Thyroid Hormone Deficiency in Dogs

Thyroid hormone deficiency, or hypothyroidism, is a disorder characterized by atrophy to of the thyroid gland and the reduction of the hormones triiodothyronine (T3) and thyroxine (T4) in the blood. It requires lifelong treatment with an oral synthetic hormone replacement therapy. Once hormone replacement therapy has begun, the prognosis for primary hypothyroidism is usually quite good. The prognosis for secondary hypothyroidism is more guarded and the final outcome will depend on the underlying cause of the disorder.

When the thyroid gland fails to produce enough of the hormones triiodothyronine (T3) and thyroxine it is called hypothyroidism, and can trigger a number of metabolic problems.

There are many symptoms that can be indicative of decreased thyroid function. These signs can include any or all of the following symptoms: 

• Decreased heart rate
• Dry skin
• Exercise intolerance
• Hair loss/ excessive shedding
• Intolerance to cold
• Lethargy
• Mental dullness or confusion
• Secondary infections
• Sensitive skin
• Sensitivity to cold 
• Weakness
• Weight gain

If a tumor develops on the thyroid itself, additional symptoms may be seen, such as: 

• Change in bark
• Coughing
• Hoarseness
• Loss of appetite
• Round mass seen or felt on throat area

Types

Primary hypothyroidism

• Primary hypothyroidism is the most common form of hyperthyroidism and is caused by issues with the thyroid itself
• The most common origins for primary hypothyroidism are immune system disorders, tumors or growths on the thyroid, and idiopathic atrophy

Secondary hypothyroidism

• Secondary hypothyroidism is less common in dogs than primary hypothyroidism and involves a decrease in thyroid-stimulating hormone (TSH) produced by the pituitary which then leads to the atrophy of the thyroid gland

Most cases of primary hypothyroidism are caused by the canine’s own immune system targeting the thyroid as if it were foreign tissue. This disorder is known as lymphocytic thyroiditis and is characterized by a diffuse infiltration of foreign cells, such as lymphocytes, plasma cells, and microphages. Production levels of two of the body’s important hormones, triiodothyronine (T3) and thyroxine (T4), are diminished by the inflammation and atrophy of the thyroid gland. Cancers such as squamous cell carcinoma and thyroid carcinoma are also known to invade the thyroid. As the thyroid is progressively diminished, symptoms of hypothyroidism will appear. Environmental and dietary elements may also be involved in the development of primary hypothyroidism, although there are cases where the reason for the atrophy is not obvious. In cases where the origin of the damage cannot be reliably determined, it is diagnosed as an idiopathic atrophy. 

Secondary hypothyroidism is generally a disease that attacks the pituitary gland instead of the thyroid itself. The decreased amounts of TSH in the blood can result in the atrophy of the thyroid gland. There are many origins to secondary hypothyroidism, such as congenital malformations, cancer on the pituitary gland, Cushing’s disease, and malnutrition.

In order to make a diagnosis your veterinarian will want a full history of the animal, as well as a general physical exam. Helpful information will include when the symptoms started, your pet’s usual diet, and any medications your dog is on. A complete blood count, biochemistry profile, and urinalysis will be ordered as well, to expose any underlying or concurrent diseases that may require treatment. If the results of these tests seem to point to hypothyroidism a test will be done to get a measurement of the thyroxine concentrations. 

The veterinarian may choose to conduct additional tests and use advanced imaging techniques, to get more information about the hormone composition in the blood and more information about the visible health of the thyroid. These could include a thyroid stimulating hormone stimulation test, ultrasonography, or radiography. A modified equilibrium dialysis test can test the levels of T4 in the patient’s blood. That information, combined with the amount of TSH circulating in the blood, will help to confirm the diagnosis of hypothyroidism.

Hypothyroidism, whether primary or secondary, is treated by addressing the hormone levels in the blood. This is done by administering daily doses of a synthetic version of the T4 hormone by mouth. The dog would need this medication to be administered either once or twice daily depending on the severity of the deficiency. Your dog will also need to visit the veterinarian for follow-up care and testing to ensure that the dosage adjusting the hormone levels correctly.  Any underlying conditions such as tumors or environmental concerns will need to be addressed at this time as well to ensure the best possible outcome. 

Within just one or two weeks pet owners generally see a marked improvement in energy and mood. Overall condition, such as fur health and skin health, continues to improve over several months and weight loss can be achieved with healthy diet and exercise. If clinical improvement is absent after three months and blood levels of thyroxine are presenting normally, the diagnosis of hypothyroidism will need to be reevaluated.

Usually the majority of symptoms resolve completely with the daily hormone therapy. Your pet will need to have their blood tested regularly to ensure that the circulating hormone levels are remaining normal. Initially these tests will probably occur every four to six weeks, but the frequency will decrease as the hormone levels stabilize. The daily hormone replacement therapy is a lifelong treatment for your canine. With this treatment It is important to note that the altered metabolism from hypothyroidism can intensify and lengthen the effects of some medications, such as anesthetic agents, so care should be taken when determining dosages for the hypothyroid pet.