Surgical Thyroidectomy for Cats with Hyperthyroidism: Preoperative Preparation

Thyroidectomy is a highly curative treatment for cats with hyperthyroidism. However, these patients are generally senior cats, with 95% being older than 10 years of age (1-3).

Performing surgery and general anesthesia on these older to geriatric cats has inherent risks. Hyperthyroid cats frequently have disorders of other organ systems (e.g., heart, kidneys, liver) that should be recognized and appropriately treated prior to surgery (1-3). Anesthesia on hyperthyroid cats can be challenging as a result of their hypermetabolic state and underlying thyroid tumor, as well as their advanced age. Without adequate preoperative preparation, thyroidectomy can be associated with significant morbidity and mortality, especially in those cats with severe or long-standing hyperthyroidism (4-6).

A thorough diagnostic evaluation of the hyperthyroid cat is mandatory prior to the surgery. Using short-term antithyroid drugs to restore euthyroidism before surgery will make these cats much better candidates for anesthesia and surgery (1-3,7,8).

Preoperative Evaluation of the Hyperthyroid Cat

All hyperthyroid cats in which surgical thyroidectomy is planned should have a thorough physical examination and complete laboratory evaluation, including a complete blood count, serum chemistry profile, and complete urinalysis. Often this database simply lends support to the diagnosis of hyperthyroidism, but these screening tests are essential in determining if a hyperthyroid cat also has concurrent medical problems (e.g., kidney, heart, or liver disease) that will need to be addressed prior to surgery.

Many hyperthyroid cats will develop secondary cardiac problems, including a rapid heart rate, heart murmurs, heart enlargement, or abnormal heart rhythms (1-4). Occasionally, especially in cats with chronic or severe hyperthyroidism, congestive heart failure will develop. Therefore, if signs of underlying heart disease is found on physical examination, one or more heart procedures, such as a chest radiograph (x-ray), electrocardiogram (ECG), or echocardiogram, may also be indicated in the presurgical workup of a cat with hyperthyroidism.

Preoperative Medical Management

Antithyroid drug treatment

Ideally, the cat would be treated preoperatively with antithyroid drugs (i.e., methimazole or carbimazole) to lower the high serum T4 levels down into the normal reference range limits (4-9). Initial doses of the antithyroid drug vary depending on the cat’s pretreatment serum T4 value and goiter size (i.e., size of the thyroid tumor). In general, however, most cats are started on 1.25-2.5 mg of methimazole or 2.5-5 mg of carbimazole, both administered twice daily. This dose is adjusted according to T4 measurements done at 2-week intervals, as well as the cat's clinical response. (7-9)

After methimazole or carbimazole treatment has maintained euthyroidism for at least 1-3 weeks, anesthetic and surgical complications will be greatly minimized. The last dose of methimazole or carbimazole should be given on the morning of surgery.

Beta-adrenergic blocking agents (Beta blockers)

In cats that cannot tolerate antithyroid drug treatment, alternate preoperative stabilization with beta-adrenoceptor blocking agents (e.g., propranolol or atenolol) should be used. The basis for use of beta blockers in hyperthyroidism is that excessive thyroid hormone mimics many of the effects of excess adrenalin. Beta blockers act to block the action of endogenous catecholamines epinephrine (adrenaline) and norepinephrine (noradrenaline), which mediates the fight-or-flight response (7,8).

Propranolol and atenolol are used to symptomatically control the tachycardia (rapid heart rate), tachypnea (rapid respiratory rate), hypertension and hyperexcitability prior to surgery when the hyperthyroid cat cannot be made euthyroid with antithyroid drugs. In addition, some cats that have persistent tachycardia (heart rate >220 beats per minute) on antithyroid drugs will also be treated with beta blockers prior to surgery to decrease the cardiac risks associated with general anesthesia.

Alternative drugs used to prepare the hyperthyroid cat for surgery
In cats with advanced or chronic hyperthyroidism that do not tolerate antithyroid drugs, there are a number of alternative drugs, including stable iodine, iopanoic acid, or L-carnitine, that can be added to the beta blocker treatment (7,8,10).

None of these drug are as effective as methimazole or carbimazole for preoperative preparation of the hyperthyroid cat.

Monitoring of Thyroid and Renal Values before Surgery

As serum thyroid levels normalized, it is important to monitor for worsening of serum kidney values as renal disease can sometimes be unmasked by treatment. Clinically significant rises in renal values warrant medical management prior to surgery to avoid postoperative renal failure (1-4).

Hemorrhage from jugular venipuncture sites can discolor and obscure the parathyroid glands. Therefore, jugular venipuncture should be avoided the week prior to surgery. Venipuncture can always be done, however, from either the cat’s saphenous or cephalic veins, located on the inside of the rear leg or top aspect of the foreleg, respectively.

Bottom Line

In most cats, thyroidectomy is relatively simple surgical procedure for an experienced veterinary surgeon to perform. However, thyroidectomy can range from a straightforward operation to one that is fairly complex, and it can be associated with significant morbidity and mortality, especially in cats with severe or chronic hyperthyroidism.

Considering that the average hyperthyroid cat is a geriatric patient with potential for high blood pressure and heart disease, quite a bit of patient preparation is necessary to reduce anesthetic risk.

A thorough preoperative evaluation of the hyperthyroid cat is mandatory. Using short-term antithyroid drugs to restore euthyroidism before surgery will make these cats much better candidates for anesthesia and surgery (1-3,7,8).

References:

1. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; 199-203.
2. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012;571-592.
3. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
4. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, pp 327-342, 2006.
5. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21, 29-33. 
6. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
7. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 2007;37:775-788. 
8. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22-28. 
9. Animal Endocrine Blog. Treating Cats with Hyperthyroidism: Antithyroid drugs. October 23, 2011.  
10. Animal Endocrine Blog. Alternative Medical Treatments for Hyperthyroid Cats. September 13, 2012.