Hyperthyroidism is a disease caused by overproduction of thyroxine, a thyroid hormone that increases metabolism in the body. The thyroid gland normally produces thyroid hormones in response to stimulation by the pituitary gland, the “master gland” of the body. Thyroid hormones normally increase chemical processes occurring within the cells of the body, especially those related to metabolism; however, in hyperthyroidism, the excessive hormone levels push the cells and body into overdrive, resulting in increased metabolism with concurrent weight loss, anxiety, and diarrhea, among other symptoms.
Hyperthyroidism is rare in dogs, and most commonly occurs as a result of carcinoma of the thyroid. It may also result from medication given to treat hypothyroidism, an underproduction of essential thyroid hormones in the body.
Symptoms and Types
- Involves many organ systems due to the overall increase in metabolism
- Weight loss
- Increased appetite
- Unkempt appearance
- Poor body condition
- Increased thirst (polydipsia)
- Increased urine (polyuria)
- Rapid breathing (tachypnea)
- Difficulty breathing (dyspnea)
- Heart murmur; rapid heart rate; particularly an abnormal heart beat known as a “gallop rhythm”
- Enlarged thyroid gland, which can be felt as a lump on the neck
Some dogs suffering from hyperthyroidism are referred to as apathetic. These patients exhibit atypical signs such as poor appetite, loss of appetite, depression, and weakness.
- Overfunctioning thyroid nodules (where the thyroid nodules produce excess thyroid hormones outside of the control of the pituitary gland)
- Secretion of the T3 (triodothyronine) or T4 (tetraiodothyronine) thyroid hormones as a result of thyroid cancer; tumor then interferes with normal functioning of the thyroid gland, prompting the gland to produce more thyroxine
- Response to medications used for hypothyroidism may lead to overproduction of thyroxine
A preliminary diagnosis can often be made based on palpitation of the gland, which becomes enlarged as it progresses. Standard tests will include chemical blood profile, a complete blood count, and a urinalysis. A high concentration of T4 in the blood serum is the most common finding of the profile, confirming a diagnosis of hyperthyroidism. In some cases, however, the T4 levels may be in the normal range, making a diagnosis of hyperthyroidism more difficult. This is especially true in the early stages of this disease. If your dog is showing the symptoms of hypothyroidism but the blood tests are not conclusive, you will need to return to your veterinarian for further blood tests.
Your veterinarian may need to conduct a battery of tests to zero in on a reliable diagnosis. Thyroid gland scintigraphy (a diagnostic test in which a two-dimensional picture of a body radiation source is obtained through the use of radioisotopes) can be used to diagnose hyperthyroidism and to determine the location of abnormal thyroid tissue.
Thoracic radiography and echocardiography may be useful in assessing the severity of myocardial disease, and chest X-rays can be used to detect pulmonary metastasis.
Outpatient management is usually sufficient if drugs that inhibit the production of thyroid hormones can be used. In cases where the thyroid is overactive as the result of medications given for hypothyroidism, which is more common in dogs, the medication dosage can be adjusted and the symptoms will generally diminish.
Surgical removal of the thyroid gland, or treatment using a radioactive form of iodine will require inpatient treatment and monitoring. Surgical removal of the thyroid gland is best performed when only one thyroid gland is affected, as removal of both can possibly lead to hypothyroidism. Another complication that can occur after surgical removal of the affected thyroid gland is the successive hyper-activity of the remaining thyroid gland.
If the hyperthyroidism is related to a tumor of the thyroid, the option for surgery will be dependent on the invasive nature of the tumor. The proximity of the tumor to the esophagus and major arteries can make surgery difficult, or even impossible, but in some cases a portion of the tumor may be removed, and the the dog can be treated further with radioactive therapy. The prognosis is dependent on how far the tumor has metastasized into the surrounding tissues.
The use of radioiodine is restricted to a confined medical facility, since the treatment itself is radioactive. Depending on the state in which you live and the guidelines in place, your dog will need to be hospitalized from several days to a few weeks after being treated with radioactive medicine, to allow the radioactive material to clear most of the body before the dog can be handled by family members. Precautions will still need to be taken after taking your dog home, to reduce your risk of having a toxic reaction to the radioactive treatment. Your veterinarian will advise you in precautionary measures.
Once the major symptoms resulting from excessive levels of thyroid hormones in the body have been resolved, dietary modifications often do not need to be strictly enforced. Even so, dietary modifications may be necessary to treat or control complications that can occur in conjunction with hyperthyroidism, such as kidney damage.
Living and Management
Once treatment has begun, your veterinarian will need to reexamine your dog every two to three weeks for the initial three months of treatment, with a complete blood count to check for serum thyroid hormone concentration of T4. The dosage of the medications will be adjusted to maintain T4 concentration in the low-normal range.
If your dog has had surgery, particularly removal of the thyroid gland, your veterinarian will want to closely observe the dog’s physical recovery. Development of low blood-calcium levels and/or paralysis of the voice box during the initial postoperative period are complications that will need to be watched for and treated, should they occur. Your doctor will also be measuring thyroid hormone levels in the first week after surgery and every three to six months thereafter, to check for recurrence of thyroid gland over activity.
In cases in which a tumor of the thyroid is found, the prognosis will be dependent on whether it is malignant or benign. Malignant tumors of this type often metastasize quickly into the surrounding tissues and organs, making treatment difficult and prognosis poor. Benign tumors can generally be managed, and have a much better prospect for future health.
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