failure will measure in at low specific gravity (or diluted urine). (best-cat-art.com)
(ref) But persistently low specific gravity urine in a pet is always a cause for my concern and an indication that further tests are needed to determine the underlying cause. (2ndchance.info)