The objective of treatment is to return circulating thyroid hormone
concentrations to the normal range (called the "euthyroid state").
are 3 methods of treatment for hyperthyroidism :
1. Surgical removal of the affected gland(s)
Renal complications are common in animals with hyperthyroidism so screening
for renal disease should be performed prior to surgery. Some authors
administer aqueous iodine (Lugols solution) orally 3-5 drops daily for 1-2
weeks pre-surgery as this reduces thyroid size
Thyroidectomy is relatively simple surgery, however care must be taken to
preserve the parathyroid glands and to avoid damage to local adjacent
structures such as major nerves and blood vessels. Carcinomas are much
more invasive locally than adenomas and they can be more difficult to remove.
If thyroid tissue extends into the chest this too can be difficult to remove.
Surgery can be followed by hypoparathyroidism if the parathyroid glands
have been removed/damaged and for this reason circulating calcium
concentrations should be monitored carefully for 4-5 days during the
post-operative period. If concentrations fall below 1.2 mmol/l supplementation
is needed with intravenous 10% calcium gluconate at a dose of 1-1.5 ml/kg body
weight, followed by oral maintenance doses of 500-750mg/kg/day calcium
gluconate tablets .
Horner's syndrome may occur if the cervical nerves have been damaged and
voice changes can occur due to altered pressures around the larynx.
2. Antithyroid drugs
These drugs reduce the production of thyroid hormone by the thyroid gland..
They all can produce side-effects such as itchiness, bleeding disorders,
jaundice, vomiting and anorexia.
Carbimazole is the drug of choice in the UK to stabilise a patient
pre-thyroidectomy or for the long term control of hyperthyroidism. This drug
is metabolised to to the active ingredient methimazole (which is available in
Dogs 10-15mg/day orally divided into 2-3 doses. Increase dose to
maintain normal T3/T4 in circulation
Cats 10-15mg/day orally given in divided doses 2-3 times daily for 1-3
weeks. Then maintain euthyroid state with minimal dose needed in 1-2 daily
Propylthiouracil is less popular because is causes more side-effects
including immune-mediated haemolytic anaemia.:
Cats 50mg three times daily orally. Then adjust dose to maintain
Sometimes propranolol is used in combination with anti-thyroid drugs to
control the cardiovascular effects of hyperthyroidism.
3. Radioactive iodine
In licensed centres radioactive iodine (I131) can be used to
destroy the neoplastic thyroid tissue. Over-dosage can result in
hypothyroidism, but it has been used successfully in cats.
4. Nutritional intervention
Recently a new special diet (Hill's Precription diet Feline y/d) has been
introduced which is very low in iodine content (below NRC minimum requirements)
. If this is fed exclusively and the cat
does not get any other source of iodine it will induce regression of the benign
thyroid adenoma responsible for hyperthyroidism. Clinical signs resolve in a few
weeks of starting the diet.