Hyperthyroidism is a metabolic disorder that results from excessive circulating concentrations of thyroid hormones. It is considered the most common endocrine disorder of cats, generally occurring in middle-aged to older cats. The disorder is caused by hyperfunctioning nodules of the thyroid gland that secretes thyroid hormones. Hyperthyroidism is extremely uncommon in dogs, but has been documented in dogs with thyroid carcinoma or with over-supplementation of exogenous thyroid hormone. Some of the main clinical features of the disorder are: weight loss, polyphagia, polydipsia, vomiting, diarrhoea, hyperactivity and aggression. An unkept hair coat, matting and excessive shedding of hair especially in long-haired breeds are also signs. Enlarged lobes of thyroid gland are palpable in most cases (>90%) by extending the cat’s neck, tilting the neck back slightly and using the thumb and the forefinger to gently palpate the tissues on either side of the trachea. Feline hyperthyroidism can be treated with long-term antithyroid drugs aimed at blocking thyroid hormone synthesis. Recent research shows that methimazole can also be used to treat feline hyperthyroidism with relatively few side-effects. The best treatment will be determined by evaluating the patient’s age, coexisting medical problems and availability of treatment. Surgical thyroidectomy may be a recommended treatment for hyperthyroidism in cats. Surgery is aimed at removing all abnormal thyroid tissue while preserving at least one of the parathyroid glands. The major complication of a bilateral thyroidectomy is hypoparathyroidism that may arise after inadvertent removal of or damage to the parathyroid glands. If the parathyroid gland has been removed it should be transferred to a nearby muscle belly so that it may revascularise. During surgery, injury to the recurrent laryngeal nerve must be avoided as this will result in laryngeal paralysis. Horner’s syndrome is another less common surgical complication that may arise. Permanent hypothyroidism as a result of a thyroidectomy is rare, but long-term T4 replacement therapy can be given to cats that develop clinical signs of it. After a thyroidectomy patients should be closely monitored for the development hypocalcaemia during the initial postoperative period. Hypocalcaemia is caused by removal or damage to the parathyroid glands or their blood supply. Early signs are lethargy, anorexia, panting and facial rubbing. Acute hypocalcaemia may be treated with 10% calcium gluconate that can be administered slowly intravenously whilst monitoring cardiac rate and rhythm. If bradycardia develops, calcium administration should be discontinued immediately. Alternatively, the intravenous dose can be diluted in the same volume of saline and administered every 6 to 8 hours until the animal is eating. Calcium administration should be discontinued when serum calcium is above8 mg/dl. Maintenance therapy consists of oral calcium and vitamin D supplementation. Serum T4 concentrations should be measured in the first week after surgery and every 3 to 6 months thereafter to check for recurrence.
REFERENCES: 1. Birchard, SJ & Sherding, RG (eds) 2006, ‘Saunders manual of small animal practice’ 3rd ed., Saunders Elsevier, Philadelphia, pp. 331-342. 2. Tilley, LP & Smith, FWK 2004, ‘The 5-minute veterinary consult : canine and feline’ 3rd ed., Lippincott Williams & Wilkins, Philadelphia, pp. 644-645. 3. Naan, EC, Kirpensteijn, J, Kooistra, HS & Peeters, ME 2006, ‘Results of thyroidectomy in 101 cats with hyperthyroidism’, Veterinary Surgery, vol. 35, no. 3, pp. 287-293. 4. Peterson, ME, Kintzer, PP & Hurvitz, AI 2008, ‘Methimazole treatment of 262 cats with hyperthyroidism’, Journal of Veterinary Internal Medicine, vol. 2, no. 3, pp.150-157. 5. Fossum, TW, Hedlund, CS, Hulse, DA, Johnson, AL, Seim, HB, Willard, MD & Gwendolun, LC 2002, ‘Small animal surgery’, 2nd ed., Mosby, St Louis, pp. 519-525.
Metadata assigned by Dr. M. van Schoor, Senior Lecturer, Dept. of Companion Animal Clinical Studies