Dear OncoLink "Ask The Experts,"
I have an 11-year-old neutered male Labrador retriever. He has a markedly elevated FT4 and very low TSH picked up on routine lab exam. His only noticeable symptom of hyperthyroidism is a 7-pound weight loss in a year. What is the efficacy of therapy for thyroid cancer? Is I-131 the only systemic therapy for canine thyroid cancer?
Lili Duda, VMD, Section Editor of the OncoLink Veterinary Oncology Menu, responds:
Only about 10% of canine thyroid tumors are functional, meaning that they secrete excessive levels of thyroid hormone. This can result in hyperthyroidism, a condition in which the body's metabolism is revved up, resulting in symptoms such as rapid heart rate, elevated blood pressure, hyperactivity, and weight loss despite a voracious appetite. These thyroid tumors are usually located within the main thyroid glands, which sit on either side of the trachea (windpipe), just below the larynx (voice box). However, carcinoma can also arise anywhere there are thyroid tissue remnants, extending from the base of the tongue, down the neck, through the thoracic inlet (where the windpipe and esophagus enter into the chest) and into the mediastinum (the area within the chest cavity in front of the heart and between the lung lobes).
About one third of thyroid cancers have already metastasized (spread) to the draining lymph nodes and lungs at the time of initial diagnosis. The best way to evaluate the location and extent of functional thyroid tumors is by using a nuclear medicine scan. This test uses a radioactive tracer, such as technetium or iodine, which binds to any residual thyroid tissue. The areas of radiation uptake are then measured with a gamma camera. If this test is not available, an alternative is to perform a contrast CT (computerized tomography) scan of the neck and chest, which uses intravenous dye and digital X-ray pictures to outline any cancerous tissues. If a CT scan is not available, radiographs (X-rays) of the neck and chest combined with an ultrasound of the neck are a reasonable alternative.
Please see our previous question on treatment of thyroid cancer in dogs for some information on treatment options. Another treatment option that is effective for thyroid tumors in both cats and dogs is the administration of radioactive iodine (I-131). However, while this treatment is widely available for cats, it is very limited for dogs, due to the high dosages of radiation needed. Because of this, very few institutions have the facilities and regulatory clearance to handle the dosages necessary to treat dogs.