Hypothyroidism in Dogs: Symptoms and Treatment [Jarosław Sałek]

Medically reviewed by: Kinga Wojtczak, Doctor of Veterinary Medicine, specialist in canine and feline diseases

Did you know that, contrary to popular belief, hypothyroidism, which occurs in humans, also affects companion animals?

In this article, you will learn about the causes of hypothyroidism in dogs, how to recognize it in your pet and what the treatment of this condition involves. Check whether your dog is predisposed to hypothyroidism—it is a very serious condition. Hypothyroidism is being diagnosed more and more often in dogs, whereas in cats it is extremely rare. Insufficient hormone production slows down the metabolic rate.

Typical signs include:

• lethargy,
• sluggishness,
• obesity,
• hair loss (alopecia).

Fortunately, the signs of the disease can be stopped and even reversed with thyroid hormone supplementation.

Hypothyroidism most often affects these dog breeds:

• Labrador Retriever,
• Boxer,
• Golden Retriever,
• Beagle,
• Miniature Schnauzer,
• Great Dane.

Hypothyroidism in dogs: causes

The types of thyroid failure in dogs are classified according to where the disorder is located.

The forms most commonly seen are:

• the primary form,
• the secondary form,
• less often, the tertiary form,
• hypothyroidism resulting from other disorders.

The causes of the disease that lead to each type of gland dysfunction are shown in the table below:

The most common causes of hypothyroidism in dogs

Primary	Secondary	Tertiary	Other
Congenital hypoplasia	Pituitary dwarfism	Impaired secretion of thyrotropin-releasing hormone (TRH)	Iodine deficiency
Atrophy due to lymphocytic thyroiditis	Pituitary tumor		Impaired conversion of T4 to T3
Tumors	Euthyroid sick syndrome		Binding protein abnormalities
Surgical removal of the gland	Radiation therapy		Tissue resistance
Antithyroid drugs	Pituitary cyst

In cats, hypothyroidism is most often the result of:

• removal of both thyroid lobes,
• radioactive iodine treatment,
• an overdose of antithyroid drugs.

In cats, the primary form is very rare and is caused by abnormal development of the gland or impaired iodine incorporation.

Hypothyroidism in dogs: symptoms

The signs of hypothyroidism in dogs are typical of a slowed metabolism and appear in middle age, at around 4–6 years.

These include:

• lethargy,
• obesity,
• cold intolerance (constant shivering),
• behavioral disturbances,
• in cats, a lack of appetite and of the desire to eat is very common.

The most characteristic are the dermatological signs: the dog or cat has symmetrical alopecia (hair loss) on the trunk. Bald patches also appear at the tip of the tail—known as "rat tail" (dog)—and on the ears, the sides of the neck and the abdomen (cat).

You may also see:

• skin discoloration,
• seborrhea,
• dry hair or hair that is slow to regrow.

Eye changes are also characteristic:

• a drooping eyelid (ptosis),
• choroiditis (inflammation of the choroid),
• corneal ulceration,
• lipid deposits in the cornea.

In addition, there are systemic signs involving the cardiovascular system, such as:

• a slow heart rate (bradycardia),
• an irregular heartbeat (arrhythmia).

Reproductive signs include:

• infertility,
• decreased libido,
• absence of heat (estrus),
• testicular atrophy.

Larger dogs show:

• paresis (weakness or partial paralysis due to hypothyroid neuropathy),
• laryngeal paralysis,
• facial nerve paralysis (the face loses its expression),
• urination disorders.

This disease very rarely occurs in puppies or kittens. When it does, the condition is called cretinism. Affected puppies and kittens have a disproportionate body build, a large, broad head, short limbs and a square trunk.

Diagnosing thyroid disease

To diagnose thyroid disease in a dog, thyroid hormone levels are measured. In addition, a complete blood count and blood chemistry panel can be performed, which, together with the clinical signs, give a full picture of the animal's condition. When the gland fails, thyroid hormone levels drop.

Remember that thyroid function should never be assessed on the basis of a single hormone level or routine bloodwork alone. Interpretation of the results should be left to a veterinarian.

Hypothyroidism in dogs: treatment

The prognosis for dogs with hypothyroidism depends on the cause, but it is usually good. Properly treated dogs and cats have the same life expectancy as healthy animals. Treatment is based on oral tablets containing levothyroxine at a dose of 10–20 µg/kg 2 times a day. Levothyroxine is the drug of choice and should restore normal hormone levels.

Triiodothyronine can also be used at a dose of 6–8 µg/kg 3 times a day, and iodine can be supplemented. Hormone treatment should be continued for at least 6–8 weeks before its effectiveness is finally assessed, although clinical improvement is visible as early as the first week of treatment. The clinical response is monitored by measuring thyroid hormone levels 4–8 weeks after the start of treatment.

It is important to perform this test 4–6 hours after levothyroxine is given. This lets the veterinarian determine whether the dose of the hormone is sufficient.

Summary

Today many dogs are diagnosed with hypothyroidism, whereas cats with this condition are rare. Keep in mind that gland failure has various causes, so you must not rely on the animal's age alone when this condition is suspected. Early on, the signs may not be typical, but once the necessary tests are done, a diagnosis can be made quickly and treatment started.

When the veterinarian's recommendations are followed, the animal stays in very good health, clinical signs resolve very quickly and our pet's quality of life is satisfactory. I think that after reading this article you now know how to recognize hypothyroidism in your dog and will take the necessary steps if you suspect this disease. If you would like to learn more about this topic, I encourage you to comment on the article.

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