Complications of Thyroidectomy in Cats: Postoperative Hypocalcemia

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 (1-6).

Many potential complications are associated with thyroidectomy, including hypoparathyroidism, Horner's syndrome, laryngeal paralysis, and persistent or recurrent hyperthyroidism. The most serious complication is hypocalcemia, which develops after the parathyroid glands are injured, devascularized, or inadvertently removed in the course of bilateral thyroidectomy. Since only one parathyroid gland is required for maintenance of normocalcemia, hypoparathyroidism develops only in cats treated with bilateral thyroidectomy (4-6).

Pathogenesis of hypoparathyroidism (calcium crisis) after thyroidectomy
Hypocalcemia is the most serious complication associated with thyroidectomy. This adverse effect develops almost exclusively in hyperthyroid cats with bilateral thyroid disease who must have both thyroid lobes removed (bilateral or total thyroidectomy).

Under normal circumstances, the circulating calcium concentration is tightly regulated to remain within a narrow normal range, as calcium is required both for adequate muscle and nerve function. When circulating calcium falls, the parathyroid glands secrete parathyroid hormone (PTH), which leads to an increase in serum calcium concentration back to normal (7). PTH acts on several organs to increase calcium levels, including the intestinal tract, kidney, and bone.

A discussed in my recent blog post on thyroid and parathyroid anatomy, the 4 parathyroid glands are located within or around the cat’s thyroid gland (the prefix para is from Greek, meaning “at or to one side of, beside, side by side”). However, only the external parathyroid glands will be visible at time of thyroidectomy, whereas the internal parathyroid gland will be embedded within the tumor itself (Figure 1).

Figure 1: Identifying and preserving the external parathyroid gland in a hyperthyroid cat.
(Note: this cat has concurrent kidney disease, which led to secondary enlargement of the parathyroid gland, making it easier to find).

If the parathyroid glands are removed or damaged, the loss of parathyroid hormone secretion leads to a condition called hypoparathyroidism (the prefix hypo is again from Greek, meaning “under”). This leads to a low circulating calcium concentration which can progress to a hypocalcemia crisis due to hypoparathyroidism (7-10).

Clinical signs of hypoparathyroidism (calcium crisis) in cats
Hypocalcemia causes the major clinical manifestations of hypoparathyroidism by increasing the excitability of both the central and peripheral nervous systems (7-10).

Early signs of hypocalcemia due to iatrogenic hypoparathyroidism include anxiety, appetite loss, depression and weakness, twitching, muscle tremors, and facial itch (4-10). Later in the course of hypoparathyroidism, these signs can progress to tetany, collapse, and seizures—hence, the importance of monitoring serum calcium levels during the postoperative period.

Monitoring for hypoparathyroidism after thyroidectomy
After bilateral thyroidectomy, the serum calcium concentration should be monitored on a daily basis until it has stabilized within the normal range. In most cats with iatrogenic hypoparathyroidism, clinical signs associated with hypocalcemia will develop within 1 to 3 days of surgery, but it may take as long as 5 days in some cats (4-10).

Although mild hypocalcemia (6.5-7.5 mg/dl) is a common finding during this immediate postoperative period, laboratory evidence of hypocalcemia alone does not require treatment. However, if accompanying signs of muscle tremors, tetany, or convulsions develop, therapy with vitamin D and calcium is indicated (7-12).

Treating hypoparathyroidism 
If symptomatic hypocalcemia develops, the cat needs to be treated with large doses of calcium and vitamin D. Calcium is initially administered as an intravenous infusion, followed by daily oral administration. Large doses of oral vitamin D also need to be given daily to increase the intestinal absorption of calcium (7-12).  Supplementation with calcium and vitamin D may be needed for only a few days or for the rest of the cat’s life, depending on the extent of damage to the parathyroid glands.

Although hypoparathyroidism may be permanent in some cats, spontaneous recovery of parathyroid function usually occurs days to months after surgery. Therefore, in most cats with surgically-induced hypoparathyroidism, oral calcium and vitamin D supplementation can eventually be tapered and withdrawn (typically, after a few weeks of treatment).

In most cases, such transient hypoparathyroidism probably results from reversible parathyroid damage and ischemia incurred during surgery. Alternatively, accessory parathyroid tissue may secrete PTH and compensate for the damaged parathyroid glands to maintain normocalcemia, or accommodation of calcium-regulating mechanisms in the absence of PTH may occur (7,13).

Incidence of postoperative hypoparathyroidism
Several studies have evaluated the incidence of hypoparathyroidism after thyroidectomy in cats. In an early study (14), 4 out of 53 cats (7.5%) that had a total thyroidectomy performed with an intracapsular technique developed hypoparathyroidism.

A much higher rate was found in another study that compared the complication between different surgical techniques (15). In that study, extracapsular dissection resulted in an 82% incidence of hypocalcemia, whereas intracapsular dissection resulted in a 36% incidence of hypocalcemia. Staged bilateral thyroidectomy, in which two thyroidectomy procedures were performed a few weeks apart, resulted in an 11% incidence of hypocalcemia (15). However, another study found lower (and similar) rates of hypocalcemia between techniques—23% with a modified extracapsular technique and 33% with a modified intracapsular technique (16).

In the most recent study of thyroidectomy in cats, performed using the modified intracapsular dissection technique, a very low incidence of hypoparathyroidism was reported (17). In that study, only 5 (5.8%) of 86 cats developed postoperative hypocalcemia and none required permanent treatment with calcium and vitamin D.

Bottom Line

No matter which surgical technique is chosen, hypoparathyroidism will develop in a significant proportion of cats treated with bilateral thyroidectomy. However, the very low occurrence of postoperative hypocalcemia in some, but not all, studies suggests that surgeon experience may be the most important factor in determining the outcome for hyperthyroid cats undergoing thyroidectomy.

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. Baral RM. Disorders of calcium metabolism In: Little SE, ed. The Cat: Clinical Medicine and Management. St. Louis: Elsevier Saunders, 2012;625-642.
8. Peterson ME. Hypoparathyroidism, in Kirk RW (ed): Current Veterinary Therapy IX. Philadelphia, WB Saunders. 1986; 1039-1045.
9. Peterson ME. Hypoparathyroidism and other causes of hypocalcemia in cats, in Kirk RW (ed): Current Veterinary Therapy XI. Philadelphia, WB Saunders. 1992; 376-379.
10. Skelly BJ. Hypoparathyroidism In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association. 2012;56-62.
11. Chew D, Nagode L. Treatment of hypoparathyroidism, in Bonagura JD (ed): Kirk’s Current Veterinary Therapy XIII. Philadelphia, WB Saunders. 2000; 340-345. 
12. Henderson AK, Mahony O. Hypoparathyroidism: treatment. Compend Contin Educ Vet 2005; April:280-287.  
13. Flanders JA, Neth S, Erb HN, et al. Functional analysis of ectopic parathyroid activity in cats. Am J Vet Res. 1991 Aug;52(8):1336-40.  
14. 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. 
15. 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. 
16. 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. 
17. Naan EC, Kirpensteijn J, Kooistra HS, et al. Results of thyroidectomy in 101 cats with hyperthyroidism. Vet Surg 2006;35:287-293. 

Thyroidectomy for Cats with Hyperthyroidism: Surgical Techniques

As discussed in my last post we have several different techniques that can be used when performing a thyroidectomy in cats with hyperthyroidism (1-8). These include the following:

• Extracapsular thyroidectomy technique
• Modified extracapsular technique
• Intracapsular thyroidectomy technique
• Modified intracapsular technique

The surgical technique chosen depends both on the surgeon’s preference and whether one or both thyroid lobes need to be removed. In other words, if we know that the cat has unilateral thyroid disease (a single thyroid tumor), only a unilateral thyroidectomy is needed to cure the hyperthyroidism. On the other hand, if the cat has bilateral thyroid disease (tumors in both lobes), a bilateral or total thyroidectomy is needed to effect a cure (1-7).

The aim of all of these thyroidectomy techniques is to remove all abnormal thyroid tissue and preserve at least 1 parathyroid gland. In addition, care are should be taken to avoid trauma to the adjacent vessels and nerves, as well as to the parathyroid glands.

The 4 Surgical Techniques for Thyroidectomy in Cats

Extracapsular technique
The “original” extracapsular technique (1) is most useful for cats with unilateral thyroid disease, in which only one thyroid lobe needs to be removed. With this technique, the affected thyroid tumor together with the associated external and internal parathyroid glands are removed.

This surgical procedure here is simple: once the thyroid tumor is identified, the cranial and caudal blood supply to the affected thyroid tumor is ligated, and the entire thyroid lobe is excised along with its capsule (Figure 1). Again, no attempt is made to preserve the external parathyroid gland with this method (1,2).

Figure 1: Performing a unilateral thyroidectomy in a cat with the extracapsular technique

Closure of the incision is by simple continuous suture pattern in the sternohyoideus muscle using absorbable suture, simple continuous pattern in the subcutaneous tissues with absorbable suture, and interrupted sutures in the skin with nonabsorbable sutures. As an alternative to skin sutures, use a continuous absorbable intradermal suture layer.

Because the entire thyroid tumor and its capsule are removed with this technique, the cure rate is high, with little chance of local recurrence. However, because the external parathyroid gland is also removed, this technique is not recommended for cats in which bilateral thyroidectomy is needed because of the very high incidence of hypoparathyroidism (4,5,9).

Modified extracapsular technique
The “modified” extracapsular technique was developed to decrease the risk of postoperative hypoparathyroidism and hypocalcemia that can develop when both thyroid lobes are removed in cats with bilateral thyroid adenomas (3,5,7,8).

Compared to the original extracapsular technique, this surgical procedure is more difficult. Once the affected thyroid tumors and external parathyroid glands are identified, the thyroid gland capsule is incised approximately 300 degrees around the external parathyroid gland (Figure 2), being careful to preserve the blood supply to the parathyroid gland.

Figure 2: Extracapsular dissection for removal of a thyroid tumor in a cat. 
Figure from reference 8 (with permission)

The surgeon then uses a moistened cotton-tipped applicator to dissect the external parathyroid gland and attached capsule away from the the main thyroid tumor. Ideally, bipolar cautery (rather than ligatures) is used to control hemostasis in order to reduce blunt dissection near the external parathyroid gland. After the caudal thyroid vein is ligated, the affected thyroid tumor and remaining capsule are removed. Skin closure is routine, as described above.

With the modified extracapsular technique, the thyroid tumor and approximately 90% of its thyroid capsule are removed, leaving a small rim of thyroid capsule around the external parathyroid gland (3,5,7,8). Use of this modified extracapsular technique helps ensure that the external parathyroid gland with its blood supply remain intact, greatly lessening the incidence of hypoparathyroidism. However, because a small amount of thyroid capsule is not removed, small remnants of adenomatous tissue that are left behind may regrow with time, leading to recurrence.

Intracapsular technique
With the intracapsular technique for thyroidectomy, a small nick incision is made in an avascular area of the thyroid capsule on the middle to caudal aspect of the gland (Figure 3A). This longitudinal incision is extended with a scalpel blade or fine iris scissors until the entire thyroid capsule is opened (Figure 3B).  The incised thyroid capsule is reflected off the gland with tissue forceps (Figure 3C). The thyroid tumor tissue is then gently teased away from the inside aspect of the thyroid capsule with a sterile cotton-tipped applicator, leaving the thyroid capsule and external parathyroid gland intact.

Figure 3: Intracapsular dissection for removal of a thyroid tumor in a cat. 
Figure from reference 8 (with permission)

Extreme care is required during manipulation of the cranial pole of the thyroid to avoid injury to the blood supply of the external parathyroid gland (located outside of thyroid capsule). As with the extracapsular technique, meticulous hemostasis is critical to maintain good visualization of the surgical field. The incision is closed as described under the extracapsular technique.

The advantage of the intracapsular technique over the extracapsular techniques, described above, are that this is a technically simple method to help ensure that the external parathyroid gland and delicate blood supply are preserved. Because of this, the risk of hypoparathyroidism is greatly reduced. However, because of retained remnants of abnormal thyroid tissue that remain attached to the thyroid capsule, the rate of recurrent hyperthyroidism is highest with this method (4,10,11).

Modified intracapsular technique
Because the intracapsular technique has the potential to leave a significant amount of thyroid tumor tissue behind, a “modified” intracapsular technique was subsequently developed for use in hyperthyroid cats.

The procedure is as described above for the intracapsular technique, but as a final step, the thyroid capsule (caudal to the parathyroid gland) is resected following removal of the thyroid tumor. This leaves only a small rim of thyroid capsule around the external parathyroid gland, thereby greatly lessening the chance of recurrence (3,7).

Staged Bilateral Thyroidectomy

Some surgeons recommend that bilateral thyroidectomy be performed in two stages or separate surgical procedures, in order to lessen the risk of hypoparathyroidism (4). A period of at least 3-4 weeks between procedures gives time for vascular or parathyroid damage to heal.

The necessity of two anesthetic episodes is the major drawback of the technique, considering the older age of the hyperthyroid cats often affected.

Parathyroid Gland Autotransplantation

Parathyroid autotransplantation has also been described as a treatment for accidental removal of the parathyroid or if complete devascularization occurs during thyroidectomy (7,9).

If the parathyroid glands are removed or damaged, the parathyroid gland can be minced into small 1-mm pieces and inserted into a small pocket made in the cervical musculature. With time, such transplanted parathyroid tissue can start to function again. This will decrease the severity and duration of postoperative hypocalcemia.

My Bottom Line

I greatly prefer the extracapsular technique for thyroidectomy in cats because it helps to ensure a permanent cure of the cat’s hyperthyroidism (i.e., the recurrence rate is extremely low). With the intracapsular techniques (especially the original technique), remnants of thyroid tissue are commonly left behind, which can regrow to cause recurrence of hyperthyroidism in some cats.

However, in cats in which the parathyroid glands can not be identified, we still rely on intracapsular dissection when an external parathyroid gland cannot be identified. With this technique, we open the thyroid capsule, which makes it easier to locate and preserve at least one of the parathyroid glands and prevent hypoparathyroidism.

Finally, in cats in which identifying or saving the parathyroid gland is difficult, use of the staged bilateral thyroidectomy approach, parathyroid gland transplantation, or both can help lessen the incidence of postoperative hypocalcemia. However, considering the increased cost and morbidity of a second operation, our preference is to perform bilateral thyroidectomy in one procedure if at all possible.

References:

1. Black AP, Peterson ME: Thyroid biopsy and thyroidectomy, In: Bojrab MJ (ed): Current Techniques in Small Animal Surgery. Philadelphia, Lea and Febiger. 1983; 388-396. 
2. 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. 
3. Birchard, SJ. Thyroidectomy in the cat. Clinical Techniques in Small Animal Practice 2006;21:29-33. 
4. 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. 
5. Flanders JA. Surgical therapy of the thyroid. Veterinary Clinics of North America. Small Animal Practice 1994;24:607–621. 
6. Flanders JA. Surgical options for the treatment of hyperthyroidism in the cat. Journal of Feline Medicine and Surgery 1999;1:127–134. 
7. Padgett S. Feline thyroid surgery. Veterinary Clinics of North America. Small Animal Practice 2002;32:851–859. 
8. 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.
9. Padgett SL, Tobias KM, Leathers CW, et al. Efficacy of parathyroid gland autotransplantation in maintaining serum calcium concentrations after bilateral thyroparathyroidectomy in cats. Journal of the American Animal Hospital Association 1998;34:219-224. 
10. Swalec KM , Birchard SJ. Recurrence of hyperthyroidism after thyroidectomy in cats. Journal of the American Animal Hospital Association. 1990;26:433–437. 
11. 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.