Canine Hypothyroidism

Elizabeth Adkins, DVM, MS, DACVO
NSCA Health Committee Chair

I wrote an article on this topic a few years ago but have been asked to write a new one since NSCA has many new members and much interest has recently been expressed about this disease.

Dogs with hypothyroidism do not produce the normal levels of thyroid hormone (called T4).  The most common type of hypothyroidism is primary hypothyroidism.  In most cases of primary hypothyroidism the thyroid gland is being destroyed by inflammation.  Other causes of primary hypothyroidism include atrophy of the thyroid gland or cancer of the thyroid gland.  The other major category of hypothyroidism is secondary hypothyroidism.  Secondary hypothyroidism is due to a disease of the pituitary gland (cancer or anatomical abnormalities).  Congenital hypothyroidism (the dog is born with hypothyroidism) is rare in dogs. 

Primary hypothyroidism is common in dogs and typically occurs between four and ten years of age.  It is more common in medium and large breeds than in small breeds.  A number of symptoms can be present in hypothyroid dogs including: lethargy, weight gain, intolerance to cold, a dry hair coat, dandruff, hair loss, excess skin pigmentation and skin infections.  This disease can also affect the nervous, cardiovascular and reproductive systems.  Rarely a life-threatening coma can be caused by severe hypothyroidism.

Thyroid hormones affect metabolism and almost every organ in the body can be affected; however, symptoms can vary a great deal in affected dogs.  As with testing for most endocrine (hormonal) diseases testing for hypothyroidism can be frustrating as there is no single test that determines whether or not the patient is hypothyroid.  Multiple tests may be required to diagnose hypothyroidism and the veterinarian must interpret the test results carefully. 

Since the symptoms of hypothyroidism can be caused by a number of different diseases there is concern that hypothyroidism is over diagnosed in veterinary medicine.  Many conditions can cause thyroid hormone levels to fluctuate and concurrent illness may cause these levels to be low when the patient is not truly hypothyroid (called euthyroid sick syndrome). 

Abnormalities that may be seen on blood work of hypothyroid dogs include mild anemia (low red blood cell count) and elevated cholesterol, triglyceride, alkaline phosphatase and creatine kinase levels.  Measurement of T4 can be a useful screening test for hypothyroidism; however additional tests such as a TSH (thyroid stimulating hormone) response test are required to confirm the diagnosis.  One frustration in diagnosing hypothyroidism is that the TSH response test is intermittently unavailable on the commercial market.

The initial treatment of hypothyroidism in dogs is oral supplementation with L-thyroxine, a synthetic thyroid hormone replacement.  The patient’s clinical signs usually improve two weeks to three months after treatment is started.  The thyroid level should be re-evaluated about eight weeks after initiating supplementation as some dogs will require an adjustment of the dosage.  Initially treatment is twice daily; however, once the patient has improved they may be able to be successfully maintained on once daily therapy.  In most cases supplementation is required for the patient’s lifetime. 


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