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Hypothyroidism in the Dog
by Jennifer Fry, VMD

Hypothyroidism is a multi-systemic disorder resulting from deficient production thyroid hormone. Thyroid hormone regulates metabolism, and therefore, affects almost every system in the body. Hypothyroidism is a very common endocrine disorder that affects middle-aged dogs. Genetics can play a role in the development of hypothyroidism and certain breeds such as Golden Retrievers, Doberman Pinschers, Irish Setters, Cocker Spaniels, Boxers, Miniature Schnauzers, Dachshunds and Poodles are more predisposed to the condition.

The most common signs of hypothyroidism are: weight gain despite limited food intake, lethargy, weakness,
poor skin coat (dull or dry coat, oily skin, scales), hair loss or slow hair regrowth, ear and skin infections, cold intolerance, diarrhea, constipation, fatty deposits in the eye, and occasionally seizures. Clinical signs are slow to develop and are only evident when 75% of the thyroid gland is not functioning.

Decreased production of thyroid hormone is associated with destruction or degeneration of the thyroid gland. In most cases the cause of hypothyroidism is due to lymphocytic thyroiditis (an immune-mediated disorder) or idiopathic atrophy (unknown shrinkage of the gland). Less than 5% of the cases of hypothyroidism are related to disease within the pituitary gland. In addition, hypothyroidism can be induced by prolonged administration of glucocorticoids (steroids).

Hypothyroidism is diagnosed by a blood test to measure the level of thyroid hormone. Other blood tests may be performed to evaluate other body systems since concurrent diseases such as Cushing's disease (characterized by an abnormal production of steroids) or Diabetes can cause low levels of circulating thyroid hormone. However, controlling these disease processes usually eliminates the need for hypothyroidism treatment. Other systemic illnesses such as kidney and liver insufficiency, heart failure, or immune system disorders can falsely lower thyroid levels and this phenomena is known as euthyroid sick syndrome.

If your pet is not treated, s/he will continue to exhibit the signs of hypothyroidism. Spontaneous remission does NOT occur; however, with the proper veterinary treatment your dog can resume a normal life with normal life expectancy.

Treatment involves supplementing with a synthetic form of thyroid hormone (levothyroxine) once to twice daily for life. Activity usually improves after 7 to 10 days of treatment; however, improvement of skin conditions may take up to 6-8 weeks. The first few months of treatment require repeated blood tests to assure the proper dosage of medication is being given. After the correct dosage is achieved, yearly blood tests are used for monitoring the condition.

Overdose of thyroid hormone can result in panting, nervousness, increased thirst and urination, increased appetite and weight loss. If you should notice any of these symptoms, please contact your veterinarian so the dose of your pet's medication can be adjusted.

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