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Treating and Monitoring Feline Hyperthyroidism
Feline hyperthyroidism is the clinical syndrome that exists when there is over-production of circulating thyroid hormones. It is the most common endocrine disorder in cats which are the only non-human species in which spontaneous thyrotoxicosis develops.
In the UK, it is estimated that one per cent of cats develop the condition(1) and all breeds are predisposed with middle-aged to older cats (>10 years old) more susceptible(2). Although the exact cause of the condition is still unknown, heredity, environmental concerns and diet have all been cited as possible aetiologies.
Feline hyperthyroidism occurs as a result of excessive production of the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) by the thyroid gland which is situated in the neck (both lobes are affected in 70 per cent of cases)(3). Both hormones are essential to the proper growth of body cells and the development of these cells for specific roles in the body. They also help control the metabolism of protein, fat and carbohydrate and are involved in the regulation of heat production and oxygen consumption.
In 98 per cent of cases, the enlargement of the lobes is benign. Some 70 per cent of cats will have bilateral disease (both thyroid glands are affected) and the remainder will have only one hyperfunctional gland. A small proportion of hyperthyroid cats have a malignant tumour (approximately two per cent), with mixed, compact and follicular adenocarcinomas being the most common.
Clinical signs associated with hyperthyroidism can be quite dramatic and cats can become seriously ill with the condition. Male and female cats are affected with equal frequency and although no studies have shown any breeds to be more prone to developing the condition than others, there is some evidence to suggest that it is less common in Siamese cats(4). Most cases of hyperthyroidism are treatable using medication, surgery or radioactive iodine therapy. Clinical signs include weight loss despite polyphagia, polyuria, polydipsia, palpable goitre, tachycardia, anxious facial expression and increased activity.
Secondary complications relate directly to the effect of thyroid hormones on the heart, liver, kidneys and muscles. The increased levels of circulating thyroid hormone may eventually result in deleterious effects on many organ systems. Left-sided hypertrophic cardiomyopathy is commonly associated with hyperthyroidism as the muscles struggle to cope with the heart rates(5). Most cats will have increased heart rates (>250 bpm) and a significant proportion will have a gallop rhythm as a result of turbulent blood flow in the heart.
Other potential complications include hypertension and chronic renal failure. Whilst the latter does not occur as a direct result of hyperthyroidism, the two diseases often occur together. Hypertension can cause additional damage to several organs including the eyes, kidneys, heart and brain. If it is diagnosed alongside hyperthyroidism, drugs will be needed to control the blood pressure to reduce the risk of damaging other organs. Where chronic renal failure is suspected, care is needed, as hyperthyroidism tends to increase the blood supply to the kidneys, which may improve their function. Thus blood tests taken to assess kidney function in a hyperthyroid cat may show normal or only mild changes, but potentially more severe renal failure may be ‘masked’ by the presence of the hyperthyroidism and then be ‘unmasked’ by the treatment of hyperthyroidism; be it by medical, surgical or radioactive iodine modalities.
On examination, one or two enlarged thyroid glands can be palpated in 80-95 per cent of cats(6) as a small, firm mass in the neck. However, in some cats there is no palpable thyroid enlargement, and this can be because the gland has migrated toward the thoracic inlet(6) or because overactive tissue is present in an ectopic site.
The diagnosis is determined by the level of thyroid hormones in the blood. Routine haematology and biochemistry are essential to help confirm a diagnosis as well as rule out the presence of any concurrent disorders.
Most cases can be confirmed by the measurement of serum total T4 (TT4). Measurement of TT4 is also useful for establishing a baseline value prior to treatment. Early cases or those with concurrent disease may have a TT4 concentration within the reference range – often at the high end of that range. Free T4 (FT4) measurement by equilibrium dialysis can be used in these situations to help confirm the diagnosis of hyperthyroidism. FT4 is the most sensitive test for hyperthyroidism, however it should not be used in place of the TT4 test since up to 12 per cent of normal cats have an elevated serum FT4 concentration. Total T3 (TT3) is not routinely used as a diagnostic test as over 30 per cent of hyperthyroid cats have a serum level of TT3 within the reference range.
Other laboratory tests may also be abnormal – for example liver enzymes are commonly increased secondary to hyperthyroidism, and assessment of routine blood and urine tests is usually advised to help rule out any other concurrent disease (such as renal failure). Where possible, blood pressure should also be checked and if secondary heart disease is suspected then an ECG, chest X-ray and ultrasound may be helpful.
Diagnosis of feline hyperthyroidism ~
There are three main treatment therapies for cats diagnosed with hyperthyroidism.
1) Medical therapy
Anti-thyroid drugs are available in tablet form and can help reduce the production of hormone by the thyroid gland. They do not provide a cure for the condition, but they do allow either short or long-term control of hyperthyroidism which is non-invasive and reversible. Thiamazole (Felimazole from Dechra Veterinary Products) is the most commonly prescribed product on the market to treat such a condition. The starting dose is 2.5 mg twice daily for three weeks. After three weeks, the response to therapy should be assessed and adjusted accordingly. To maintain control of hyperthyroidism, treatment needs to be given for the rest of the cat’s life.
2) Surgical thyroidectomy
Surgical removal of the affected thyroid tissue (thyroidectomy) can produce a permanent cure and is a common treatment for many cats. However, this is only considered appropriate following an initial course of thiamazole which is necessary to stabilise the cat’s condition. This is important for two reasons. First, to induce euthyroidism and help to reduce the surgical and anaesthetic risk and second, to unmask any underlying conditions that may only be revealed after the establishment of euthyroidism (e.g. chronic renal failure).
Because with around 70 per cent of cases both thyroid lobes are affected, it will necessitate a bilateral thyroidectomy. However, as many as 15 per cent of cats with bilateral disease have one thyroid lobe that appears grossly normal and if left in-situ results in recurrent hyperthyroidism usually within 12 months(6).
The major risk associated with surgery is inadvertent damage to the parathyroid glands – these are small glands that lie close to, or within, the thyroid glands and have a crucial role in maintaining stable blood calcium levels. Damage to these glands can result in a life-threatening fall in blood calcium concentrations (hypocalcaemia). This is most likely to occur when both thyroid glands are removed simultaneously, since this can result in damage to both parathyroid glands. To minimise the risk of this complication, meticulous surgical technique is required and in those cats that require removal of both thyroid glands, it may be appropriate to perform the procedure in two stages – removing the most affected gland first and allowing six to eight weeks for recovery of parathyroid hormone production before removing the second thyroid gland.
3) Radioactive iodine therapy
Radioactive iodine (131I) can also be used as a safe and effective cure for hyperthyroidism. Like surgical thyroidectomy, it has the advantage of being curative in most cases with no ongoing treatment required.
Radioactive iodine is administered through injection – the iodine is then taken up by the active (abnormal) thyroid tissue, but not by any other body tissues, resulting in a selective local accumulation of radioactive material in the abnormal tissues. The radiation destroys the affected abnormal thyroid tissue, but does not damage the surrounding tissues or the parathyroid glands.
Because the injection itself is of a radioactive substance, it needs to be handled with great care, particularly for the people who come into close contact with the cat. Treatment can only be carried out at licensed facilities and the animal will need to remain hospitalised until the radiation levels fall to an acceptable limit – this usually means cats must be hospitalised for around two to three weeks, however, this may be longer depending on the home environment (animals will be kept longer if children are present, etc).
The licensed facilities currently available in the UK are at the Animal Health Trust near Newmarket, the university veterinary schools at Bristol and Glasgow, the Barton Veterinary Hospital in Canterbury and Bishopton Veterinary Group in North Yorkshire.
For those cats receiving medical therapy, a full clinical examination should be carried out and TT4, haematology and biochemistry measured three weeks after starting treatment and then at 6, 10 and 20 weeks and every three months thereafter.
TT4 monitoring does not have to be carried out at any specific time post-pill, but it is important that the morning tablet is administered on the day of the blood test. Good owner compliance with the dosing regime is essential, as TT4 will increase within 24-72 hours if tablets are missed.
All treatment regimes for hyperthyroidism can potentially ‘unmask’ renal disease. Each case should be assessed individually using the following guidelines:
• Renal function should be assessed prior to treatment
• If a mild but stable azotaemia develops, continue treatment
• If a worsening azotaemia develops, it may be beneficial to decrease the dose rate to maintain mild hyperthyroidism(1).
If severe renal disease exists pre-treatment, the treatment plan should be subject to a risk benefit assessment.
For further information about feline hyperthyroidism or treatment protocols, please contact your local Dechra territory manager or phone 01743 441 632.
This article was provided by Dechra Veterinary Products, makers of Felimazole and first published in September 2010:
1. Data on file at Dechra Veterinary Products – survey of small animal practices
2. Wakeling, J. et al. Subclinical hyperthyroidism in cats. Endocrine Abstracts 13 P315, 2007
3. Mooney, C. T. Pathogenesis of Feline Hyperthyroidism. J. Feline Med. Surg. 4: 167–169, 2002
4. Feline Advisory Bureau. Hyperthyroidism in cats. January 2006.
5. Nelson, RW and Couto, CG. (eds). Small Animal Internal Medicine (2nd Edition). Mosby, St Louis.
6. Ettinger & Feldman, Eds., 5th Ed. Textbook of Veterinary Internal Medicine, pp. 1400- 1415.