Long-Term Effects of Thyroid Disease in Dogs

Canine thyroid disease, or hypothyroidism, occurs when the thyroid gland does not produce adequate amounts of T4 and T3 hormones. Certain breeds, such as Great Danes, boxers, Irish setters and cocker spaniels, are more predisposed to the condition. Basic symptoms include lethargy, weight gain, hair loss, scaling, skin infections and cold intolerance. While synthetic hormones treat hypothyroidism, long-term effects of the illness can be expected if the condition goes untreated.

Dogs with hypothyroidism often suffer from abnormal heart rates and rhythms. However, the rare cardiovascular condition related to hypothyroidism is atherosclerosis. Normally uncommon in dogs, atherosclerosis occurs when lipids, cholesterol and calcium build up in the arteries, leading to blockage or causing blood clots.

Often, hypothyroidism affects tear glands in the eyes, leading to keratoconjunctivitis sicca. In this condition, normal tears are replaced with thicker mucus, resulting in dryness. Without treatment, this dry eye can lead to cornea ulceration or keratitis. Keratitis is an inflammation of the cornea that causes a cloudy appearance. Over time, complete blindness can develop.

Two major neurological conditions are associated with hypothyroidism in dogs. Megaesophagus affects the muscles of the esophagus, causing them to lose muscle tone. This lack of muscle tone affects food movement into the stomach, often resulting in regurgitation. Laryngeal paralysis affects the larynx. The larynx is responsible for preventing aspiration and allows for airflow as well as vocalization. When paralysis occurs, common symptoms include aspiration, change in barking, coughing and noisy breathing.

One of the most common symptoms of hypothyroidism is hair loss. With treatment, this hair typically grows back. However, the skin in the areas of hair loss may suffer from other long-term complications. Hyperpigmentation, or increased pigment, may affect these areas. Lichenification is a thickening of the skin, often leaving a dog with patches of elephantlike skin.

Two common endocrine conditions connected to hypothyroidism include diabetes mellitus and Addison’s disease. Addison’s disease occurs when the adrenal gland produces insufficient hormones, such as cortisol. Without adequate cortisol, dogs have difficulty dealing with stress, often leading to aggression. In diabetes mellitus, the pancreas produces inadequate amounts of insulin and affects the ability of the body to regulate blood sugar.