Introduction

Hyperthyroidism is considered the most common endocrine disorder in cats, with histopathological examination of affected tissues indicating that most of cases are caused by functional thyroid adenomas, and a small proportion being caused by thyroid carcinoma (Naan et al. 2006). The diagnosis of hyperthyroidism is usually made using a combination of clinical signs, thyroid palpation, appropriate laboratory tests (serum total thyroxine concentration (TT4)) and/or imaging findings. Thyroid palpation is a standard part of a physical examination and can be an important aid in the diagnosis of hyperthyroidism. Treatment options for hyperthyroidism in cats include use of methimazole, radioactive iodine or gland surgical removal. Reported success rates are high for all these treatment methods (Harvey et al. 2008, Naan et al. 2006). Our study purpose was to determine the factors that influencing the choice for thyroidectomy and surgical success rate in Brazilian hyperthyroid cats.

Materials and Methods

Forty hyperthyroid patients were evaluated from cat veterinary clinic in Brazil between January 2008 and December 2008. The clinical signs, cervical enlarged thyroid palpation and laboratory test were done to confirm hyperthyroidism. Oral management was indicated to lower serum thyroxine and to establish euthyroidism before surgical treatment. Only fifteen patients (37%) were submitted to thyroidectomy. Factors influencing the choice for surgical management were evaluated. Extracapsular technique with parathyroid gland transplantation into ipsilateral sternohyoid muscle was done (Norsworthy 1995). Success rates were recorded. Thyroid scintigraphy wasn't done because this nuclear procedure is not available in Brazilian Veterinary Medicine.

Results

The age of the cats submitted to thyroidectomy was ranged from 9 to 18 years (mean 14 years). Twelve cats were domestic shorthairs and three were Siamese. Eight were neutered males and seven were neutered females. Clinical signs included weight loss (ten cases), excessive vocalization (two cases), aphasia (two cases), polyphagia (one case), vomiting (one case), diarrhea (one case) and agitation (one case). All cats had palpable thyroid gland on examination. Nine cats had unilateral large palpable goiter and six had bilateral enlarged glands. Serum total thyroxine concentration was increased in all cats. Medical management of hyperthyroidism was introduced. The factors influencing the choice for surgical treatment included difficulty in medicating (two cases), owners interest (four cases), inability to stabilize with oral medical management (three cases), and adverse effects of methimazole (six cases). Side effects of methimazole were vomiting (three cases), anorexia (two cases) and severe leucopenia (one case). Radioactive iodine therapy wasn't available in Brazilian Veterinary Medicine. Unilateral thyroidectomy was done in fourteen cats and bilateral thyroidectomy was done in one cat. Hypocalcemia occurred in cat submitted to bilateral thyroidectomy. Unilateral thyroidectomy was successful in nine cats. Relapse of hyperthyroidism occurred in five cats and three of them were submitted to surgery to remove the other enlarged thyroid gland. Other complications of surgery weren't observed.

Discussion and Conclusions

Three options are available for treating hyperthyroidism in cats and include use of methimazole, radioactive iodine treatment or surgical removal of the gland. The most appropriate treatment option for any individual hyperthyroid cat is determined by evaluation of age, concurrent medical problems, availability of therapy, the client's opinion, financial options and the presence of multiple foci of abnormal thyroid tissue or ectopic thyroid tissue (Harvey et al. 2008, Naan et al. 2006). In Brazil, treatment options for hyperthyroidism are oral management, which is not curative, or surgery. When enlarged thyroid glands are palpable, the hyperthyroid cats are indicated to thyroidectomy unless concurrent renal problem or systemic diseases are present. But some owners are resistant about surgery because the old age of their cats and prefer the oral management until side effects occur or oral management become a difficulty practice. The most frequent reason for thyroidectomy was adverse effects of oral medication management. The main side effects included vomiting (three cases), anorexia (two cases) and severe leucopenia (one case). Other side effects weren't observed like facial excoriation and thrombocytopenia (Trepanier 2006). Owner's interest in stopping the oral medication was the second reason for thyroidectomy. Surgical complications weren't observed when it was done with parathyroid gland transplantation and a staged technique. Recurrence of hyperthyroidism was associated with unilateral thyroidectomy on this study. But based on a study, between 1/4 and 1/5 hyperthyroid cats have multiple areas of hyperfunctional thyroid tissue and/or intrathoracic hyperfunctional thyroid tissue where surgical thyroidectomy would not be curative (Harvey et al. 2008) and for this reason thyroid scintigraphy should be considered. Veterinary surgeons should, therefore, routinely warn owners about the possibility of ectopic thyroid tissue before thyroidectomy is performed. Unilateral or staged thyroidectomy extracapsular with parathyroid gland transplantation may be a good option for long-term resolution of the hyperthyroidism in cats with enlarged palpable gland even though scintigraphy is not available.

References

1.  Naan EC, Kirpensteijn J, Kooistra HS, Peeters M. 2006 Results of thyroidectomy in 101 cats with hyperthyroidism. Vet Surg 35: 287-293.

2.  Harvey AM, Hibbert A, Barrett E, Day MJ, Quiggin AV, Brannan RM. 2008. Scintigraphic findings in 120 hyperthyroid cats. Article in press: J Fel Med Surg. 1-12.

3.  Norsworthy GD. 1995. Feline thyroidectomy: a simplified technique that preserves parathyroid function. Vet Med 95: 1055-1063.

4.  Trepanir LA. 2006. Medical management of hyperthyroidism. Clin Tech Small Anim Pract 21: 22-28.