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Hypothyroidism is the natural deficiency of thyroid hormone and is the most common hormone imbalance of dogs. This deficiency is produced by several different mechanisms. The most common cause (at least 95% of cases) is immune destruction of the thyroid gland. It can also be caused by natural atrophy of the gland, by dietary iodine deficiency, neoplasia (primary or metastatic) of the thyroid gland or (rarely) as a congenital problem. Hypothyroidism is most common in medium to large breeds of dogs that are middle aged (4 to 10 years) but can occur in any dog. Hypothyroidism is extremely rare in cats.
Thyroid hormone serves as a sort of volume dial for metabolism. Since virtually every cell in the body can be affected by thyroid hormone it is not surprising that reduced levels of thyroid hormone can lead to symptoms in multiple body systems. A recent published survey of hypothyroid dogs showed the following percentages of symptoms:
88% had some kind of skin abnormality
40% had hair loss (often on the tail or on both sides of the trunk and flanks).
22% had skin infections
14 % had dry brittle coats with hair that could easily be pulled out
49% were obese
48% were described as lethargic or listless at home
36% were anemic
80% had an increase in blood cholesterol
Hypothyroidism is treated with the oral administration of thyroid hormone, usually given twice daily for the life of the dog. Periodic blood testing is recommended; it is important to know if the medication dose is too low or too high. Thyroid supplement is a safe medication but if it is not given in sufficient doses the patient will not be adequately treated. If the dose is too high excessive water consumption, weight loss, and restlessness can result. Once a pet is started on thyroid supplementation, it is recommended to check a thyroid hormone (T4) level in two to three weeks, with the blood draw between 4 to 6 hours after the morning dose, with the dog fasted for 12 hours. Once the correct dose is found, it is recommended to perform a T4 every six to twelve months.