Hypothyroidism, or low levels of thyroid hormone in the body, is the most common endocrine system disorder of dogs. Its prevalence has been estimated to be higher than 1 in every 200 dogs, most often occurring in dogs 4-10 years of age. Hypothyroidism is rarely seen in cats, and when it does occur is usually congenital in nature.

The thyroid gland is a bilobed organ which is located just behind the larynx (the "voicebox"). Its function is to produce the thyroid hormones T3 and T4. Most thyroid hormone is found as T4 in the blood, and is converted to T3 in the body's cells. These hormones act to regulate the body's metabolic rate.

Hypothyroidism can occur for several reasons. The most common type (>95%) is primary hypothyroidism wherein the thyroid gland itself atrophies or is attacked by the pet's own immune system. Primary hypothyroidism may also occur if a cancerous growth destroys the gland. Secondary hypothyroidism is seen if there is a lesion in the pituitary gland (a part of the brain) reducing the amount of Thyroid Stimulating Hormone (TSH). Additional (rare) causes of hypothyroidism include: an inability to convert T3 to T4 in the cells, iodine deficiency and congenital abnormalities. This last form is usually noted early in life, with the patient showing dwarfism and mental dullness.

The symptoms of hypothyroidism may be very apparent or relatively subtle. There is no one sign which is specific for the condition.

Common symptoms include:

Hair loss

Hyperpigmentation and/or thickening of the skin

Seborrhea

Recurrent skin infections

Obesity

Lethargy/Depression

Cold intolerance

Less frequently seen symptoms:

Decreased libido in males and lack of reproductive cycling in females

Neuropathies (nerve dysfunction) such as facial nerve paralysis and vestibular disease

Myxedema- a potentially life threatening state marked by edema, weakness, and comatose attitude

Diagnosis of hypothyroidism involves integration of clinical appearance and laboratory testing. Blood samples are taken to check thyroid hormone levels. The most common test is for Total T4. A more specific test measures Free T4. Rarely a test measuring the body's response to Thyroid Stimulating Hormone may be run.

Hypothyroidism may occasionally be mimicked by a condition known as euthyroid sick syndrome. This is a suppression in thyroid hormone levels due to concurrent illness or drug use. Prolonged steroid use, Cushing's Disease (hyperadrenocorticism), chronic illness, neoplasia (cancer), fever, anti-seizure therapy, and use of some types of antibiotics may all lead to misleading thyroid test results.

Treatment for this disease is through supplementation of thyroid hormones. Synthetic T4 is given orally. Regular testing of thyroid hormone levels will be required to make sure that the level of supplementation is meeting the pet's needs. Most cases see improvement of clinical signs within a few months.