Abstract

Unlike the situation in dogs, thyroid disease in cats is benign in the majority of cases. However, concurrent hyperthyroidism will completely disrupt the animal's metabolism if left untreated. Medical, surgical, or radioactive therapies can be used for the treatment of thyroid disease in cats. The availability of radioactive treatment, the technical expertise of the surgeon, and the patient's general condition must all be considered when the choice for the best therapy is made.

Introduction

The most common thyroid disease in cats is benign adenomatous hyperplasia resulting in hyperthyroidism.^1,2 One or both thyroid glands may be affected or ectopic thyroid tissues become hyperplastic. Thyroid carcinomas in cats are rare but have been described in a minority of cases.³ A sole underlying cause for the development of benign thyroid hyperplasia has not been identified to date. The three most commonly used treatment options for feline hyperthyroidism are: anti-thyroid drugs, surgical thyroidectomy, or radioiodine (¹³¹I) ablation of the abnormal thyroid tissues.^4-6 Most hyperthyroid cats are middle aged or old and may have concurrent cardiovascular or renal disorders. The least invasive available treatment should be recommended for these patients. If available, the treatment of choice is radioiodine ablation. Most cats are rendered euthyroid with a single treatment with ¹³¹I and no side effects have been observed. The parathyroid glands are not affected by ¹³¹I. Inactive thyroid tissue will have no iodine uptake and therefore is also not affected by ¹³¹I. Reported side effects of anti-thyroid drugs (methimazole or carbimazole) include neutropenia, thrombocytopenia, facial excoriation, anorexia, and vomiting.^4,6,7 Medication has to be given daily to keep plasma T₄ levels in a low normal range. If radioiodine treatment is not available surgical thyroidectomy also provides a permanent solution for hyperthyroidism.

Surgical Thyroidectomy

Surgical candidates need a thorough evaluation for the existence of co-existing illness. Surgical and anesthetic complications can be minimized by stabilizing the patient's cardiac and renal functions and by preoperative correction of hypokalemia. Thyrotoxicosis should be controlled preoperatively by anti-thyroid drugs. In cases where anti-thyroid drugs cause unwanted side effects beta-blockers may be used preoperatively to control tachycardia.^9,10 Anesthetic protocols for this procedure should avoid the use of drugs that induce tachycardia or potentiate adrenergic activity.¹¹ All patients are endotracheally intubated and ECG is monitored during the procedure. Preoperative thyroid scans (pertechnetate ^99mTcO₄^-) will identify all hyper functioning thyroid tissues, including ectopic hyperplasia. The author strongly advises the use of magnifying glasses and bipolar electrocautery for thyroid dissection.

Patients are positioned in dorsal recumbency with a small neck support.^9,10 The front legs are retracted caudally. A ventral midline incision extending from the larynx to the thoracic inlet is used for cervical thyroidectomies. The cervical muscles are separated in the midline and the trachea is exposed. The abnormal thyroid tissue is carefully dissected from the surrounding structures while avoiding trauma to the recurrent laryngeal nerve and the carotid artery. The author uses a modified intracapsular technique for thyroid resection.¹² This involves excision of nearly the entire thyroid capsule, except for a small part that adheres to the external parathyroid gland and its blood supply. The thyroid tissue is carefully dissected from the capsule using small scissors and a moistened cotton-tipped applicator. Hemostasis is provided by bipolar electrocautery. Caudal cervical or mediastinal ectopic hyperplastic tissue is approached by extending the incision more caudally into the thoracic inlet. For the resection of mediastinal hyperplasias in the pre-cardiac areas an intercostal thoracotomy is warranted. The presence of ectopic hyperplastic thyroid tissue significantly increases the incidence of recurrence of hyperthyroidism after surgery.¹³

Postoperative Care

After bilateral thyroidectomy oral substitution with L-thyroxine is provided (50 μg per cat twice daily, starting on the fourth day after surgery). Plasma T4 levels are measured after one month and then every six months. The dosage of L-thyroxine is adjusted if necessary to maintain plasma T₄ within the normal range.⁴

Postoperative hypocalcemia may be caused by damage to the parathyroid glands or their blood supply and is reported in 6% to 82% of bilaterally operated cases, depending on the surgical technique that was used and the experience of the surgeon.^4,5,14 Hypocalcemia may be life-threatening. In all cases of bilateral thyroid surgery the plasma Calcium levels should be monitored by measuring preoperatively and at about 20 h after surgery. In cases with severe hypocalcemia and clinical symptoms (tremors, tetany, convulsions) IV administration of 0.5 mmol Ca²⁺/kg as calcium(boro)gluconate is given under close ECG monitoring. The same dose that was needed to control tetany and tremors is diluted with at least an equal volume of saline 0.9% and administered subcutaneously, 2–4 times a day. In cases without clinical symptoms of hypocalcemia the following protocol is used: if plasma calcium is < 2.0 mmol/l or more than 10% below the preoperative value, calcium(boro)gluconate (1–2 ml/kg) is administered subcutaneously, 2–4 times a day, diluted with at least the same volume of 0.9% saline. As soon as the cat is eating oral supplementation is started. Calcium carbonate powder (15–20 mg/kg) is added to each meal and dihydrotachysterol is provided at a starting dose of 0.05 mg per cat once a day during the first 3 days, and then decreased to 0.025 mg once a day. In cases where substitution therapy has to be continued dihydrol is tapered even more to prevent hypercalcemia. Subcutaneous calcium(boro)gluconate is gradually tapered when cats are eating. Plasma calcium levels are measured at least twice daily in the beginning of the supplementation therapy and later as often as necessary but at least once a week. After 4–10 weeks of maintaining plasma calcium levels within the normal range an attempt can be made to further taper the supplementation.

Auto transplantation of parathyroid tissue during surgery involves the placement of small pieces of cut parathyroid tissue in a sternohyoideus muscle bed. Revascularization (and therefore functioning) will take at least 14 days.^4,10 Calcium levels should be monitored carefully and supplementation should be given consistently during that period until parathyroid function has been completely restored.

Prognosis of Thyroid Surgery in Cats

The prognosis after successful surgical thyroidectomy is good in cases with benign disease without renal or cardiac complications. Recurrence of hyperthyroidism is increased in cases where ectopic hyperplasias were removed compared to cases with non-ectopic hyperplasias.¹³ The prognosis in cases with malignant thyroid disease depends on the histological tumor type and the presence of metastatic disease and is probably similar to the situation in dogs.

References

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