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.
Background
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.
Causes
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.
Iodine
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 isoflavones
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.Goitrogens
are chemicals that disrupt thyroid hormone production. Chemicals such as bisphenol A and polybrominated diphenyl ethers have been shown to exist in indoor cats at heightened levels and are hypothesised to be a potential cause of hyperthyroidism. Bisphenol A is common in canned cat foods, although no association between it and hyperthyroidism has currently been demonstrated. Polybrominated diphenyl ethers are often used as fire retardants and are known to be a goitrogen; however, levels of these chemicals have been shown to be the same in euthyroid cats as hyperthyroid cats. No association has been proven in a study for the association between PBDEs and hyperthyroidism.Signs and symptoms
Hyperthyroidism is a disease that slowly progresses and symptoms are very subtle at first. It can take up to two years from onset of symptoms for a diagnosis of hyperthyroidism to be made.The most common symptom is weight loss, occurring in 98% of hyperthyroid cases. Other symptoms that occur more than 50% of the time, include: a palpable thyroid gland ; polyphagia, without obesity; restlessness ; tachycardia ; polydipsia and polyuria ; emesis; and cardiac murmur.
Less common symptoms include: diarrhoea, increased volume of faeces, anorexia, polypnoea, myasthenia, muscle tremors, congestive heart failure, excessive nail growth, and dyspnoea.
Systolic hypertension is reported in 10-15% of cats that have been diagnosed with hyperthyroidism. Some normotensive hyperthyroid cats develop hypertension after having their hyperthyroidism treated. The reasons for this are not currently known.
Cachexia is a possible symptom in hyperthyroid cats but due to improved screening and diagnostics it is less common for a cat to become cachectic before starting treatment.
Ventroflexion of the head is a rare symptom observed in hyperthyroid cats but the last case report is from 1994. Due to the rarity the pathogenesis behind this is not known.
Cardiac conditions have become less common over the years in hyperthyroid cats due to earlier diagnosis and improved treatment. The most common cardiovascular issues are tachyarrhythmias, cardiac murmurs, and gallop rhythms. The cause of these conditions is due to how triiodothyronine increases cardiac output.
Thyroid cysts can occur in cases of adenoma, adenomatous hyperplasia, or carcinoma. These cysts have a high concentration of thyroxine. Cysts should not be treated with radioactive iodine and instead be surgically removed.
Symptoms of hyperadrenocorticism can occur in hyperthyroid cats. Adrenocorticotropic hormone levels are raised in some hyperthyroid cats and some studies have documented acromegaly in hyperthyroid cats.
Risk factors
Multiple case control studies have looked at diet and hyperthyroidism. These studies have found associations between commercial diets and hyperthyroidism. Other risk factors identified include non-Siamese related breeds, lack of outdoor access, flea medication, pesticides, certain cat litters, female sex, sleeping on the floor, organic fertiliser, human baby food, carpet cleaners, natural gas, lack of deworming, and a fish diet. These mixed results suggest a multifactoral cause with diet being having an important role.Age of onset
The age of onset has been reported to be between 4–22 years with an average of 13 years, with some individual cases occurring in cats below the age of 4 years. More than 95% of cases occur in cats 8 years and older.Diagnosis
Testing for hyperthyroidism is routine for elderly cats and standard blood tests that can be performed in clinic allow for measurement of serum thyroxine levels. With this advancement diagnosis is often made before clinical signs are well noticeable and severe.Diagnosing hyperthyroid cats is not just to confirm the presence of the condition but also what symptoms are present. For example, more serious cardiac conditions require special treatment compared to a cat without any serious cardiac abnormalities. Cats with hyperthyroidism should have a complete blood count test, a biochemistry profile of blood serum, urinalysis, measurement of thyroxine serum levels, thoracic radiography, and measurement of blood pressure. If heart disease is suspected electrocardiography and echocardiography should be performed.
Measurement serum thyroxine levels is the best method of confirming hyperthyroidism. Measurement of serum concentrations triiodothyronine are not useful for diagnosing hyperthyroidism and neither is measurement of serum thyroid-stimulating hormone levels. Thyroxine levels on their own should not be used to diagnose hyperthyroidism: if a patient is displaying no symptoms of hyperthyroidism but test results show heightened serum thyroxine levels the possibility of other causes such as an error in the test have to be considered and excluded before establishing a diagnosis.
Complete blood count
Common findings of a complete blood count on a hyperthyroid cat include a small increase in packed cell volume, macrocytosis, and Heinz bodies. Anaemia is rare. Increased erythrocyte count is due to thyroid hormone stimulating erythropoietin secretion. Leukocytes and thrombocytes usually have normal results, some changes may occur but these are not specific and are not consistent.Serum chemistry
The most commonly noticed change in serum chemistry is a non-major increase in liver enzymes. More than 90% of hyperthyroid cats have an increase in either serum concentration of alanine aminotransferase and alkaline phosphatase, with more than 75% of cats having an increase in both. These increases are small and typically below 500 IU/L. Higher increases may be due to hepatic disease, although many other conditions can cause an increase in liver enzymes.Fructosamine concentrations are lowered by serum protein metabolism, which is increased in hyperthyroid cats. Fructosamine concentrations are significantly lower in hyperthyroid cats, with half of hyperthyroid cats having levels outside the reference range. Following treatment for hyperthyroidism these concentrations rise to normal levels. Diabetes mellitus also affects fructosamine concentration and this should not be relied on in diabetic cats. Hyperglycaemia can occur in hyperthyroid cats independent of diabetes mellitus. Frucotsamine measurement cannot reliably differentiate non-diabetes related hyperglycaemia.