Affects: Cats
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.
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 (91%); polyphagia (81%), without obesity; restlessness (76%); tachycardia (66%); polydipsia and polyuria (60%); emesis; and cardiac murmur (53%).
Less common symptoms include: diarrhoea (33%), increased volume of faeces (31%), anorexia (26%), polypnoea (25%), myasthenia (25%), muscle tremors (18%), congestive heart failure (12%), excessive nail growth (12%), and dyspnoea (11%).
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.
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.
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.
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.
Treatment: Hyperthyroidism can be either managed with anti-thyroid drugs or restricting iodine intake, or it can be treated with radioactive iodine or thyroidectomy for a permanent solution. Choice of treatment depends on a multitude of factors including cost, availability of the treatments, and age and health of the patient. In areas where available, radioactive iodine treatment is the preferred method of treating hyperthyroidism due to the low risks and high efficacy.
Prognosis: Between 93 and 95% of cats treated with radioactive iodine become euthyroid without further need for treatment. Reasons for failing to achieve euthyroidism can be due to adenoma, adenomatous hyperplasia, carcinoma, or an error in the administration of the treatment.
Rarely some cats will redevelop hyperthyroidism up to 6 years after treatment, this is likely due to new foci caused by new mutations in the tissue than any failure of treatment.