How is Canine Hypothyroidism Diagnosed?

Even though hypothyroidism is the most frequently recognized canine endocrine disorder in dogs, it is still difficult to make a definitive diagnosis of the condition. Since the thyroid gland regulates metabolism of all of the body’s cellular functions, reduced thyroid function can produce a wide range of clinical signs (see our last blog post). Many of these signs mimic those of other disorders and illnesses, making recognition of a thyroid condition and proper interpretation of thyroid function tests confusing and problematic for veterinarians.

The dog’s signalment can be important!

The first step in diagnosis of hypothyroidism is to review the dog’s signalment (i.e., age and breed). Most hypothyroid dogs are young adults, and certain breeds are predisposed to developing the condition (see our last last blog post on this topic). That said, we can see hypothyroidism in any breed of dog, and very young or old dogs may also be affected.

Dog’s history, clinical signs, and physical examination findings

Another part of how we diagnose is a review of the history, and clinical symptoms the dog with suspected hypothyroidism is exhibiting. The most common clinical features include mental dullness or lethargy, weight gain, and skin changes. (See our last blog post for a list of clinical signs and images of affected dogs)

General laboratory evaluation

All dogs with suspected hypothyroidism should have a general laboratory panel done prior to determination of any specific thyroid function tests. Ideally, this should include a complete blood count (CBC), comprehensive serum chemistry panel, and complete urinalysis.

• Complete blood count (CBC) – It is not uncommon for hypothyroid dogs to be anemic (normocytic, normochromic, nonregenerative anemia).

• Serum chemistry profile – Hypothyroid dogs have decreased fat metabolism and commonly have high levels of blood cholesterol (hypercholesterolemia) and lipids (hyperlipidemia).

• Urinalysis – A complete urine analysis is important for assessing whether other diseases are affecting your dog either in addition to, or instead of hypothyroidism. This test is normal in hypothyroid dogs.

Although we frequently see changes in the general laboratory work that point to hypothyroidism as a cause of the dog’s clinical signs, the most important reason that we must do these test in to rule out other diseases that may mimic hypothyroidism. Many common non-thyroid diseases (e.g., diabetes, Cushing’s syndrome, kidney or heart disease, severe infections) can also cause circulating thyroid hormone levels in the blood to decrease.

Your veterinarian must perform these general lab tests to rule these diseases out before confirming a diagnosis of hypothyroidism.

Once these general laboratory tests have been done, the next step in diagnosis is to run one or more specific thyroid tests. That will be the topic of my next blog post.