Hyperthyroidism in Dogs - Causes, Symptoms and Treatment

As with humans, the thyroid gland is located in the neck. It is behind the trachea and the dog's Adam's apple, the later being causes by the cartilage which surrounds the thyroid. As it is located near the surface of the skin, it is possible to feel an increase in size from a physical examination. The size of an individual dog's thyroid gland is variable and depends largely on the intake of iodine in the diet.

Commercial dog food has developed to balance a dog's nutritional requirements. This includes a certain amount of iodine. If a dog is given a diet with either too much or too little iodine, they could be at greater risk of hyperthyroidism. Since iodine amounts are regulated in dog food formulae, the condition is relatively uncommon.

The thyroid is accompanied by four parathyroid glands which are usually located at the back of the thyroid. Thyroid surgeries need to be very careful not to remove too much from these glands. As a gland, the thyroid releases certain hormones which fulfill various important functions related to metabolism and a dog's development. Therefore, any modification or change to its function can result in observable symptoms.

The hormones which the thyroid secretes are called triiodothyronine (T3) and thyroxine (T4). When the thyroid produces too little of these hormones, it leads to a condition known as hypothyroidism. When too much hormone is produced it is called hyperthyroidism. This can affect dogs in a number of different ways.

When a dog has hyperthyroidism, changes to the clinical picture can be difficult to register in the early stages. This is partly dependent on the amount of hormone overproduction occurs, but the health status of the dog is also a factor. Generally a dog with hyperthyroidism will display the following symptoms:

• Increased appetite (polyphagia) and water consumption (polydipsia), with the concurrent increase in urination (polyuria).
• Weight loss even though the dog is eating more than usual.
• Heart rhythm problems, such as tachycardia.
• General nervousness.
• In some cases a lump on the neck can be detected. The size of this lump is variable, but it can be observed with a physical examination of the neck. This increase in size can put pressure on the trachea (windpipe) which leads to coughing or dyspnea (shortness of breath).
• Chronic diarrhea and vomiting.
• If the hyperthyroidism is caused by a tumor, it can affect the vagosympathetic trunk and lead to Horner syndrome. This results in various other symptoms including protrusion of the third eyelid or enophthalmia (eyes sunk in the sockets).

Hyperthyroidism in dogs usually more often affects medium or large sized dogs. The onset of the disease is often difficult to detect because it can develop slowly. Symptoms of hyperthyroidism in dogs can worsen progressively.

Unfortunately, the most common cause of hyperthyroidism in dogs appears to be a tumor on the thyroid gland. If the tumor is malignant it can metastasize to other parts of the body. Luckily, thyroid tumors in dogs are relatively rare overall. In most cases, they do not even affect hormone production levels, often lowering them rather than creating more. Cases of hyperthyroidism are relatively rare, even compared to hypothyroidism.

To arrive at an appropriate diagnosis, it is necessary to observe the clinical symptoms and perform a physical examination. An x-ray or ultrasound may be required to verify the size of tumor and whether or not it has metastasized to other parts of the body. Also, a blood sample will likely be taken to determine the values of T4 produced.

It is also possible to aspirate gland cells to determine their typology, although a biopsy will provide a more reliable result. An elevation of the T4 hormone will undoubtedly indicate a problem with hyperthyroidism. Unfortunately, since the development of a tumor is progressive, symptoms do not occur until after the tumor has metastasized. This has a detrimental effect on the dog's prognosis. Ideally treatment needs to occur as soon as possible.

Once a diagnosis of hyperthyroidism in dogs has been confirmed, the veterinarian will inform us about the most appropriate course of treatment. As is the case with dogs, it is common for the thyroid hyperactivity to be caused by a tumor. For this reason, the treatment will be directed towards the tumor and the hyperthyroidism is treated as a symptom of the larger condition.

Treatment of the dog's tumor will likely require surgical intervention, but this will depend on the its size, progression and other factors. Surgery is seen as a necessary treatment due to the high potential for malignancy in the tumor. It is possible to treat hyperthyroidism in dogs with medication or radiotherapy. The medication is in the form of chemotherapy which is designed to decrease the tumor's size, often before surgical removal. Both chemotherapy and radiotherapy are relatively uncommon treatment for thyroid tumors.

For surgical treatment, it is very important to preserve the parathyroid glands. At least two of the four must be retained to avoid subsequent hyperthyroidism. Additionally, there are a lot of veins and arteries in this area, so blood loss is a key concern. A transfusion may be required during surgery to improve success rates. After the operation, treatment may need to include more medication. Monitoring T4 levels will be necessary regardless.

If the thyroid gland is removed partly or entirely, the dog may develop hypothyroidism which will also require medication to treat. If the tumor is malignant and has progressed, it is necessary to weigh up the risk of surgical intervention. If treatment is not likely, palliative care will be required.

This article is purely informative. AnimalWised does not have the authority to prescribe any veterinary treatment or create a diagnosis. We invite you to take your pet to the veterinarian if they are suffering from any condition or pain.

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