Feline hyperthyroidism
[1] More studies and greater awareness would follow and today hyperthyroidism is a common condition in small animal practice.
The most commonly identified abnormalities of the thyroid gland in hyperthyroid cases are follicular cell adenoma and multinodular adenomatous hyperplasia.
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.
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%).
[2] 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%).
[2] Cardiac conditions have become less common over the years in hyperthyroid cats due to earlier diagnosis and improved treatment.
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.
[2] Common findings of a complete blood count on a hyperthyroid cat include a small increase in packed cell volume (40-50%), macrocytosis (20%), and Heinz bodies.
[2] Common abnormalities found during urinalysis of hyperthyroid cats include: decreased urine specific gravity, proteinuria, symptoms of urinary tract infections, and ketonuria.
[2] In a 2009 study most cats that had been treated for hyperthyroidism with radioactive iodine did not have changes to urine specific gravity levels.
[2] The most common echocardiographic abnormalities in hyperthyroid cats is hypertrophy of the left ventricular caudal wall and interventricular septum.
Scintigraphy can also be used to confirm hyperthyroidism when clinical symptoms are present but serum thyroxine levels remain within reference.
[2] Polyphagia and weight loss are two concurrent symptoms that have multiple causes, including: diabetes mellitus, malnutrition, malabsorption, and maldigestion.
In areas where available, radioactive iodine treatment is the preferred method of treating hyperthyroidism due to the low risks and high efficacy.
[2] Surgery is a cheaper option than radioactive iodine treatment and does not require any equipment not found in a standard veterinary clinic.
A bad surgery can result in nerve damage or fail to fully remove all the abnormal tissue.
These risks can be prevented with a more experienced surgeon and proper imaging of the thyroid gland before removal.
Ergocalciferol is a cheaper alternative that is sometimes used but is not recommended due to risk of hypercalcaemia[2] Following unilateral thyroidectomy thyroxine levels may fall into the hypothyroid reference range.
Supplementation retards the growth of the thyroid tissue and will prevent the patient from reaching a euthyroid state.
Anti-thyroid drugs do not address the underlying thyroid issues and has several side effects such as anorexia, emesis, lethargy, thrombocytopaenia, granulocytopaenia, and hepatopathy.
[2] Of these drugs propylthiouracil is not recommended due to the common side effects including: anorexia, emesis, lethargy, immune-mediated haemolytic anaemia, and thrombocytopaenia.
[2] Thioureylenes should not be used in patients with thyroid carcinoma these drugs increase the release of thyroid-stimulating hormone which aggravates growth of the tumour.
Methimazole can also be used to stabilise patients with severe hyperthyroidism prior to surgery or radioactive iodine treatment.
Other mild side effects include haematological condition: eosinophilia, leukopaenia, and lymphocytosis occur at rates of 15% for oral and 5% for topical.
No studies compare the two but evidence suggests lower rates of adverse effects with no reports of severe blood disorders.
There is no evidence to support for this theory in felines; however, it is still recommended that anti-thyroid drugs should be discontinued for a week or two before starting treatment.
Iodine limited diets present an alternative for owners with financial issues and in cats with concurrent illness that would make other options more dangerous.
When hyperthyroidism is treated these decrease and an increase of serum creatinine often occurs, which may cause azotaemia and in some cases renal failure.
Thus the best practice is to trial methimazole then measure serum biochemistry and analyse urine before starting permanent hyperthyroid treatment (radioactive iodine and thyroidectomy).
Reasons for failing to achieve euthyroidism can be due to adenoma, adenomatous hyperplasia, carcinoma, or an error in the administration of the treatment.
