[header] How to keep your dog or cat at its correct weight [title] Know the issues related to obesity. [step] The first step in combating pet obesity is to make sure your pet is not suffering from a medical illness that either contributes to obesity or is a complication from being overweight. In dogs and rarely in cats, hypothyroidism, or low thyroid hormone levels, can cause a pet to gain weight. 
Hypothyroidism can also cause a dog's hair to thin or fall out in patches. The dog may also become less playful and energetic.