Surgical Thyroidectomy for Cats with Hyperthyroidism: Intraoperative Considerations

Surgical thyroidectomy is a highly effective treatment for hyperthyroidism in cats. While thyroidectomy is most often successful, it can be associated with significant morbidity and mortality (1-4). Hyperthyroid cats are older to geriatric, can suffer from marked weight loss, and may have cardiac complications. Both before and during surgical thyroidectomy, a number of factors must be considered to ensure a successful outcome (4-7).

As discussed in a recent post on preoperative management of the hyperthyroid cat, all cats should, therefore, be prepared for surgery by administration of an antithyroid drug, a ß-adrenoceptor blocking drug, or iodide to decrease the metabolic and cardiac complications associated with hyperthyroidism. Establishing euthyroidism in these cats preoperatively will help make them much better candidates for anesthesia and surgery (5-7).

Just prior to anesthesia and surgery, it is important to rule out concurrent kidney problems that may have been masked by the untreated hyperthyroidism. The cardiovascular status should also be reevaluated to ensure that any cardiac issues or hypertension are controlled. Both renal and cardiac disease complicate anesthesia and may even mean that surgery is not a good option for a particular hyperthyroid cat.

Anesthetic Considerations
Anesthetic management of the hyperthyroid cat should include the judicious use of agents that have minimal cardiac arrhythmic effects (1-4,8). A variety of anesthetic agents and techniques can be used and none has advantages that exclude use of all others, especially if the hyperthyroid state has been controlled with methimazole prior to surgery. Due to their weight loss, poor body condition, older age, and overactive metabolic state, hyperthyroid cats tend to be very sensitive to many common drugs used to induce surgical anesthesia (8).

During the anesthetic period, continuous monitoring of the anesthetic level, blood pressure, and electrocardiogram is essential. Cardiac arrhythmias are common, especially in cats not rendered euthyroid prior to surgery. If arrhythmias develop, the anesthetic concentration should be lowered and the cat ventilated with a higher concentration of oxygen. If the arrhythmia persists, small doses of intravenous ß-adrenoceptor blocking drugs  (e.g., propranolol) usually helps to restore normal sinus rhythm.

Unilateral vs. Bilateral Thyroid Tumors?
About 30 percent of hyperthyroid cats have disease in only one thyroid lobe (unilateral tumor), whereas the remaining 70 percent have tumors in both thyroid lobes (i.e., bilateral tumors) (5-7).

As discussed in my last post on thyroid imaging for preoperative staging of hyperthyroid cats, nuclear scintigraphy is ideally done prior to surgery to determine with certainty which thyroid lobe should be removed or if bilateral thyroidectomy is needed. If this is not feasible, then the surgeon will have to make a decision to remove one or both thyroid lobes based on the visual appearance of the glands during surgery.

Unilateral disease
In cats with unilateral thyroid tumors, the adenomatous thyroid will be obviously large and abnormal, whereas the other “normal” thyroid lobe is of normal size or even small (Figure 1) (1-4). Removing the single abnormal thyroid lobe cures the hyperthyroid state in these cats. Relapse of hyperthyroidism will not occur in these cats unless they develop a “new” adenoma in the remaining thyroid lobe in the future. However, relapse is rare and, when it does occur, hyperthyroidism generally takes years to redevelop.

Figure 1: Unilateral thyroid adenoma (notice the large tumor on right).
The smaller thyroid lobe (on left) was normal.

After unilateral thyroidectomy, a low serum calcium will not develop, even if both of the parathyroid glands associated with the excised thyroid lobe have also been removed. Cats can easily maintain a normal serum calcium with the remaining 2 parathyroid glands that are still intact, associated with the other thyroid lobe that was not surgically removed.

Bilateral disease
With bilateral thyroid tumors, enlargement of both lobes can easily be identified at surgery in most cats (Figure 2) (1-4). However, about 15% of cats with bilateral lobe involvement have one lobe which is only slightly enlarged and may be easily mistaken as normal. If a cat has bilateral lobe involvement but the smaller thyroid lobe is mistaken as normal and not removed, many of these cats experience a temporary cure but relapse of hyperthyroidism will usually occur within 6 to 12 months of surgery (9).

Figure 2: Bilateral thyroid adenomas.
Notice that both thyroid lobes are large and nodular.

In cats with bilateral thyroid adenomas, removal of both thyroid lobes with preservation of at least a single parathyroid gland is necessary to cure hyperthyroidism and avoid postoperative hypocalcemia (low serum calcium level) secondary to parathyroid damage (hypoparathyroidism).

Uncertain if unilateral or bilateral disease
If it’s not clear if the cat has unilateral or bilateral thyroid tumors and preoperative thyroid imaging has not been done, I recommend removal of only the obviously enlarged thyroid tumor. However, the associated external parathyroid gland should be preserved. By saving a parathyroid gland during the first unilateral thyroidectomy, this helps minimizes the risk of hypoparathyroidism should removal of the second thyroid lobe be required in the future.

Figure 3: Hyperthyroid cat with one thyroid tumor (bottom of photo) that was clearly large,
whereas the other thyroid lobe is equivocally enlarged (top smaller lobe).
The final diagnosis was bilateral thyroid disease (bilateral adenomas).

In my next post, I’ll be discussing the specifics of the ins and outs of thyroid surgery for cats with hyperthyroidism.

References

1. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
2. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21, 29-33. 
3. Padgett S. Feline thyroid surgery. Veterinary Clinics of North America. Small Animal Practice 2002;32:851–859. 
4. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Ed), Philadelphia, Saunders Elsevier, pp 327-342, 2006.
5. Kintzer PP: Considerations in the treatment of feline hyperthyroidism. Veterinary Clinics of North America. Small Animal Practice 1994;24:577–585.
6. 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.
7. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders. 2012;571-592. 
8. Peterson ME: Considerations and complications in anesthesia with pathophysiologic changes in the endocrine system. In: Short CE (ed), Principles and Practice of Veterinary Anesthesiology. Philadelphia, Williams and Wilkins Co. 1987;251-270. 
9. Peterson ME, Randolph JF, Mooney CT:  Endocrine diseases, In: Sherding RG (ed): The Cat: Diagnosis and Clinical Management (2nd Ed) New York, Churchill Livingstone. 1994;1404-1506. 
10. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21,10-16.