Low thyroid function in dogs is called hypothyroidism. It is the most commonly diagnosed endocrine disorder in dogs. Affected dogs tend to develop hair or skin problems, gain weight and become lethargic.
Low thyroid production occurs for different reasons. The most common reasons are destruction of the thyroid gland by the immune system and atrophy of the thyroid gland. Other potential reasons are iodine deficiencies in the diet and birth defects. Except in congenitally affected cases, dogs are typically middle-age or older when they develop hypothyroidism. Certain breeds are predisposed to developing hypothyroidism, including golden retrievers, Doberman pinschers and Irish setters. The decreased thyroid hormone output leads to a lower metabolic rate.
Signs of Hypothyroidism
Weight gain is a common sign of hypothyroidism. Skin problems are commonly associated. If your dog is affected, you may notice hair loss or coarser hair. Hypothyroid dogs are more prone to developing bacterial and fungal skin infections. Some dogs develop oily, smelly coats. You may notice your dog becoming agitated or listless at home, or even lethargic. Red blood cell production may be decreased, causing your dog to become anemic. Your dog’s heart rate may slow, the lower thyroid output leading to less stimulation of heart contractions. Neurological problems rarely develop but, when they do, they can be noticeable, including a head tilt or facial paralysis -- noticeable by drooping skin on your dog’s face.
Your veterinarian may have difficulty diagnosing hypothyroidism. Non-hypothyroid dogs may have thyroid levels that fluctuate into hypothyroid ranges, making ruling out hypothyroidism difficult. Veterinarians typically start by measuring total T4 hormone, the inactive hormone that circulates in the blood stream before body tissues activate it. If a total T4 count is normal, the dog is unlikely to be hypothyroid. If the level is low, your dog may be hypothyroid or the level may have been lowered by a concurrent illness or certain medications such as phenobarbital. Free T4 tests may help confirm hypothyroidism, and TSH -- thyroid-stimulating hormone -- may be elevated in hypothyroid dogs. These tests can be utilized in combination to help diagnose your dog with hypothyroidism.
Hypothyroid dogs are treated with thyroid supplements. Dogs often feel better overnight with thyroid supplementation, but it may take weeks to months to see noticeable changes in a dog’s coat or weight. Affected dogs must be treated for life, and they are typically treated twice daily. Most veterinarians will treat a dog for a period of time and then retest their thyroid levels to ensure they are being treated adequately. Most dogs are retested periodically, such as annually, from that point forward.