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Feline hyperthyroidism
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Feline hyperthyroidism
Feline hyperthyroidism is an endocrine disorder in which the thyroid gland produces too much thyroid hormone. Hyperthyroidism is the most common endocrinopathy of cats. The complete pathogenesis is not fully understood.
In 1979 the first clinical report of a cat with hyperthyroidism was reported. More studies and greater awareness would follow and today hyperthyroidism is a common condition in small animal practice. Whether that is due to increased prevalence or better testing is not entirely agreed upon.
A study in 1987 transplanted thyroid tissue from affected cats into nude mice. The mice were administered levothyroxine, which suppresses thyroid-stimulating hormone. The thyroid cells remained in the hyperthyroid state. This study helped provide evidence for thyroid dysfunction as the cause rather than thyroid stimulation.
The majority of cases of hyperthyroidism in cats are the result of benign tumours. The most commonly identified abnormalities of the thyroid gland in hyperthyroid cases are follicular cell adenoma and multinodular adenomatous hyperplasia. 1% to 3% of feline hyperthyroid cases are caused by malignant tumours. These are not always able to be distinguished from benign tumours.
It is believed that the cause of hyperthyroidism is multifactorial. Studies have identified different mutations as causing thyroid cell autonomy.
Excessive iodine administration has been hypothesised as causing thyrotoxicosis in cats, as it does in humans; however, cats have been shown in multiple long-term studies to be able to regulate their levels of thyroid hormone within safe ranges when administered iodine. Iodide intake and its effects are unknown.
Iodine amounts in cat food vary by as much as a factor of 30. Low, high, and variable iodine diets have been hypothesised as a cause of feline hyperthyroidism.
Soy is often used in commercial cat food diets as a vegetable protein. In one study more than half of commercial cat foods surveyed contained soy isoflavones. Genistein and daidzein, both of which occur in soybeans, inhibit the enzymes thyroid peroxidase and thyroxine 5-deiodinase. This causes decreased thyroxine and triiodothyronine concentrations. In response to decreased triiodothyronine levels the body will produce more thyroid-stimulating hormone to normalise triiodothyronine levels, this has been shown to result in increased thyroxine levels. In addition these effects are heightened when a cat is suffering from iodine deficiency. However, further research is needed to confirm a link between soy isoflavones and hyperthyroidism.
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Feline hyperthyroidism
Feline hyperthyroidism is an endocrine disorder in which the thyroid gland produces too much thyroid hormone. Hyperthyroidism is the most common endocrinopathy of cats. The complete pathogenesis is not fully understood.
In 1979 the first clinical report of a cat with hyperthyroidism was reported. More studies and greater awareness would follow and today hyperthyroidism is a common condition in small animal practice. Whether that is due to increased prevalence or better testing is not entirely agreed upon.
A study in 1987 transplanted thyroid tissue from affected cats into nude mice. The mice were administered levothyroxine, which suppresses thyroid-stimulating hormone. The thyroid cells remained in the hyperthyroid state. This study helped provide evidence for thyroid dysfunction as the cause rather than thyroid stimulation.
The majority of cases of hyperthyroidism in cats are the result of benign tumours. The most commonly identified abnormalities of the thyroid gland in hyperthyroid cases are follicular cell adenoma and multinodular adenomatous hyperplasia. 1% to 3% of feline hyperthyroid cases are caused by malignant tumours. These are not always able to be distinguished from benign tumours.
It is believed that the cause of hyperthyroidism is multifactorial. Studies have identified different mutations as causing thyroid cell autonomy.
Excessive iodine administration has been hypothesised as causing thyrotoxicosis in cats, as it does in humans; however, cats have been shown in multiple long-term studies to be able to regulate their levels of thyroid hormone within safe ranges when administered iodine. Iodide intake and its effects are unknown.
Iodine amounts in cat food vary by as much as a factor of 30. Low, high, and variable iodine diets have been hypothesised as a cause of feline hyperthyroidism.
Soy is often used in commercial cat food diets as a vegetable protein. In one study more than half of commercial cat foods surveyed contained soy isoflavones. Genistein and daidzein, both of which occur in soybeans, inhibit the enzymes thyroid peroxidase and thyroxine 5-deiodinase. This causes decreased thyroxine and triiodothyronine concentrations. In response to decreased triiodothyronine levels the body will produce more thyroid-stimulating hormone to normalise triiodothyronine levels, this has been shown to result in increased thyroxine levels. In addition these effects are heightened when a cat is suffering from iodine deficiency. However, further research is needed to confirm a link between soy isoflavones and hyperthyroidism.