There are several diseases and conditions that can cause rapid weight gain in dogs. Hypothyroidism occurs when the thyroid gland doesn't produce enough thyroid hormone, which can cause weight gain and other symptoms. Cushing's disease, also known as hyperadrenocorticism, is a condition that occurs when the adrenal glands produce too much cortisol. Symptoms can include weight gain, increased appetite, and increased thirst and urination. Some types of heart disease, such as dilated cardiomyopathy, can cause fluid buildup in the body, leading to rapid weight gain. Dogs with diabetes may experience weight gain, especially if their blood sugar levels are not well controlled. Dogs with kidney disease may experience weight gain due to fluid retention. Some types of tumors, such as those in the liver or pancreas, can cause weight gain.