Veterinary Information:
Hyperthyroidism in Dogs

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Diagnosis of Canine Hyperthyroidism:
-Palpation of an enlarging mass in the ventral cervical region.
-Rule out other possibilities for masses in this region by fine-needle biopsy and cytology, excision biopsy and histopathology.
-Blood and urinary screening can help to exclude other differential diagnoses for pu/pd.
-Chest radiographs or CT-scans should be made in dogs suspected of having a thyroid tumor. 1/3 of these dogs already show pulmonary metastases when presented for veterinary consultation.
-ECG: showing a high voltage in all leads.
-A high serum T4 (T4>100 nmol/l) concentration combined with a low to low-normal serum TSH concentration.
Note:The main differential diagnose for hyperthyroidism in dogs is the presence of auto-antibodies for T4, T3 or both, resulting in a false blood test result with apparent elevated thyroid hormones serum concentration. These dogs however commonly suffer from hypothyroidism.
-Scintigraphy: Pertechnetate (99mTc) scans can help localizing abnormal thyroid tissue.

Therapy:
Surgery is the first treatment of choice of canine hyperthyroidism. Small non-invasive encapsulated tumors can be removed entirely. In most of the cases however, the malignant and invasive character of these tumors in dogs make a total curative resection impossible. In these cases a palliative relief is the main goal of surgical tumor resection. In some cases cobalt irradiation therapy can be applied 2-3 months before surgical resection to reduce tumor volume. The main complications that can occur with radical surgical resection of the tumor mass result from damage to the parathyroid glands, laryngeal recurrent nerves and major blood vessels. Removed tissue should be send for histopathological examination.

Additional Treatment Options:
-Chemotherapy: This is a good adjunctive therapy option when total resection of the malignant primary tumor is not possible or when metastases are apparent. Drug of choice is Doxorubicin, 30 mg/m2 body surface area IV every 3-6 weeks. Treatment with Doxorubicin should be continued until total remission of the tumor occurs or negative reactions to Doxorubicin (like cardiac toxicity) occur. Other chemotherapy options are combinations with Cyclophosphamide or Viscristine if Doxorubicin alone is not effective enough.
-External Beam (Cobalt) Radiation: Can be a good treatment option if combined with surgical resection and possibly followed by chemotherapy. It rarely has any adverse effects.
-Radioactive Iodine Therapy: In most cases the malignant tumors are still capable of concentrating iodine. Therefore a treatment with radiactive iodine (131I) is a good option for functional thyroid tumors. Because of the high doses of radioactive Iodine needed to treat these tumors in dogs, a prolonged hospitalization is required for these animals in able to collect radioactive waste.

Prognosis:
Because of the malignant character of thyroid tumors in dogs the treatment is seldom curative. Surgical resection or debulking of the tumor mass possibly combined with additional therapies usually have a good palliative effect.

A.V.van Marrewijk, DVM

Literature: Ettinger SJ, Feldman EC, Peterson ME, Textbook of Veterinary Internal Medicine, WB Saunders Company, Philadelphia (2000) 5th edition: 1415-1417

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