Dogs gain weight slowly, so rapid weight gain is usually a cause for concern. More often than not, rapid weight gain is caused by a medical condition rather than too much food.
Normal Weight Gain
Many dogs are overfed and under exercised, and become overweight as a result. To determine if your dog is overweight, rub your hands over his ribs when he is standing. You should be able to easily feel his ribs through only a thin layer of skin. If you cannot, your dog may be anywhere from slightly overweight to completely obese.
If this is the case, reduce your dog's food intake, including treats, or find a lower calorie diet for your dog. Gradually increase exercise until your dog is getting 40 to 80 minutes of high-energy exercise daily. This can include walking, jogging, running with your bicycle or playing fetch. Consult your veterinarian about an effective weight loss program for your dog.
Thyroid Condition
Sometimes a medical problem can cause sudden weight gain that may not be obvious to you right away. One of the most common causes of this is thyroid disease.
The thyroid gland releases hormones that regulate your dog's metabolism, so a thyroid condition could cause your dog to rapidly gain weight if it reduces the amount of hormones being produced. This condition is known as hypothyroidism and can cause weight gain, lethargy, dull or dry coat, or oily, greasy skin.
If your dog experiences rapid weight gain during a short period, visit your veterinarian for a blood panel. Though you may notice decreased energy level, rapid weight gain without a change in food could be the only symptom.
Cushing's Disease
Cushing's disease is caused by the overproduction of glucocorticoids, steroids which are responsible for several normal body functions. This is more common in middle age and older dogs, but it can affect any dog.
One of the most common symptoms is rapid weight gain or abdominal enlargement. Additional symptoms include increased eating, drinking and urination, as well as hair loss and thin skin. This can be diagnosed with a blood test.
Bloat
Bloat occurs when the stomach rapidly expands with air, often caused by gulping while eating, and then rotates, blocking the normal movement of the digestive system. If not treated immediately, this can result in death.
Symptoms include a swollen belly that appears like rapid weight gain, as well as abdominal discomfort, unsuccessful attempts to vomit, pacing and heavy panting.
Deep-chested dogs such as Rottweilers, Labrador retrievers, golden retrievers, great Danes, doberman pinschers, Newfoundlands, German shepherds and standard poodles are more likely to develop bloat.
Other Causes
A potbelly appearance can look like sudden weight gain when instead it is the indicator of a serious internal illness. Causes of this potbelly appearance include heart or liver disease, different types of cancer, a ruptured bladder or enlarged abdomen. If you notice a potbelly appearance, immediately consult a veterinarian to prevent the problem from further developing.
Though weight gain is most often caused by overeating and under exercising, rapid weight gain can be caused by an illness. If you notice a sudden weight gain or pot-bellied appearance in your dog, consult your veterinarian.