Complications of Thyroidectomy in Cats: Persistent Hyperthyroidism & Relapse

Most hyperthyroid cats are readily cured quite easily with the use of surgical thyroidectomy (1-6).
Occasionally, however, cats treated with thyroidectomy remain persistently hyperthyroid (7). Others improve temporarily to become euthyroid, only to experience a relapse of hyperthyroidism days to months after surgery. Such problematic hyperthyroid cats can be frustrating to manage, especially as their disease becomes more severe and they develop complications of advanced and poorly controlled hyperthyroidism (1,2,7).

After successful surgical thyroidectomy in a cat with hyperthyroidism, the serum thyroid hormone concentrations (both T4 and T3) should fall to low-normal or low concentrations by 24 hours postoperatively (6,8). I recommend checking a serum T4 concentration before the cat is discharged from the hospital (within 1-2 days of surgery) to ensure that the procedure has been successful in removing all adenomatous thyroid tissue.

If the serum T4 concentration remains high or has only fallen into the high-normal range, it is very likely that remaining adenomatous tissue remains and that the cat will require additional treatment.

Persistent Hyperthyroidism After Thyroidectomy

Occasionally, cats undergoing thyroidectomy will remain hyperthyroid or develop relapse very shortly after surgery. If unilateral thyroidectomy was performed, it is likely that the other thyroid lobe is also adenomatous and was missed at surgery. Thyroid imaging can be very helpful in identifying all adenomatous thyroid tissue (1,9), no matter where its location (see Figure 1).

Figure 1: Bilateral thyroid adenoma in a cat with hyperthyroidism. Notice that gravity has pulled the larger thyroid tumor ventrally, through the thoracic inlet into the chest cavity. At surgery, this large thyroid tumor could easily be missed, resulting in persistent hyperthyroidism.

Alternatively, especially in those cats undergoing bilateral thyroidectomy, ectopic thyroid tissue, intrathoracic thyroid tissue, or thyroid carcinoma must be suspected (1,2,7). In these cats, use of thyroid scintigraphy (Figure 2) is again the best way to identify the location of the remaining hyperfunctioning thyroid tumor tissue and to help diagnose thyroid carcinoma (1,9).

Figure 2: Thyroid carcinoma in a cat with hyperthyroidism (thyroid scan on left). Notice the 3 thyroid masses, with the larger two being located within the chest cavity. The horizontal yellow line indicates the area of the thoracic inlet (top opening of the chest cavity). At surgery, these large thoracic tumor could easily be missed, resulting in persistent hyperthyroidism.

Recurrent Hyperthyroidism After Thyroidectomy

Unilateral Thyroidectomy
Only 30% of all hyperthyroid cats have unilateral disease. In these cats, removal of the one affected thyroid tumor will result in complete cure of the hyperthyroid state. Although it is possible for "new" adenomatous changes to develop in the remaining thyroid lobe sometime in the future, this is uncommon and would take many months to years to occur (8).

Although 70% of hyperthyroid have bilateral disease, some of these cats have asymmetrical thyroid enlargement, with one thyroid lobe being very large and the other being only minimally enlarged. In these cats, unilateral thyroidectomy generally restores euthyroidism, at least for a few weeks, and it may take a up to 6 months for the remaining lobe to grow to a size for hyperthyroidism to recur (8). However, unless preoperative thyroid scintigraphy is performed to verify that unilateral disease is indeed present (Figure 3), the owner must be aware of the possibility that hyperthyroidism could persist or recur after unilateral thyroidectomy is performed.

Figure 3: Bilateral asymmetric thyroid adenomas in a cat with hyperthyroidism. At surgery, it could be easy to mistake the slightly enlarged thyroid tumor (on the left) as being normal size. 

If hyperthyroidism recurs following unilateral thyroidectomy, reoperation to remove the remaining thyroid lobe can be performed, but care must be taken to preserve parathyroid function or hypoparathyroidism will develop after the second thyroid tumor is removed.

Bilateral Thyroidectomy
Because most hyperthyroid cats have involvement of both thyroid lobes, bilateral thy­roidectomy is generally indicated if long-term cure is the goal. The two major techniques for bilateral thyroidectomy include the intracapsular and extracapsular methods (3-6,10,11).

The major problem with the intracapsular technique for thyroidecto­my, as noted in my previous post on surgical techniques for thyroidectomy, is that it can be difficult to remove the entire thyroid capsule (and therefore all abnormal thyroid tissue) while con­currently preserving parathyroid function. Small remnants of thyroid tissue that remain attached to the capsule may regenerate and produce recur­rent hyperthyroidism (10,11). With the extracapsular tech­nique, the incidence of relapse is much less than because the entire thyroid capsule is removed at time of surgery.

When recurrent hyperthyroid does occur after bilateral thyroidectomy, it generally takes many months to years for the serum T4 to increase or clinical signs of hyperthyroidism to redevelop (10,11).

Prognosis and Long-Term Follow-up

The prognosis for hyperthyroid cats after thyroidectomy is good. Treated cats show improved behavior and significant weight gain.

Relapse of hyperthyroidism can occur but is uncommon if all involved tissue is removed at time of the original surgery. The lowest rates have been associated with the extracapsular technique.  Postoperative hypocalcemia is more common after reoperation, so alternative treatment methods (e.g., methimazole or radioiodine) should be considered in cats that experience relapse of the hyperthyroidism.

References:

1. Mooney CT, Peterson ME. Feline hyperthyroidism In: Mooney CT,Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association. 2012;92-110.
2. Baral RM, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. St. Louis: Elsevier Saunders, 2012;571-592.
3. 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.
4. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
5. Padgett S. Feline thyroid surgery. Veterinary Clinics of North America. Small Animal Practice 2002;32:851–859. 
6. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21:29-33. 
7. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. 2011; 104-106. 
8. 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.
9. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. J Vet Intern Med 2012; 26:754.
10. 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. 
11. Swalec KM, Birchard SJ. Recurrence of hyperthyroidism after thyroidectomy in cats. J Am Anim Hosp Assoc 1990;26:433-437.