The thyroid glands are located on either side of the trachea (wind pipe) half way down the neck. Thyroid hormones ,thyroxine (T4) and triiodothyronine (T3) ,influence many of the organs and the rate of metabolism of the body. Increases in these hormones are almost always a result of a functional enlargement (adenomatous hyperplasia) of one or both gland and rarely due to tumors (thyroid adenocarcinomas). The cause of thyroid hyperplasia is unknown.

Most cats effected by hyperthyroidism are over 10 years of age however it has been recorded in cats as young as three years of age.

A wide range of clinical signs have been observed in varying combinations. Weight loss with normal to increased appetite is common due to a faster metabolism. This may also result in poor heat tolerance. Anxiety and behavioral changes such as increased excitability may be recognized.

A number of cardiac (heart) conditions including tachycardia (increased heart rate), arrhythmia's (irregular rhythm), cardiomegaly and cardiomyophathy (enlarged heart and weakened heart muscle) have been seen associated with hyperthyroidism. Accompanying panting and dyspnoea (shortness of breath) may lead to weakness and even collapse following periods of stress.

Polydypsia and polyuria (increased drinking and urinating) may be observed possibly due to changes in blood flow to the kidneys.

Vomiting, increased flow through the intestine with greater volume and frequency of faeces, and diarrhea are the potential intestinal symptoms.

Generally poor coat condition with lack of grooming is a minor sign.

Over 80% of cases of hyperthyroid effected cats will have one or both thyroids discernibly enlarged however palpably normal thyroids do not rule out this condition.

Diagnosis is made through measurement of thyroid hormone and thyroid function tests. Thyroxine (T4) concentration is a commonly used test however fluctuations during the day may result in periods of normal T4 concentration in a hyperthyroid cat. Where clinical sign indicate hyperthyroidism with a normal T4 result a T3 suppression test may be performed. This test measures the level of T4 following the administration of oral doses of T3. Normal cats will display a lower of T4 levels however this response is absent in the hyperthyroid cat.

Reduction of thyroid hormone may be achieved by three methods chemical suppression , surgical removal , and irradiation of the thyroid glands.

Carbimazole and methimazole are the two most commonly used drugs for thyroid suppression. They act by stopping the formation of thyroid hormone but have required continued administration to prevent return to the hyperthyroid state. There is usually a delay of three to four weeks between the commencement of therapy and signs of improvement. Side effects involving damage to blood cells must be monitored during the initial phase of treatment.

Surgical removal of effected glands has been a widely used treatment for hyperthyroidism. Cats will generally produce enough thyroid hormone from other sites in the body to maintain normal levels despite having both glands removed. The main complication of this surgery is damage to the parathyroid glands located next to the thyroids. The parathyroid glands are responsible for the calcium balance in the body and damage can lead to hypocalcemia (low blood calcium) a potentially serious condition.

Radioactive Iodine (131I) can be used to destroy the hypertrophic thyroid tissue. Iodine is concentrated by the body in the thyroid gland. The hypertrophic tissue has an even greater uptake of 131I than the normal thyroid tissue thus being destroyed by the radiation. This is a very effective low risk treatment with an up to 85% cure rate without recurrences.