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Canine Hypothyroidism

by Dr. Bill Ehlers, D.V.M.

Hypothyroidism is the result of impaired production and secretion of thyroid hormone from the thyroid gland. It is a disease which occurs frequently in middle aged dogs of medium to large size, and the Golden Retriever is a breed predisposed to this disorder. Thyroid hormone deficiency causes decreased function at the cellular level of multiple organ systems.

Golden retriever image - Jenna

There are three categories of hypothyroidism, however the majority of cases are due to the category labeled primary hypothyroidism. Primary hypothyroidism can be divided into Iymphocytic hypothyroidism, which is present most commonly, idiopathic atrophy and neoplastic destruction. Lymphocytic hypothyroidism is due to a problem with the immune system which leads to the destruction of the thyroid gland. Idiopathic atrophy is a term used to describe the condition where there is an unknown cause of fat replacing functioning thyroid tissue. Neoplastic destruction is due to an expanding tumor of the thyroid gland. Clinical signs occur when more than 75% of the gland is destroyed.

Many body systems can be affected with resultant lethargy, mental dullness, exercise intolerance, weight gain, cold intolerance, infertility, weak or still-born pups, lack of labido, testicular atrophy, weakness, stiffness, muscle wasting, scaly skin, dull coat, hair loss, 'Rat tail' appearance to the tail, puppy type coat in an adult, skin infection, slow heart rate, arrhythmias to the heart, ocular abnormalities, constipation, diarrhea and vomiting. Lethargy, weight gain and symptoms involving the coat and skin seem to be among the most common.

The most common test of thyroid function is a blood test measuring total T4, a hormone of the thyroid gland. Total T4 is T4 hormone bound and unbound to protein in the blood. Variation in this value is possible due to certain medications such as cortisone, age, presence of other illness, obesity and breed type. When there is question about the validity of a low total T4, other nonthyroidal illness would be explored as well as a blood test to determine the levels of Free T4 (T4 unbound to protein) and Canine Thyroid Stimulating Hormone.

The initial treatment of choice is administration of synthetic T4 (sodium levothyroxine) which comes in a tablet form. Follow-up blood serum levels would be assessed 4 to 6 weeks after initiating supplementation and the dosage would be adjusted accordingly. Clinical improvement for non-dermatologic symptoms may be seen as early as I to 2 weeks, while dermatologic conditions may take up to 4 weeks to improve after initiation