Canine Hyperthyroidism: Diagnosis and Treatment

There are several veterinary issues that can affect a dog's thyroid gland — such as hyperthyroidism, which needs to be treated as soon as possible to keep our furry friend's health from worsening dramatically.

At Soyunperro we'll walk you through this condition, so you can better understand its implications for your dog's health, covering the possible causes, symptoms, and treatments.

What Is Canine Hyperthyroidism?

As its name suggests, this disease affects the thyroid gland, an organ located anatomically on both sides of the trachea, surrounded by other glands called the parathyroids. Unlike in humans, where the two lobes are connected, in dogs there are two separate portions. Its job is to regulate metabolism, affecting calorie use and heart rate.

Its function is tied to iodine intake, so it's important to make sure our pet has a good diet to prevent this and other metabolic imbalances. In any case, most commercial dog foods include a standardized amount of this element, so this shouldn't be a major concern.

When the thyroid gland produces more thyroid hormone than normal, hyperthyroidism occurs, while if the opposite happens and there's a deficiency, we call it hypothyroidism. Both disorders produce different symptoms and are treated differently.

The causes of hyperthyroidism in dogs can vary, such as a malformation of the thyroid gland or neoplasia (a tumor). Thyroid neoplasms, despite being the most common cause of hyperthyroidism, don't usually trigger changes in the synthesis of thyroxine hormone.

It's a disease that particularly affects older dogs (over 9 years old) and appears to have a higher incidence in medium or large breeds, such as the Boxer, Beagle, or Labrador. Females are generally more prone to this disorder than males.

If symptoms appear that make us suspect our dog has hyperthyroidism, we should take it to a vet so they can run the appropriate diagnostic tests and prescribe the right treatment.

How to Tell If a Dog Has Hyperthyroidism

The main symptoms we might see in a case of hyperthyroidism in dogs are as follows:

• Increased appetite (polyphagia) and water intake (polydipsia), as well as more frequent urination, meaning the dog urinates more often (polyuria).
• Loss of appetite or weight loss (despite eating more).
• Tachycardia and tremors.
• Skin and/or coat changes (hair loss, dull-looking fur).
• Restlessness and nervousness.
• A lump in the neck may appear (goiter).
• Coughing or trouble breathing (dyspnea).
• Vomiting and/or chronic diarrhea.
• Low tolerance to heat and lethargy as a response to it.
• Dysphonia, or a change in the dog's normal bark or voice (you'll notice it sounds off). This happens because the thyroid tumor can affect the vocal cords.

And which breeds are affected most? Usually medium or large breeds, such as Golden Retrievers, Labradors, Cocker Spaniels, or Dobermans.

On the other hand, hyperthyroidism tends to develop gradually, which is why it may take us a while to notice something's wrong. Typically we start to suspect it once symptoms become obvious and the disease is already advanced.

How Is Hyperthyroidism Diagnosed?

After spotting these symptoms, we need to take our dog to the vet, who will run the appropriate diagnostic tests. Besides a physical exam, these include additional tests such as an X-ray and/or ultrasound to confirm whether there's a tumor and whether it has metastasized — that is, spread to other parts of the body. A thyroid scan, after administering radioactive isotopes, is another diagnostic option.

If hyperthyroidism is suspected, a blood sample is needed to confirm the diagnosis through bloodwork, which will determine the levels of T4 hormone produced by the thyroid (in cases of hyperthyroidism, these will appear elevated). A biopsy may also be necessary to assess the type of tumor.

Treatment for Canine Hyperthyroidism

Let's say we've been to see the vet, our furry friend has already had the bloodwork done, and the diagnosis has been confirmed: it has hyperthyroidism, and the disorder is caused by a tumor. You're probably wondering, now what? Well, the next step is to start treatment, which will depend on the type of tumor, its size, and whether the thyroid gland needs to be fully removed or not.

Most likely, surgery will be recommended, though the vet may also prescribe chemotherapy to shrink the tumor if it isn't operable. If surgery is performed to remove the thyroid, one possible complication is the development of iatrogenic hypothyroidism (due to reduced or absent thyroxine production), or excessive blood loss, since the thyroid sits in a highly vascularized area. In that case, a transfusion may be needed.

The patient's prognosis will be determined mainly by the type and extent of the tumor, especially by whether or not metastasis has occurred. Unfortunately, it's worth noting that malignant thyroid tumors (carcinomas) occur far more often than benign ones (adenomas), which carry a much better prognosis. In any case, early diagnosis will be key to ensuring our pet's survival.

Once the dog is out of surgery, we'll need to make sure it takes the prescribed medication. We'll also need to periodically monitor its blood T4 hormone levels and, above all, take great care of our furry friend so it recovers as soon as possible.

References consulted: Avellaneda, A. Font. A. Closa, José Mᵃ and Mascort, J. 2002. Hipertiroidismo canino: tres casos clínicos. Avepa vol. 22, nº 3. pp. 239-245.