Treating Cats with Hyperthyroidism: Surgical Thyroidectomy

Performing a thyroidectomy in a cat with hyperthyroidism

Surgical thyroidectomy is an operation that involves the surgical removal of all or part of the thyroid gland. In cats with hyperthyroidism, this entails either removal of one (unilateral) or both (bilateral) thyroid lobes or parts (see Figure below). Because about 70% cats have tumor involvement of both thyroid lobes, bilateral thy­roidectomy is indicated in most hyperthyroid cats (1-6).

The thyroid gland in humans consists of 2 thyroid lobes, which are connected at the base by an isthmus. 
In cats, the 2 thyroid lobes are not connected and are totally separate.

Surgical thyroidectomy is an extremely effective means of treatment for hyperthyroidism. Because the goal of surgery in these cats is to remove all of their thyroid tumor tissue, this procedure can result in a complete cure of the cat’s hyperthyroidism.

In most cats, thyroidectomy is relatively simple and quick for an experienced veterinary surgeon to perform. In practice it is often considered the treatment of choice, particularly if radioactive iodine is not readily available (the only other form of treatment that will ablate the thyroid tumor(s) and cure the hyperthyroid state).

However, thyroidectomy can range from a straightforward procedure to one that is fairly complex, and it can be associated with significant morbidity and mortality, especially in cats with severe hyperthyroidism (1-8). Serious complications are uncommon, however, especially when an experienced surgeon performs this procedure.

Considering that the average hyperthyroid cat is a geriatric patient with potential for high blood pressure, renal and heart disease, quite a bit of patient preparation is necessary to reduce anesthetic risk (4,5,9).

Treatment options for cats with hyperthyroidism
Four treatment options are available for cats with hyperthyroidism:

1. long-term antithyroid drug administration
2. nutritional support with a iodine-deficient diet
3. surgical thyroidectomy
4. destruction of the tumorous thyroid tissue by radioactive iodine.

The best treatment option for a hyperthyroid patient is determined by evaluation of age, concurrent medical problems (such as cardiovascular or renal disease), availability of therapy, and the client’s opinion and financial options (4,5,9,10).

Assuming that the cat is a good anesthetic and surgical candidate, surgical thyroidectomy may be a good option for long-term resolution of the condition.

Advantages of surgical thyroidectomy as treatment of feline hyperthyroidism

• Thyroidectomy can be a highly effective and curative treatment for hyperthyroidism.
• Most veterinary practitioners can perform this surgical technique, but knowledge of the principles of anesthesia for the thyroidectomy patient, as well as the technical aspects of thyroidectomy is key. 
• Treatment is generally permanent after recovery from this surgical procedure. 
• Unless complications arise, no further treatment (other than thyroid hormone replacement) is needed. 
• No special facilities are required for this treatment, so this procedure can be preformed by most veterinary facilities. 

Disadvantages of surgical thyroidectomy as treatment of hyperthyroidism

• Performing surgery and general anesthesia on any older or geriatric cat with hyperthyroidism carries inherent risks, including morbidity or even death.
• Postoperative bleeding or infection is always possible but is extremely rare.
• The monitoring before and after surgical thyroidectomy, in addition to the surgery itself, is relatively expensive.
• Care must be taken to preserve at least 1 of the cat's parathyroid glands during bilateral thyroidectomy to prevent occurrence of iatrogenic hypoparathyroidism (1-8). Cats normally have 4 parathyroid glands, which are located just adjacent to, or within, the thyroid glands (11). If the parathyroid glands are damaged or removed, a temporary or permanent drop in serum calcium concentration (hypocalcemia) will develop, resulting in weakness, tetany and seizures. This is an emergency situation, requiring intensive care to monitor and properly treat these hypocalcemic cats. 
• The recurrent laryngeal nerve is located adjacent to the thyroid gland. If it is damaged during surgery, the cat may experience a voice change or hoarseness, which may be permanent in some cats (2-6). If both laryngeal nerves are damaged (extremely rare), breathing difficulties and airway obstruction caused by bilateral vocal cord paralysis may develop.
• The sympathetic trunk (another neurologic tissue that is a fundamental part of the sympathetic division of the autonomic nervous system) is also located adjacent to the thyroid gland. If this nerve trunk is damaged during surgery, the cat may develop Horner's syndrome (2-6).  This syndrome is characterized by a combination of drooping of the eyelid, constriction of the pupil, and prolapse of the third eyelid (see Figure, below). 
• Most cats will develop temporary hypothyroidism and may require supplementation with thyroid hormone (L-T4). Cats with total (bilateral) thyroidectomy may require lifelong thyroid hormone replacement. 
• If abnormal thyroid tumor tissue is left behind, hyperthyroidism is likely to persists or recur within 6 to 12 months (8,12). Continued monitoring of serum T4 concentrations  typically twice a year, is frequently recommended. 

Horner's syndrome affecting left eye in cat following surgical thyroidectomy

Bottom Line: 

Surgical thyroidectomy represents a good means to treat hyperthyroid cats. It is readily available to most practitioners, and assuming that the cat is a good anesthetic and surgical candidate, surgical thyroidectomy may be a good option for long-term cure of this disorder. However as with any surgical procedure, thyroidectomy is not without potential complications.

In my upcoming posts, I’ll be reviewing thyroid and parathyroid anatomy, and I will be discussing the preoperative preparation that we use for these feline patients. I’ll then move on to the peri-operative management and discuss actual surgical techniques that can be used. Finally, I’ll end this section on thyroidectomy by discussing how to deal with the potential complications of thyroidectomy in more detail.

References:

1. Birchard SJ, Peterson ME, Jacobson A: Surgical treatment of feline hyperthyroidism: Results of 85 cases. Journal of the American Animal Hospital Association 1984;20:705-709. 
2. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21, 29-33. 
3. Flanders JA. Surgical options for the treatment of hyperthyroidism in the cat. Journal of Feline Medicine and Surgery 1999;1:127–134. 
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. 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.
6. Padgett S. Feline thyroid surgery. Veterinary Clinics of North America. Small Animal Practice 2002;32:851–859. 
7. Flanders JA, Harvey HJ, Erb HN. Feline thyroidectomy. A comparison of postoperative hypocalcemia associated with three different surgical techniques. Veterinary Surgery 1987;16:362–366. 
8. Welches CD, Scavelli TD, Matthiesen DT, Peterson ME: Occurrence of problems after three techniques of bilateral thyroidectomy in cats. Veterinary Surgery 1989;18:392-396. 
9. Kintzer PP: Considerations in the treatment of feline hyperthyroidism. Veterinary Clinics of North America. Small Animal Practice 24:577–585, 1994. 
10. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012;571-592.
11. Nicholas JS, Swingle WW. An experimental and morphological study of the parathyroid 
glands of the cat. American Journal of Anatomy 1925;34:469-508. 
12. Swalec KM, Birchard SJ. Recurrence of hyperthyroidism after thyroidectomy in cats. Journal of the American Animal Hospital Association. 1990;26:433–437. 

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