What is the canine thyroid gland? 

The canine thyroid gland is composed of two parts, or lobes, each located in the front of the neck along the windpipe (or trachea). Thyroid hormones are responsible for maintaining a healthy metabolism, allowing all the body’s organs to function normally.  

What is hypothyroidism? 

Hypothyroidism is an endocrine (hormonal) disease that occurs when the thyroid glands produce lower-than-normal amounts of thyroid hormones.   

How does it happen? 

More than 90% of dogs that are diagnosed with hypothyroidism have what is called primary hypothyroidism, which can result from two processes: 

• Thyroiditis: Healthy thyroid tissue is lost due to the dog’s own immune system creating inflammation within the glands. The process can occur over months or even years, and it often has a hereditary origin.   

• Atrophy: Normal thyroid tissue is replaced by fat and connective tissue. This may also be a final stage of thyroiditis.  

There are other causes that are much less common, and these include cancer of the thyroid gland, diseases of the pituitary gland, congenital defects and external trauma.  

Other diseases can also cause a condition known as Euthyroid Sick Syndrome. Dogs with this illness may have abnormal thyroid tests, but this is not the same as hypothyroidism itself.  

What dogs are at greatest risk? 

Hypothyroidism most commonly occurs in dogs between the ages of 4 and 10 years (on average, ages 6-7). 

Males and females are equally affected, but it may be more prevalent in spayed females and neutered males. 

Breeds most commonly diagnosed:

• English Setter, Dalmatian, Basenji, Rhodesian Ridgeback, Old English Sheepdog, Boxer, Maltese Dog, Chesapeake Bay Retriever, Beagle, Cocker Spaniel, Shetland Sheepdog, Siberian Husky, Border Collie, Husky, Akita, Golden Retriever 

Breeds least commonly diagnosed:

• Chihuahua, Lhasa Apso, Pomeranian, Miniature Pinscher, Cairn Terrier, Basset Hound, Schnauzer, Yorkshire Terrier, Boston Terrier, Norwegian Elkhound, Greyhound, Portuguese Water Dog, Newfoundland, Bichon Frise, Welsh Corgi, Miniature Schnauzer, Cavalier King Charles Spaniel, Flat Coated Retriever 

What are the signs of hypothyroidism? 

Most common signs  

• Lethargy, dullness, decreased activity level  

• Weight gain 

• Cold intolerance 

• Dermatological signs: abnormal hair loss not associated with itching (most commonly along the trunk, base of tail, chest, bridge of nose), poor coat quality (dry, brittle, dull), darkening of the skin, recurrent skin and ear infections, slow hair regrowth after clipping  

Less common signs

• Gastrointestinal: vomiting, diarrhea, constipation, regurgitation  

• Nervous system: weakness, disorientation, seizures, coma 

• Heart: low heart rate, abnormal rhythms 

• Reproductive: reduced fertility, low birth weight 

• Eyes: lipid deposits on the cornea 

How is it diagnosed? 

• Observation of clinical signs 

• Routine lab work may show abnormalities such as anemia, elevated kidney and cholesterol levels  

• Definitive diagnosis is made by measuring thyroid values in the blood:   

  • Thyroid hormones: Total T4, Free T4, Total T3, TSH 

  • Thyroid autoantibodies (indicates a hereditary origin) 

How is it treated? 

Your veterinarian will prescribe a synthetic thyroid hormone called levothyroxine. It is usually given twice daily, but some dogs will do well with once daily administration. This medication will need to be given for the rest of your dog’s life.  

After starting medication, your veterinarian will want to see your dog in a few weeks for an additional exam and blood tests. Periodic exams and blood tests will be recommended for life.  

What is the prognosis for hypothyroidism? 

With an accurate diagnosis, as well as appropriate treatment and monitoring, hypothyroidism carries a good prognosis (overall success rate). 

Many of your dog’s abnormal signs will show improvement within a few weeks, although improvement in skin and haircoat may take several months.