Clinical History
The cat was first presented for episodes of collapse and ataxia. An ECG revealed a heart rate of 260, sinustachycardia. A radiograph showed massive cardiomegaly. Systolic blood pressure measured at 178 - borderline hypertensive. The left thyroid was enlarged on palpation. Blood samples showed thyroxine > 300 nmol/l ( normal 19 - 65), elevated ALT and Alk Phos, and urea and creatinine normal. The cat was prescribed carbimazole 5 mg (NeoMercazole, Roche) one tab every 8 hrs, and propranolol 10 mg (Inderal, Zeneca), a quarter of a tablet three times daily.

The cat initially responded to medical treatment with a heart rate of 175 and systolic blood pressure falling to 142, and a weight gain of 0.1 kg in 3 weeks. However, the cat continued to be polyphagic and no further improvement was seen. In view of this and that the cat was only 10 years old it was decided to try surgical treatment. The cat was booked for a unilateral left thyroidectomy. The owners were warned of the risks of a general anaesthetic and of possible post-operative complications, and signed a consent form before surgery was undertaken.

Surgical preparation
Premedication consisted of 0.5 mg of morphine (Morphine Sulphate, Evans) by intramuscular route 1 hour prior to surgery. A 22 g intravenous catheter was placed aseptically into the right cephalic vein and an infusion of 200 mls of Hartmann's was started at a rate of 10mls/kg/hr. Induction was achieved using 1.5 mg diazemuls, followed by 20 mg propofol (Rapinovet, Schering Plough) given to effect, both by intravenous route.

The cat was intubated with a 5 mm uncuffed endotracheal tube and anaesthesia was maintained using a semi-closed Ayre's T piece with a Jackson Rees modification. Flow rates of 2L of nitrous oxide and 1L of oxygen were used, with halothane as the inhalation agent. Additional analgesia was given with 10 mg of carprofen (Rimadyl, Pfizer) by sub- cutaneous route and antibiotic cover was provided with 85 mg ampicllin LA (Amfipen LA). The ventral neck was clipped and prepared as in paragraph 1 of Appendix 1, extending caudally to the cranioventral thorax and cranially to include all the ventral neck.

The cat was moved to theatre and placed in dorsal recumbency with the abdomen supported in a bubble wrap lined cradle. The neck was extended and a sand bag placed underneath. The forelimbs were extended and tied caudally. The skin was then prepared as in paragraph 2 of Appendix 1. Four drapes were placed to leave a rectangular window, leaving the surgical site as the only part of the cat exposed. See figure 1 on the right. A standard kit was laid out as listed in Appendix 2. Additional instruments included iris scissors and fine rat tooth forceps.

Surgery
An incision through the skin was made from the larynx, on the ventral neck, extending for 5 cms. The sternohyoid and sternothyroid muscles were bluntly separated to expose the left thyroid gland. The right gland was not visible at this point as it was located behind the trachea, which explains why it was not clinically palpated despite also being large. A decision was made at this point to perform a bilateral thyroidectomy. Using iris scissors and fine rat toothed forceps the thyroid gland was removed from both the caudal and cranial parathyroid glands by careful sharp and blunt dissection. Both the cranial and caudal thyroid blood vessels were preserved and did not need ligating. All the visible thyroid tissue was removed from the parathyroids. The process was repeated with the left side and although no ligating was necessary there was suspicion that the caudal parathyroid might be damaged. The muscles were closed using 3 m chromic catgut (Ethicon) using a simple continuous suture pattern and the skin was closed with 2 m polyamide (Nylonamide, Animus) using simple interrupted sutures. The cat had an uneventful recovery and went home the same day, with instructions to the owner regarding post-operative complications. The cat was seen two days later and was eating well and had no signs of hypocalcaemia. Sutures were removed after 10 days and there were still no signs of hypocalcaemia. Three weeks after surgery the cat had increased it's weight by 0.3 kg and a further blood test showed T4 levels at 7.6 nmol/l (slightly low). Heart rate was 172 p min and systolic blood pressure down to 130.

Discussion
Hyperthyroidism usually occurs in cats over 8 years old, with an average age of 13 years. Symptoms can include weight loss, polyphagia, tachycardia, lethargy, rough coat, change of character, and it is often associated with cardiac abnormalities, hypertension and renal failure. Blood tests will show elevated T4 levels. The condition can be treated medically, surgically, or with Iodine 131. The type of treatment selected will depend on the renal function, age, cardiac function and owner preference. Owners need to be advised of the possible post-operative complication of hypocalcaemia, more commonly occurring when bilateral thyroidectomies have been performed. Removal or disruption of the parathyroids which are closely associated with the thyroids can result in an inability to increase calcium levels in the body, leading to ataxia, tremors, coma and death. This is likely to occur within the first few days and the cat should be kept inside for observation and the owners should phone immediately if symptoms appear, as the cat may need intravenous calcium gluconate. Other surgical complications include Horner's syndrome, hypothyroidism, laryngeal paralysis and recurrence of hyperthyroidism. The draping of this patient was good, with no possibility of contamination of the surgeon by any part of the cat. Analgesia was good in the choice of an opioid and non steroidal and the cat was quite comfortable post-operatively. Pre- operative use of drugs are important for this surgery. The Inderal is given to reduce the blood pressure and for management of sinustachycardia and the NeoMercazole is given to reduce the levels of T4 production. Both of these help reduce the risks of general anaesthesia whilst performing the surgery. There are two techniques for performing a thyroidectomy in the cat. The intracapsular technique and the modified extracapsular technique. In this case the surgeon used a slight variation on the modfied extracapsular method. The surgeon feels that each thyroidectomy is different and that the technique she chooses will depend on a visual inspection of the glands. The post-operative T4 levels were slightly low but this is usually transitory and does not need treatment. Removal of both thyroids can be undertaken unilaterally 6 weeks apart. This does result in the cat having two anaesthetics and if surgery is performed competently the risks of bilateral surgery should be minimal. With regard to the antibiotics used, there was no evident pre-existing infection so a single dose of antibiotics should be adequate. Ideally the antibiotics should be working at the time of surgery and could have been given with the premedication. Placement of a sandbag under the neck assists with the surgery by moving the glands into a more accessible position. Placing the cat in a bubblewrap lined cradle and giving warmed fluids helps to preserve body temperature during general anaesthesia, particularly as this procedure is usually performed on very thin cats with little or no body fat. Alternative suture materials could have been used for this procedure, such as polyglactin 910, but cost of materials is important in a charity clinic and the surgeon has never encountered a tissue reaction using catgut for this particular procedure or had a problem with polyamide in the skin, so feels that it is not imperative to use other suture materials when performing thyroidectomies.

In summary, this was successful surgery as there were no post-operative complications and the aim of the surgery had been accomplished, ie the cat is no longer polyphagic or suffering weight loss and the T4 levels did return to the normal range after 2 months.

References:
Small Animal Surgery, T Fossum, Mosby. 1997