The thyroid and thyroid hormone
gland consists of 2 small lobes situated in the neck around the trachea
gland produces two hormones; calcitonin, which regulates calcium metabolism and
thyroid hormone, which controls many important body functions, including how
the body uses energy, regulates temperature, metabolises nutrients, and
controls growth and development.
hormone exists in two forms. Thyroxine
or levothyroxine, also known as T4 and liothyronine or L-triiodothyronine also
known as T3. Up to 90% of thyroid hormone
secreted by the thyroid gland is T4, which is inactive but is converted to the
active form of thyroid hormone T3 when it reaches its target tissue. This
conversion takes place in the liver, kidney and other tissues of the body and
T3 then interacts with a specific receptor inside the nucleus of cells, to
influence the transcription of specific pieces of DNA (genes) to produce
proteins needed to manage and control a variety of processes, including
carbohydrate, fat, protein and vitamin metabolism.
The production of
thyroid hormone is regulated by a hormone produced by the pituitary gland in
hypothalamus at the base of the brain, and is called thyroid stimulating hormone (TSH). When levels of thyroid hormone in the blood are low,
this stimulates increased production of TSH and thereby increases production of
more thyroid hormone. Conversely, when thyroid hormone levels increase,
TSH production is decreased. This is known as a negative feedback loop, which
helps maintain normal levels of thyroid hormone in the blood.
Hyperthyroidism in pets
Hyperthyroidism or thyrotoxicosis, as it is also known, is a common disorder in cats, particularly older cats and is caused by overproduction of thyroid hormone usually due to a benign enlargement of the thyroid gland. However, hyperthyroidism is a rare condition in dogs and is usually caused by a malignant tumour called thyroid adenocarcinoma.
The enlarged thyroid gland in cats is similar to toxic nodular goitre in humans, which is one of the major causes of hyperthyroidism in humans. The symptoms or signs of hyperthyroidism in cats can be dramatic and can cause serious illness. They include:
- weight loss
- increased appetite (polyphagia) and increased thirst (polydipsia)
- increased food intake
- over activity and restlessness
- increased heartrate (tachycardia)
- diarrhoea and vomiting
- heat intolerance
- change in hair coat condition
If undiagnosed or left untreated hyperthyroidism can cause more serious illness including heart failure, high blood pressure and kidney disease.
Although surgical removal of the affected thyroid tissue is an option for treating hyperthyroidism (thyroidectomy); also radioactive iodine treatment, medical treatment is available and successful. Anti-thyroid drugs known as thioamides like carbimazole are widely used to treat hyperthyroidism in animals and humans. They work by blocking the synthesis of thyroid hormone and help restore high levels of thyroid hormone to normal levels.
Medical management of hyperthyroidism with carbimazole requires daily administration and is not a cure but can be also used stabilise the animal before radiation or surgery.