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[edit]

In 1979 the first clinical report of a cat with hyperthyroidism was reported.[1] 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.[2]

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.[2]

Causes[edit]

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.[2]

It is believed that the cause of hyperthyroidism is multifactorial. Studies have identified different mutations as causing thyroid cell autonomy.[2]

Iodine[edit]

Excessive iodine administration has been hypothesised as causing thyrotoxicosis in cats, as it does in humans; however, cats have been shown in multiple long terms studies to be able to regulate their levels of thyroid hormone within safe ranges when administered iodine. Iodide intake and it's effects are unknown.[2]

Soy isoflavones[edit]

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.[2]

Goitrogens[edit]

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 (PBDEs) 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.[2]

Signs and symptoms[edit]

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.[2]

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%).[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]

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.[2]

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.[2]

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.[2]

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.[2]

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.[2]

Risk factors[edit]

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.[3] These mixed results suggest a multifactoral cause with diet being having an important role.[2]

Age of onset[edit]

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.[2]

Diagnosis[edit]

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.[2]

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.[2]

Complete blood count[edit]

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. 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.[2]

Serum chemistry[edit]

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.[2]

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.[2]

Blood urea and creatinine[edit]

In roughly 10% of hyperthyroid cats serum creatinine levels are increased to the point of azotemia. In 10-20% of cats increased serum concentration of blood urea nitrogen is observed. Although the prevalence of hyperthyroid cats with chronic kidney disease is higher than this due to how hyperthyroidism results in an increase to the glomerular filtration rate. Most cats with chronic kidney disease and hyperthyroidism have the kidney disease go undiagnosed until after treatment for hyperthyroidism has begun. Between 15% and 49% of non-azotemic hyperthyroid cats will develop azotemia following treatment for hyperthyroidism. Further complicating the diagnosis is the shared symptoms between the conditions. It is not currently known whether the effects of hyperthyroidism cause or worsen renal disease. Chronic kidney disease is not an uncommon condition in elderly cats further complicating determination of a relationship between the two conditions.[2]

Urinalysis[edit]

Common abnormalities found during urinalysis of hyperthyroid cats include: decreased urine specific gravity, proteinuria, symptoms of urinary tract infections, and ketonuria.[2]

Decreases to urine specific gravity are likely not the result of hyperthyroidism but instead due to chronic kidney disease.[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.[4]

Proteinuria is reported in 75–80% of cats with hyperthyroidism. Likely causes of this are glomerular hyperfiltration and glomerular hypertension, and structural changes in the glomerular filtration barrier.[2]

The initial step should be to determine the serum concentration of thyroxine (T4). However, in veterinary medicine, the total thyroxine concentration is typically determined, rather than that of free (non-protein-bound) thyroxine (fT4). Nevertheless, the latter is more sensitive.[5] The normal range for T4 in cats is between 1.1 and 4.5 μg/dL, for fT4 between 1.0 and 2.8 ng/dL when determined by equilibrium dialysis.[6] In approximately 20% of animals, the T4 level is normal despite the presence of disease, which may be due to fluctuations in the hormone level during the course of the day or a reduction in the T4 level as a result of other secondary diseases. The measurement of fT4 has a sensitivity of 95%, but 20 to 30% of thyroid-healthy cats also have elevated fT4 levels. Therefore, if fT4 levels are elevated, total T4 must also be measured, which should be in the upper reference range in animals with hyperthyroidism.[7] Furthermore, various medications, including glucocorticoids, non-steroidal anti-inflammatory drugs (NSAIDs), phenobarbital, and trimethoprim-sulfonamide combinations, can influence T4 levels.[8] In the event of a clinical suspicion, it is advisable to repeat the determination at a later date.

Another method is the thyroid suppression test. In this procedure, a synthetic triiodothyronine (T3, typically Liothyronine) is administered to the cat over two days. A cat with a healthy thyroid gland will respond to this treatment by reducing the secretion of TSH (negative feedback), which will result in a decline in the T4 concentration. However, as the hyperthyroidism has already led to a permanently low TSH level, the administration of T3 in sick cats does not result in a reduction in TSH and T4.[9]

Another diagnostic procedure is the TRH stimulation test. In this test, the cat is administered thyrotropin-releasing hormone (TRH), which leads to a significant increase in the T4 concentration in healthy cats. In diseased animals, on the other hand, there is no or at most a slight increase.[10] However, this test sometimes has considerable side effects in cats (salivation, vomiting, palpitations, defecation), which is why it is rarely used. The TSH test, which determines the serum content of thyrotropin, a hormone that regulates the thyroid gland, is now also available for cats.[6] As in humans, early forms of hyperthyroidism can be detected based on low or unmeasurable TSH levels. However, the TSH stimulation test, which functions in a similar manner to the TRH function test, is no longer conducted as TSH is no longer available on the market.

Imaging procedures[edit]

Thyroid sonography, a diagnostic technique that has been utilized in human medicine for decades, has only recently been employed in veterinary medicine, primarily for research purposes. The primary reasons for this are the high equipment requirements and the associated high equipment costs. High-resolution linear transducers with at least 7.5 MHz, preferably 10 to 13 MHz, with a small contact surface are utilized.[11] Sonography can be employed to diagnose thyroid enlargement in all hyperthyroid cats, whereas the diagnostic reliability of palpation is only 84%, even among experienced veterinarians.[12] Thyroid scintigraphy is a valuable diagnostic procedure, yet it is only available in a limited number of veterinary clinics. In this procedure, a cat is administered a radionuclide (e.g., the iodine isotope 131I or the technetium isotope 99mTc), and its accumulation in the adenomas is then visualized. The principal advantage of this method is that the precise location of the tumors in the thyroid gland can be determined, which is advantageous with regard to surgical therapy. On rare occasions, additional thyroid tissue may colonize outside the thyroid gland (ectopia, particularly in the mediastinum) and become diseased as a result of disorders during organogenesis. Such displaced thyroid tissue can only be detected by scintigraphy.[13]

Clinical examination[edit]

Multiple clinical signs that support a diagnosis can be observed in hyperthyroid cats during a clinical exam. The most useful symptoms for this is weight loss with concurrent polyphagia and tachycardia.[2]

A goitre is swelling from an enlarged thyroid gland. This does not occur in all cases of hyperthyroidism, even those caused by adenoma; however it occurs in most cases and is a common way to support a hyperthyroid diagnosis caused by adenoma. Other conditions can cause a cervical mass that may resemble a goitre.[2]

Differential diagnosis[edit]

Polyphagia and weight loss are two concurrent symptoms that has multiple causes, including: diabetes mellitus, malnutrition, malabsorption, and maldigestion. If these symptoms occur serum thyroxine levels should be tested to confirm or exclude a diagnosis of hyperthyroidism.[2]

Therapy[edit]

Currently, three therapy options are available for hyperthyroidism in cats: the use of thyrostatic drugs, surgical removal of the diseased thyroid tissue, and radioiodine therapy. Regardless of the procedure selected, subsequent treatment of concomitant and secondary diseases (e.g., kidney damage, high blood pressure, heart disease) is typically necessary.[14] In order to ascertain the potential adverse effects of reduced thyroid hormone levels on renal function, a 30-day course of medication is recommended prior to the implementation of more radical measures such as thyroidectomy or radioiodine therapy.[15]

Thyreostatics[edit]

Therapy with thyrostatic agents is relatively straightforward and is therefore the most commonly used. Thyrostatic drugs inhibit the formation of thyroid hormones, but, in contrast to other methods, do not eliminate the pathologically altered tissue. Nevertheless, these drugs can usually be used in long-term therapy without any problems or can also be used to stabilize patients before a surgical procedure. In veterinary medicine, thiamazole (syn. methimazole, trade names Felimazole, Felidale and Thiamatab) or carbimazole (trade name Vidalta) are employed. Carbimazole is rapidly converted into methimazole when administered orally.[16] According to the manufacturer, side effects (including vomiting, lethargy, itching, liver disease, and blood count changes) occur in approximately 20% of cats, particularly with long-term treatment. However, these typically resolve once the drug is discontinued. Additionally, thiamazole cannot be used in cats with concomitant liver disease, diabetes, or blood clotting disorders.

Iopanoic acid may also be employed in the event of intolerance to thiamazole. It inhibits the conversion of T4 to T3 and has a negligible incidence of side effects.[14]

Thyroidectomy[edit]

Although surgical removal (thyroidectomy) is an effective treatment, it is also associated with a high risk of complications, particularly in cats with severe hyperthyroidism, due to the inherent risks associated with anesthesia. Prior to the surgical procedure, it is common practice to administer thyrostatic drugs. There are several techniques for the removal of the thyroid gland, with the objective of preserving the epithelial cells to the greatest extent possible. Additionally, there is a potential risk of injury to crucial cervical nerves (recurrent laryngeal nerve, vagosympathetic trunk) during surgery. A total thyroidectomy results in a deficiency of thyroid hormones, which must be compensated for by lifelong administration.[17] In the event of unilateral removal, a transient hypothyroidism frequently develops postoperatively, although this is typically not a cause for concern.[9] Furthermore, there is a risk of recurrence with surgical removal, particularly in the presence of ectopic thyroid tissue.[18]

Radioiodine therapy[edit]

Radioiodine therapy is the treatment of choice due to its efficacy and tolerability. A single treatment is typically sufficient, eliminating the need for long-term drug treatment (which can be problematic in some cats) and the risks associated with surgical removal.[19] However, it is associated with significant radiation protection requirements and is currently only available at two veterinary facilities in Germany. In addition to the limited availability, the associated costs and the need for hospitalization represent a disadvantage. In close consultation with the responsible supervisory authorities, it has been possible to reduce the required duration of hospitalization from approximately three weeks to a few days.[20] The necessary duration of hospitalization is determined by dosimetry and is seven to ten days.[21]

Thermal or chemical destruction of the thyroid gland[edit]

Destruction of the thyroid tissue using a radiosurgical device under ultrasound control (thermal ablation) or by injection of 96% ethanol (chemical ablation) is practically no longer relevant. Both forms of treatment have increased side effects such as laryngeal paralysis or Horner's syndrome.[21]

References[edit]

  1. ^ M. E. Peterson et al.: Spontaneous hyperthyroidism in the cat. In: Proc. Am. College Vet. Intern. Med. 1979, p. 108.
  2. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac Scott-Moncrieff, J. Catherine (2015). "Feline Hyperthyroidism". In Feldman, Edward C.; Nelson, Richard W.; Reusch, Claudia; Scott-Moncrieff, J. Catharine (eds.). Canine and feline endocrinology (Fourth ed.). St. Louis, Missouri: Elsevier Saunders. pp. 137–190. ISBN 978-1-4557-4456-5.
  3. ^ Edinboro, Charlotte H; Scott-Moncrieff, J Catharine; Glickman, Larry T (2010). "Feline Hyperthyroidism: Potential Relationship with Iodine Supplement Requirements of Commercial Cat Foods". Journal of Feline Medicine and Surgery. 12 (9): 672–679. doi:10.1016/j.jfms.2010.07.011. ISSN 1098-612X.
  4. ^ van Hoek, I.; Lefebvre, H.P.; Peremans, K.; Meyer, E.; Croubels, S.; Vandermeulen, E.; Kooistra, H.; Saunders, J.H.; Binst, D.; Daminet, S. (2009). "Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine". Domestic Animal Endocrinology. 36 (1). Elsevier BV: 45–56. doi:10.1016/j.domaniend.2008.10.001. ISSN 0739-7240.
  5. ^ M. E. Peterson: Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. In: J. Am. Vet. Med. Assoc. 218(4), 2001, p. 529–536. PMID 11229503.
  6. ^ a b R. Hämmerling: Die feline Hyperthyreose. In: Der praktische Tierarzt. 86(5), 2005, p. 320–324. ISSN 0032-681X
  7. ^ Stephanie Nather et al.: Hyperthyreose der Katze. In: Kompendium Kleintier 2017, p. 52–57.
  8. ^ Eva Höfel, Thomas Rieker: Hyperthyreose – Aktuelles zur Pathogenese und Diagnose. In: Fachpraxis. No. 63, 2013, p. 6–11.
  9. ^ a b M. E. Peterson: Hyperthreodism. In: Stephen J. Ettinger, Edward C. Feldman: Textbook of veterinary internal medicine. 5th edition. Volume 2, Saunders, 2000, ISBN 0-7216-7256-6, pp. 1400-1419.
  10. ^ M. E. Peterson et al.: Use of the thyrotropin releasing hormone stimulation test to diagnose mild hyperthyreodism in cats. In: J. Vet. Intern. Med. No. 4, 1999, p. 279–286. ISSN 0891-6640
  11. ^ C. Poulsen Nautrup et al.: Schilddrüse und Nebenschilddrüsen. In: C. Poulsen Nautrup, R. Tobias (Ed.): Atlas und Lehrbuch der Ultraschalldiagnostik bei Hund und Katze. 2nd edition. Published by Schlütersche publishing company, Hannover 1998, ISBN 3-87706-663-1, p. 113–116.
  12. ^ W. Kraft et al.: Symptome bei Hyperthyreose der Katze: eine retrospektive Studie. In: Kleintierpraxis. 44(10), 1999, p. 719–732. ISSN 0023-2076
  13. ^ M. E. Peterson, D. V. Becker: Radionuclide thyroid imaging in 135 cats with hyperthyroidism. In: Vet. Radiol. 25(1), 1984, p. 23–27. ISSN 0196-3627
  14. ^ a b Beate Egner, Anthony P. Carr: Hyperthyreose bei der Katze – Welche Rolle spielen ACE-Hemmer? In: kleintier konkret. 11, 2008, p. 11–14.
  15. ^ Thomas Graves: Aktuelle Aspekte der Hyperthyreose bei der Katze. In: Vet. Focus. 19.3, 2009, p. 2–5.
  16. ^ M. E. Peterson, D. P. Aucoin: Comparison of disposition of carbimazole and methimazole in clinically normal cats. In: Res. Vet. Sci. 54(3), 1993, p. 351–355. PMID 8337482.
  17. ^ C. D. Welches et al.: Occurrence of Problems after Three Techniques of Bilateral Thyroidectomy in Cats. In: Vet. Surgery. 18(5), 1989, p. 392–396. PMID 2815557.
  18. ^ E. C. Naan et al.: Results of thyroidectomy in 101 cats with hyperthyroidism. In: Vet. Surg. 35(3), 2006, p. 287–293. PMID 16635010
  19. ^ M. E. Peterson: Radioiodine treatment of hyperthyroidism. In: Clin. Tech. Small Anim. Pract. 21(1), 2006, p. 34–39. PMID 16584029.
  20. ^ M. Puille et al.: Radiojodtherapie bei Katzen: Strahlenschutz der Kontaktpersonen. In: Tierärztl Prax. 33 (K), 2005, p. 291–295. ISSN 1434-1239
  21. ^ a b Andrea Monika Mathes, Reto Neiger: Hyperthyreose der Katze. In: Kleintierpraxis. 55, 2010, p. 685–698.

External links[edit]

Retrieved from "https://en.wikipedia.org/w/index.php?title=Feline_hyperthyroidism&oldid=1233657694"