Thyroidectomy in cats is most commonly indicated to treat hyperthyroidism because of adenomatous hyperplasia of the thyroid glands. Thyroidectomy is a very effective treatment option for hyperthyroid cats. Surgical treatment of hyperthyroidism in cats offers permanent cure without chronic medical management. No specialised equipment other than standard surgical instrumentation and facilities are necessary. With practice, feline thyroidectomy can become a routine procedure in most veterinary hospitals.
Preoperative stabilization of the hyperthyroid cat with antithyroid drugs is preferred to minimize anesthetic and surgical complications. Multiple surgical techniques for thyroidectomy have been reported, and results of surgery and complications differ between techniques. The extra-capsular technique offers the most complete removal of abnormal thyroid tissue, while still being associated with a low incidence of postoperative hypoparathyroidism. Important postoperative considerations are to carefully monitor serum calcium concentrations and treat hypocalcemia if necessary, and to monitor the cat for evidence of relapse of hyperthyroidism.
Generally speaking, there are five types of clinical presentation of hyperthyroidism in cats. The most common finding is two nodules, often one larger than the other, on either side of the trachea i.e. a left thyroid nodule and a right thyroid nodule. In a smaller number of cases, a single (unilateral) large thyroid nodule is palpable. In still other cats, there are several small thyroid nodules, of different sizes, palpable in the ventral cervical area. Cats with thyroid carcinomas usually have a single large firm thyroid nodule. Finally, some cats have cystic thyroids or parathyroids, which can become considerable in size. This latter diagnosis is made using ultrasonography, or by doing a fine needle aspirate, which typically yields a large volume of fluid. Fluid retrieved by needle aspiration should be stored frozen, as in some cases it is helpful to measure the T4 and PTH levels within this fluid to determine whether the cystic mass is thyroid or parathyroid in origin.
Cats with bilateral thyroid nodules should receive radioactive-iodine therapy. But in some situations, for example, when the referral centre is too far for the owners to travel, or where the owners will not accept a long period of hospitalization, then surgery can be considered. Another indication for bilateral thyroidectomy might be when a veterinarian wants to restore euthyroidism at a reduced cost, for example, for a pensioner client.
Extracapsular dissection of the thyroid glands resulted in an 82% rate of postoperative hypocalcemia. An intracapsular dissection technique produced a 36% rate of postoperative hypocalcemia. The incidence of postoperative hypocalcemia was reduced to 11% when bilateral thyroidectomies were done asynchronously (staged), 3 to 4 weeks apart.
Cats with a single thyroid nodule are excellent candidates for surgical thyroidectomy using an extra-capsular technique. Experienced surgeons describe it as cervical spay! The dissection on the ventral midline is anatomically simple, and although there are some structures like the carotid artery and vagosympathic trunk to be avoided, the procedure is relatively straight-forward. As with all surgeries, it is ideal to practice first on a cadaver if you have never done this operation previously. An extremely clear instructional CD ROM and ancillary notes, produced by Gary Norsworthy (Norsworthy How-To Video Series: Thyroid palpation and thyroidectomy; Teton NewMedia), is available from the Post Graduate Foundation of Veterinary Science (www.pgf.edu.au), which greatly facilitates explanation of the procedure for the novice. Extracapsular thyroidectomy requires meticulous dissection of the external parathyroid gland from the thyroid capsule, and then removal of the entire capsule. Incidence of hypocalcaemia is as high as 82% with this procedure, so a modified extracapsular approach was attempted to preserve both the external and internal parathyroid glands. In the modified extracapsular technique, cautery was used to divide the thyroid capsule between the thyroid gland and the internal parathyroid, with special care to preserve the blood supply to the parathyroid. This method preserved all four parathyroid glands, although some amount of disturbance may have occurred. Hypocalcaemia was improved at 23%.
The intracapsular approach is performed by opening the thyroid capsule and bluntly dissecting the thyroid gland out with a sterile swab, hence avoiding destruction of the blood supply to the parathyroid. Recurrence rates ranged from 8% to 22%, and hypocalcaemia incidence ranged from 15% to 26% in 129 patients. A modified method was used in a separate group of patients. In the modified approach, post removal of the thyroid gland, the caudal portion of the capsule was removed and the caudal vessels were cauterized. This method resulted in a recurrence rate reduction to 5% but hypocalcaemia incidence went up to 34%. While none of the techniques reviewed were free of risk, careful evaluation and experience has brought about clear improvements in surgical treatment in the feline patient.
In experienced hands, this operation takes less than 10 minutes from skin to skin, and accomplished surgeons would take half this time. The excised tissues should ideally be submitted to a laboratory for histological assessment, although in the vast majority of cases the nodule will be a hyperplastic lesion or an adenoma. It is well worth removing the whitish parathyroid gland that sits at the caudal pole of the thyroid and implanting it into a pocket fashioned in the strap muscles in the neck. The advantage of doing this is that if the contralateral thyroid becomes hyperplastic and functional at some time in the future, it can be removed without the risk of the patient developing hypocalcaemia due to acute hypoparathyroidism, as the transplanted parathyroid tissue will have engrafted and become functional by this time.
Although surgical texts will provide an explanation of techniques least likely to result in post-operative hypoparathyroidism, the staged procedure described by Norsworthy is the surgical technique of choice. This involves removal of the largest thyroid nodule first, with transplantation of the ipsilateral parathyroid into the strap muscles, followed 4 to 6 weeks later by a second procedure on the contralateral side.
Surgery has several disadvantages. Intra-operative problems associated with thyroidectomy include anaesthesia-induced complications such as cardiac arrhythmias in addition to hemorrhage associated with thyroid dissection. Post-operatively, Horner's syndrome and laryngeal paralysis also have been observed. The most significant life-threatening complication of thyroidectomy is severe hypocalcemia that occurs as a result of parathyroid injury, devascularisation, or inadvertent removal of all parathyroid tissue secondary to bilateral thyroidectomy.
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