What is Hypothyroidism?

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. Breeds that seem to be predisposed to this condition include the Cocker Spaniel, Miniature Schnauzer, Dachshund, Doberman Pinscher, Golden Retriever, Airedale Terrier, and Irish Setters.

Hypothyroidism is extremely rare in cats and is most commonly seen in cats following bilateral thyroid removal or radioactive iodine therapy for hyperthyroidism. This is often transient and usually does not require therapy. Rarely cats can have congenital hypothyroidism.


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. Some of the more common symptoms include:


Exercise intolerance

Mental dullness

Weight gain without a change in appetite


Cold intolerance

Changes in coat and skin, like increased shedding, hair thinning, and hair loss

Thickening of the skin

Reproductive disturbances in intact dogs


According to the Merck Veterinary Manual, hypothyroidism is one of the most over-diagnosed diseases in dogs. This is because many diseases mimic hypothyroidism and some diseases may affect the thyroid levels. Veterinarians diagnose your dog based on clinical signs and careful diagnostic testing. This typically requires several blood tests to determine which abnormalities may be present and also includes regular monitoring of your dog’s thyroid levels. 

One of these tests will most likely be a test of your dog’s T4 concentration. This is a good screening test, but other tests may be necessary to get a definitive result. 


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 T4 level in two to three weeks, with the blood drawn between 4 to 6 hours after the morning dose. Once the correct dose is found, it is recommended to perform a T4 every six to twelve months.


American Kennel Club: https://www.akc.org/expert-advice/health/thyroid-disease-in-dogs/


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