Skinny cats and fat dogs often have dysfunctional thyroids.
The thyroid gland is found in humans as well as cats and dogs. One of its most important functions is metabolism, which explains why being overweight or underweight can be the result of a thyroid disorder.
Dogs typically exhibit hypothyroidism. "Hypo" means lacking or not enough, signifying that the gland is underactive, thereby producing less than optimal amounts of the various thyroid hormones. Similar to individuals who are said to have slow metabolisms, hypothyroid dogs are often overweight.
Cats usually show signs of hyperthyroidism. The opposite of "hypo," "hyper" suggests the thyroid gland is overactive--even hyperactive. Just like humans with fast metabolisms tend to be skinnier, hyperthyroid cats are often underweight.
Iodine can affect the thyroid function in animals. Deficiency, for example, can result in hypothyroidism. Some vegetables cause complications for iodine absorption, resulting in depleted iodine levels despite a balanced diet. Veterinarians might recommend against these foods, like broccoli, kale, and soy.
Symptoms of hypothyroidism include excessive panting due to obesity, lethargy, skin issues, and hair loss or poor coat condition. Hyperthyroidism is usually showcased by an increased appetite with little to no weight gain, poor fur condition, restlessness, and rapid heart rate.
Thyroid issues in pets tend to occur in middle- to geriatric-aged individuals. Following symptoms of a thyroid disorder, blood work will confirm or deny the presence of a hormonal condition.
Medication is most often prescribed in either case; a thyroid hormone supplement in dogs or an inhibitor in cats. Radioactive iodine therapy is also sometimes recommended for felines. Nutritional changes might also be suggested by a veterinarian.
Thyroid disorders are often chronic issues that need to be monitored, but they can change over time. The underlying cause of the disorder, and why it affects dogs and cats differently, is not well known, though genetics likely play a role.