Veterinary Information: Feline HyperthyroidismPage 4 (5)Continue Diagnoses of Feline Hyperthyroidism: -T3 Suppression Test: If T4 measurements are non-diagnostic the T3 Suppression Test can be applied. In normal cats if T3 is administered this results in a suppression of the TSH release through a negative feedback loop. A decrease of the TSH release results in a low T4 concentration. In hyperT4 cats, the T4 release is autonomous and independent of the TSH release. T3 administration will therefore not result in a decreased T4 concentration in hyperthyroid cats. T3 Suppression Test Protocol: 1.)Measure total basal serum concentrations of T3 and T4 at t=0 (before T3 administration). (centrifuge sample and keep serum refrigerated or frozen) 2.)Next morning start administering 25 µg Liothyronine (T3) orally, 3x/day for 2 days. 3.)On the morning of the 3rd day (7th dosis) administer the final dosis of 25 µg Liothyronine (T3) and measure serum T3 and T4 again within 2-4 hours. 4.)In normal cats a clear T4 serum decrease (<20 nmol/L) can be seen after T3 administration. In hyperT4 cats minimal to no decrease of serum T4 (> 20 nmol/L) is seen after T3 administration. 5.)Normal cats show a suppression of 50% or more after T3 administration. -Scintygraphy: Pertechnetate (99mTc) thyroid imaging normally shows a 1-to-1 ratio in size and intensity, between the salivary glands and the thyroid lobes in healthy cats. Scans of hyperT4 cats show an enlargement of either one or both (70%) thyroid lobes. The scan is also useful for detecting metastases and thyroid lobes that have descended into the thoracic cavity. Treatment: Treatment should be focussed on controlling the excessive secretion of thyroid hormones. However the treatment of choice depends on several factors: age of the cat; concurrent diseases (renal failure; heart disease) and willingness of the owner.
1.)Surgical thyroidectomy: Curative treatment option. In severely ill patients, 3-4 weeks of an euthyroid status can be achieved with anti-thyroid drugs before surgical thyroidectomy. Possible complications are: hypoparathyroidism and hypocalcemia, Horner’s Syndrome and laryngeal paralysis. (For normocalcemia only one functional parathyroid gland is needed). In unilateral thyroidectomy supplementation of L-thyroxine is in general not necessary. In bilateral thyroidectomy there is a risk of hypocalcemia when both parathyroid glands are damaged (first signs after 1-3 days). Serum calcium levels should be measured until these are stabilised. L-thyroxine (0.1-0.2 mg/day) should be administered, starting 24-48 hours after surgery. Lifetime treatment with L-thyroxine might not be necessary as thyroxine levels might increase normally. Recurrence of hyperT4 is possible. 2.)Radioactive Iodine (131I): Curative treatment option. The radioactive iodine is concentrated in the thyroid gland tissue like normal iodine. It mostly concentrates in the hyperplastic or neoplastic tissue resulting in an irradiation and destruction of this tissue. This is therefore also a good treatment option for ectopic thyroid tissue and in cases where thyroid lobes have descended into the thoracic cavity. Most hyperthyroid cats are cured by one single dose of Iodine 131. It is also the best treatment option in cats with malignant thyroid tumours (less than 2% of the hyperthyroid cats). Continue Feline Hyperthyroidism... Page 1, 2, 3, 4, 5 |