Hypothyroidism in Dogs

Summary

Hypothyroidism is a common clinical condition in dogs resulting from reduced production of thyroid hormones by the thyroid glands. Lymphocytic thyroiditis is one common cause, but in other cases the cause cannot be determined (idiopathic thyroiditis).  

Hypothyroidism cannot be cured but it is easy to manage, and with proper veterinary care hypothyroid dogs are capable of living a normal life. The main symptoms of hypothyroidism are unexplained weight gain, lethargy, exercise intolerance, and changes to the skin and coat. 

Thyroid hormone levels vary over time and in response to a variety of medical conditions. Low thyroid hormone levels on screening blood tests suggest hypothyroidism, but a confirmed diagnosis may require additional blood tests. When diagnosed early, treatment of hypothyroidism is straightforward, requiring little more than daily thyroid supplementation and periodic monitoring.

Risk Factors

Hypothyroidism is a common condition in dogs. Most hypothyroid dogs are middle-aged at the time of diagnosis, around 6 - 7 years old. Data suggests that spayed females and neutered males are at higher risk. Analysis of the canine genome shows a specific location of an inherited predisposition for hypothyroidism in some dogs. 

Hypothyroidism is easy to treat and is not life-threatening if identified early and carefully managed. If left untreated, hypothyroidism eventually becomes life-threatening.   

Hypothyroidism progresses slowly in most cases, so ample time is available for diagnosing a dog showing early symptoms. In rare circumstances where early diagnosis and treatment do not occur, hypothyroidism presents with severe symptoms clearly indicative of an emergency: 

• Collapse • Weakness • Paralysis • Labored breathing • Seizures Hypothyroidism usually occurs in mid to large-size breed dogs between 4 and 10 years of age. Early detection is facilitated by routine wellness blood screening for senior pets.

Possible Causes

Potential causes of hypothyroidism include:

• Lymphocytic thyroiditis: in the case of lymphocytic thyroiditis the dog's immune system attacks the thyroid 

• Idiopathic atrophy of the thyroid gland: in the case of idiopathic atrophy thyroid tissue is replaced by fat and no apparent cause is identifiable.

Main Symptoms

The symptoms of hypothyroidism include: 

• Unexplained weight gain or inability to lose weight • Lethargy • Obesity • Mental dullness • Cold intolerance

• Hair loss • Increased skin pigmentation • Skin thickening • Exercise intolerance

Detailed Characterization

The thyroid gland regulates metabolism and calcium uptake. If left untreated in the long term, hypothyroidism can lead to severe and untreatable neurological disorders culminating in seizures, coma, and death.

Hypothyroidism is one of the most commonly diagnosed endocrine disorders in dogs, but other metabolic conditions may cause low thyroid hormone levels and metabolism.

Hypothyroidism is most prevalent in middle aged and older dogs. Screening T4 tests are included on senior wellness profiles for clinically healthy dogs.   Although not curable, hypothyroidism is easily treated. As long as the diagnosis is made early in the course of the disease and the dog is maintained on the recommended treatment, the prognosis is good and the dog’s life expectancy remains normal.

Testing and Diagnosis

After taking a thorough history and performing a physical examination, the next steps towards diagnosing hypothyroidism include: 

• Complete blood count (CBC) • Serum chemistry profile

• Total thyroxin level (TT4 or Total T4): thyroxin (T4) is the main hormone produced by the thyroid gland. 

Interpreting baseline thyroid hormone levels is not straightforward because levels fluctuate over time for a variety of reasons. A more specific blood test called a thyroid profile measures other aspects of thyroid hormone production and is used to clarify the diagnosis.

Steps to Recovery

Hypothyroidism is not curable but its treatment is straightforward. Treatment for hypothyroidism consists of once- or twice-daily oral administration of a synthetic thyroid hormone.

The initial dose is calculated based on the animal's weight but ongoing adjustments are necessary. Repeated thyroid profiles are necessary to regulate each dog’s individual response for the best outcome. No notable improvement of symptoms is expected within the first 4 to 8 weeks of hormone replacement treatment. 

Resolution of the clinical signs of hypothyroidism takes a few months, but with regular treatment and monitoring, a dog with hypothyroidism is expected to have a normal, full coat of hair and easily maintain a healthy weight.   

While a genetic predisposition for hypothyroidism is suggested, there is no known way to prevent it.

Prevention

Hypothyroidism is not contagious, nor is it preventable. Annual routine health screening of adult dogs aids in early detection which mitigates the development of severe symptoms.

Is Hypothyroidism Common in Dogs?

Hypothyroidism is a common clinical condition in dogs.

Typical Treatment

Daily oral synthetic hormone tablets and periodic monitoring of blood hormone levels. Ongoing treatment of hypothyroidism is required for the remainder of the dog’s life once diagnosis is confirmed.