Hypothyroidism: Thyroid Disease in Dogs

Laurie Anne Walden, DVM

The thyroid, a gland in the throat, produces hormones that regulate many body functions. In dogs, the most common thyroid disorder is hypothyroidism, or low thyroid hormone production. Naturally occurring hypothyroidism is rare in cats.
 
Causes
In dogs, hypothyroidism almost always happens either for no known reason or because of an immune system condition called lymphocytic thyroiditis. Rare causes are cancer and thyroid malformation that’s present at birth. Hypothyroidism is more common in medium and large breeds than in toy breeds. Some breeds may be more likely than others to develop hypothyroidism [1]:

• Golden retriever
• Irish setter
• Doberman pinscher
• Miniature schnauzer
• Dachshund
• Cocker spaniel
• Airedale terrier

 
Signs
Decreased thyroid hormone levels reduce the overall rate of metabolism in the body and affect many body systems. Changes in the coat and skin are the most common results of hypothyroidism, but some dogs have few or no symptoms. The signs vary from dog to dog and can include the following:

• Weight gain
• Lethargy (“slowing down”)
• Low tolerance for cold weather
• Hair loss: hair thinning, increased shedding
• Slow hair regrowth
• Dry, flaky skin
• Darkened skin in armpits and groin
• Skin infections
• Ear infections
• Droopy face
• Nervous system abnormalities causing weakness or regurgitation of food (uncommon)

 
Diagnosis
The signs of hypothyroidism are not specific; they can be caused many disorders other than hypothyroidism. Some symptoms can improve with thyroid hormone supplementation even if the dog doesn’t have hypothyroidism. For this reason, hypothyroidism can’t be diagnosed only by the signs or by response to treatment.
 
Diagnosing hypothyroidism in dogs generally requires blood tests for more than 1 type of thyroid hormone. One of the main hormones produced by the thyroid is thyroxine (T4). Animals with hypothyroidism have a low blood level of T4. Most of the T4 in the blood is bound to proteins. The blood level of protein-bound T4 can be affected by illnesses other than thyroid disease. A blood test for total T4 (which includes protein-bound T4) is sometimes used to screen for thyroid disease, but this test by itself isn’t reliable for diagnosing hypothyroidism. The blood level of free T4 (T4 that is not bound to proteins) is more accurate for diagnosing hypothyroidism. Your veterinarian might recommend additional thyroid function tests if thyroid disease is suspected but not clearly shown on the initial blood tests.
 
Treatment
Hypothyroidism is treated by giving a thyroid hormone supplement. This medication is given by mouth once or twice daily for the rest of the dog’s life. Your veterinarian will likely recommend regular blood tests to be sure your dog’s dose is correct (for example, dogs that lose weight after a few months of treatment might need a different dose tailored to the new weight). Most dogs respond well to treatment.
 
Reference

1. Peterson ME. Hypothyroidism in animals. Merck Veterinary Manual website. https://www.merckvetmanual.com/endocrine-system/the-thyroid-gland/hypothyroidism-in-animals. Accessed October 23, 2019.

 
Photo by Jamie Street on Unsplash

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