Feline hyperthyroidism

from Wikipedia, the free encyclopedia

The feline hyperthyroidism is a disorder of the endocrine system in domestic cat ( feline adjective of lat. Felis "cat") defined by an overactive thyroid ( hyperthyroidism is characterized). It is the most common hormonal disorder ( endocrinopathy ) in cats over ten years old , whereas hyperthyroidism is much less common in other pets . The disease often manifests itself as weight loss despite increased food intake, is usually detected by blood tests and is easily treatable.

Occurrence

Feline hyperthyroidism was first described in 1979 and has been increasingly diagnosed in cats ever since . It is unclear whether this is really a disease that has only emerged and is on the rise since then, or whether the increasing surveillance of the cat population for this disease is leading to the detection of more cases. The disease is the most common endocrine disorder in cats older than ten years. All cat breeds are affected, there is no increased tendency to disease for certain breeds or a dependency on gender.

Pathogenesis

In contrast to hyperthyroidism in humans , the disease is almost exclusively due to benign enlargement of the thyroid gland. Cell proliferation similar to gland tumors ( adenomatous hyperplasia ) occurs most frequently, and autonomic adenomas are less common . In about 70% of the cases there are many small foci ( multifocal ), in the remaining cases a single foci ( unifocal ). Thyroid cancer can rarely cause hyperthyroidism, but it is very rare in cats (less than 5% of thyroid diseases). Immune-related hyperthyroidism, as in humans ( Hashimoto's thyroiditis , Graves' disease ) has not been described in cats so far.

As a result of the changes, there is an increased secretion of the thyroid hormone thyroxine , in three quarters of cases of triiodothyronine (another hormone the thyroid gland). The secretion of these hormones in sick cats is independent of the thyroid gland normally regulating thyrotropin (TSH).

What triggers these adenomas remains to be seen. Mutations in the genes for the thyrotropin receptor could be responsible for the unregulated growth of the cells. Diet and environmental influences, but also genetic factors, could have an influence . According to epidemiological studies, feeding commercial cat food is a risk factor for the development of the disease, which is attributed to the high content of thyroid-enlarging ( goiter-own ) substances such as soy isoflavones or phthalates . Cats fed canned food have a 2.5–5 fold increased risk of developing hyperthyroidism. In addition, environmental factors such as the use of certain cat litter may also be involved in the development of the disease. Polybrominated diphenyl ethers (PBDE), which are used as flame retardants in textiles (they are now banned in the EU and in some US states), could also be involved in the pathogenesis through their endocrine effects: the chronically increased TSH production could lead to a Hypertrophy of the thyroid follicle.

Clinical picture

The clinical picture is very variable and also depends on the extent of the hyperfunction. Ultimately, most of the symptoms observed are signs of forced metabolic processes in the sick animals, which are caused by the excess of thyroid hormones.

The most common sign is weight loss, which occurs in 88% of hyperthyroid cats. Further signs with a frequency of around 50% are a palpable enlargement of the thyroid gland (the healthy thyroid gland is not palpable in cats), palpitations and heart murmurs as well as increased food consumption up to voraciousness . The excess supply of thyroid hormones can produce the clinical picture of hypertrophic (more common) or dilated heart muscle disease (rarely). The clinical picture is also known as thyrotoxic cardiomyopathy. The hypertrophic form is often reversible after successful treatment of hyperthyroidism.

Other symptoms that occasionally occur with an overactive thyroid are increased stool volume, vomiting , increased thirst and increased urination , increased activity (significantly less often also decreased with rapid fatigue), behavioral changes ( anxiety or increased aggressiveness ), decreased food consumption , shortness of breath and Changes in the skin (rough fur, hair loss , increased growth of the claws ). High blood pressure is observed in 5–20% of hyperthyroid cats, but a clear cause-effect relationship has not yet been proven.

diagnosis

Based on the clinical picture, a number of other diseases in older cats such as diabetes , chronic kidney disease , heart disease , liver failure , digestive disorders and chronic intestinal inflammation, as well as lymphoma in the intestine , are possible. Diseases that are rare in cats, such as exocrine pancreatic or adrenal hypofunction, should also be considered. The diagnosis can therefore only be made with certainty by means of hormone tests or scintigraphy (see below).

Laboratory tests

As a result of the stress response to high thyroxine levels, the blood count often shows an increase in the number of white blood cells ( leukocytosis ) and a decrease in eosinophilic granulocytes ( eosinopenia ) and lymphocytes ( lymphopenia ). The number of red blood cells and the content of red blood pigment are in the upper normal range. In serum , a mild to moderate increase can mostly activity of various enzymes ( ALAT , ASAT , LDH , AP ) detected. The fructosamine level is lowered due to the increased protein metabolism and is usually below 200 µmol / l.

Due to the often combined with hyperthyroidism renal impairment may urea - and creatinine -contents be elevated in the blood. If hyperthyroidism and chronic kidney dysfunction are present at the same time, however, this can be masked to a certain extent, since thyroxine increases metabolism and cardiac output and thus improves blood flow to the kidneys. This increases the glomerular filtration rate and thus promotes the excretion of toxic metabolic products. Paradoxically, therefore, after treatment of hyperthyroidism, renal insufficiency can become clinically manifest. According to Egner and Carr , these laboratory changes together with a positive palpable finding are already proof of the disease.

Special thyroid function tests must be carried out for further diagnosis.

The first thing to do is to determine the serum concentration of thyroxine (T4), although currently only the total thyroxine concentration is usually determined in veterinary medicine and not that of free (not bound to proteins ) thyroxine (fT4), although the latter is more sensitive. The normal range for T4 in cats is between 1.1 and 4.5 µg / dl, for fT4 when determined by means of equilibrium dialysis between 1.0 and 2.8 ng / dl. In about 20% of the animals, the T4 content is normal despite the disease, which may be due to fluctuations in the hormone content over the course of the day or a reduction in the T4 content as a result of other secondary diseases. The measurement of fT4 has a sensitivity of 95%, but 20 to 30% of cats with healthy thyroid gland also have elevated fT4 values. Therefore, if the fT4 values ​​are increased, the total T4 must also be recorded, which should be in the upper reference range in animals with hyperthyroidism. In addition, various drugs such as glucocorticoids , NSAID , phenobarbital or trimethoprim - sulfonamide combinations can influence the T4 level. If there is clinical suspicion, the determination should be repeated at a later point in time.

The thyroid suppression test is another method . The cat is given a synthetic triiodothyronine (T3, usually liothyronine ) over two days. A cat with healthy thyroid gland reacts to this with a decrease in TSH release (so-called negative feedback ), which in turn leads to a decrease in T4 concentration. However, since the overactive thyroid has already led to a permanently reduced TSH level, the administration of T3 does not trigger a reduction in TSH and T4 in sick cats.

Another diagnostic procedure is the TRH stimulation test . Thyrotropin releasing hormone (TRH) is administered to the cat , which in healthy cats leads to a significant increase in the T4 concentration. In the case of sick animals, on the other hand, there is no or at most a slight increase. However, this test sometimes has considerable side effects in cats (salivation, vomiting, racing heart, excrement), which is why it is rarely used. The TSH test , i.e. the determination of the serum content of the thyroid-controlling thyroid gland, is now also available for cats. As in humans, early forms of hyperthyroidism can also be detected on the basis of low or non-measurable TSH levels. The TSH stimulation test , which works analogously to the TRH function test, is no longer performed because TSH is no longer available on the market.

Imaging procedures

The sonography of the thyroid gland, which has been common in human medicine for a long time, has only recently been used in veterinary medicine and is mainly used in research. The reasons are the high device requirements and the associated high equipment costs. High-resolution linear transducers with at least 7.5 MHz, better with 10 to 13 MHz, with a small contact surface are used. Enlargement of the thyroid gland can be detected in all hyperthyroid cats by means of sonography, while the diagnostic reliability of palpation is only 84%, even in experienced veterinarians.

Thyroid scintigraphy is a valuable diagnostic procedure, but it is only available in a few veterinary clinics. The cat is given a radionuclide (for example the iodine isotope 131 I or the technetium isotope 99m Tc) and its accumulation in the adenomas is then shown. The great advantage of this method is that the exact localization of the tumors in the thyroid gland can be determined, which is advantageous in terms of surgical therapy. Occasionally, as a result of disorders during organogenesis, additional thyroid tissue can settle outside the thyroid gland ( ectopia , especially in the area of ​​the middle layer ) and become ill. Such displaced thyroid tissue can only be detected with a scintigraphy.

Magnetic resonance imaging and computed tomography have so far not been used for thyroid diagnostics in cats. Corresponding devices are only available at large veterinary clinics.

therapy

There are currently three therapeutic options for treating hyperthyroidism in cats: the use of anti- thyroid drugs , the surgical removal of the diseased thyroid tissue and radioiodine therapy . Regardless of which of these procedures is used, subsequent treatment of the accompanying and secondary diseases (kidney damage, high blood pressure, heart disease) is usually necessary. In order to test the possible negative effect of the lowered thyroid hormone level on kidney performance, a 30-day drug treatment is recommended before radical measures such as thyroid removal or radioiodine therapy.

Anti-thyroid drugs

The therapy with antithyroid drugs is simple to perform and is therefore used most often. Thyrostatic drugs inhibit the formation of thyroid hormones, but, unlike other methods, do not remove the pathologically altered tissue. Nevertheless, these drugs can mostly be used without problems in long-term therapy 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 used. Carbimazole is rapidly converted into methimazole when ingested. According to the manufacturer, side effects (vomiting, lethargy , itching , liver disease , changes in the blood count ) occur in around 20% of cats, especially with long-term treatment, but these usually disappear again after discontinuation of the drug. In addition, thiamazole cannot be used in cats with simultaneous liver disease, diabetes or blood clotting disorders.

If thiamazole is intolerant, iopanoic acid can also be used. It inhibits the conversion of T 4 to T 3 and has practically no side effects.

Thyroidectomy

The surgical removal ( thyroidectomy ) is effective, but also risky, especially in heavily hyperthyroid cats because of the high risk of anesthesia. Pretreatment with anti-thyroid drugs is usually recommended here. Various techniques exist for the removal, whereby care must be taken to preserve the epithelial bodies as far as possible . In addition, there is a risk of injury to important cervical nerves ( recurrent laryngeal nerve , vagosympathetic trunk ) during the operation . Total thyroidectomy results in a deficiency of thyroid hormones, which must be compensated for by giving them for life. With unilateral removal, a temporary hypothyroidism often develops after the operation, but this usually does not need to be treated. In addition, there is a risk of relapse during surgical removal , especially if ectopic thyroid tissue is present.

Radioiodine therapy

The radioiodine therapy is considered the treatment of choice because it is effective and well tolerated. A single treatment is usually sufficient, so that no long-term drug treatment (taking tablets is quite problematic for some cats) and the risks of surgical removal are eliminated. However, it is associated with considerable radiation protection requirements and is therefore only available at two veterinary facilities in Germany. In addition to the limited availability, the associated costs and the necessary inpatient stay are a disadvantage. In close coordination with the responsible supervisory authorities, this was reduced from around three weeks to a few days. The necessary length of stay is determined by dosimetry and is seven to ten days.

Thermal or chemical destruction of the thyroid gland

The destruction of the thyroid tissue with a radiosurgery device under ultrasound control (thermal ablation) or by injecting 96% ethanol (chemical ablation) is practically irrelevant. Both forms of treatment have increased side effects such as laryngeal paralysis or Horner's syndrome .

Literature and Sources

  1. ^ ME Peterson et al.: Spontaneous hyperthyroidism in the cat. In: Proc. At the. College Vet. Intern. Med. 1979, p. 108.
  2. a b c d R. Hämmerling: The feline hyperthyroidism. In: The practical veterinarian. 86 (5), 2005, pp. 320-324. ISSN  0032-681X
  3. a b c d e Eva Höfel, Thomas Rieker: Hyperthyroidism - updates on pathogenesis and diagnosis. In: Fachpraxis. No. 63, 2013, pp. 6-11.
  4. a b c d e 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.
  5. a b c Andrea Monika Mathes, Reto Neiger: Hyperthyroidism of the cat. In: Small Animal Practice. 55, 2010, pp. 685-698.
  6. a b c d Thomas Graves: Current aspects of hyperthyroidism in cats. In: Vet. Focus. 19.3, 2009, pp. 2-5.
  7. RW Nelson, CG Couto (Ed.): Feline Hyperthyroidism. In: Internal medicine of small animals. 1st edition. Urban & Fischer, Munich / Jena 2006, ISBN 0-323-01724-X , pp. 758-772.
  8. ^ PH Kass et al .: Evaluation of environmental, nutritional, and host factors in cats with hyperthyroidism. In: J. Vet. Int. Med. 13 (4), 1999, pp. 323-329. ISSN  0891-6640
  9. a b c J. D. Broussard et al.: Changes in clinical and laboratory findings in cats with hyperthyreodism from 1983 to 1993. In: J. Am. Vet. Med. Assoc. 206 (3), 1995, pp. 302-305. PMID 7751233 .
  10. a b c Beate Egner, Anthony P. Carr: Hyperthyroidism in cats - what role do ACE inhibitors play? In: small animal specifically. 11, 2008, pp. 11-14.
  11. ^ ME 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, pp. 529-536. PMID 11229503 .
  12. Stephanie Nather et al .: Cat hyperthyroidism. In: Kompendium Kleintier 2017, pp. 52–57.
  13. ME Peterson et al: Use of the thyrotropin releasing hormone stimulation test to diagnose mild hyperthyroidism in cats. In: J. Vet. Intern. Med. No. 4, 1999, pp. 279-286. ISSN  0891-6640
  14. C. Poulsen Nautrup et al: Thyroid and parathyroid glands. In: C. Poulsen Nautrup, R. Tobias (Hrsg.): Atlas and textbook of ultrasound diagnostics in dogs and cats. 2nd Edition. Schlütersche Verlagsgesellschaft, Hanover 1998, ISBN 3-87706-663-1 , pp. 113–116.
  15. W. Kraft et al.: Symptoms of cat hyperthyroidism: a retrospective study. In: Small Animal Practice . 44 (10), 1999, pp. 719-732. ISSN  0023-2076
  16. ^ ME Peterson, DV Becker: Radionuclide thyroid imaging in 135 cats with hyperthyroidism. In: Vet. Radiol. 25 (1), 1984, pp. 23-27. ISSN  0196-3627
  17. ^ ME Peterson, DP Aucoin: Comparison of disposition of carbimazole and methimazole in clinically normal cats. In: Res. Vet. Sci. 54 (3), 1993, pp. 351-355. PMID 8337482 .
  18. CD which among other things: Occurrence of problems after three techniques of bilateral thyroidectomie in cats. In: Vet. Surgery. 18 (5), 1989, pp. 392-396. PMID 2815557 .
  19. EC Naan et al .: Results of thyroidectomy in 101 cats with hyperthyroidism. In: Vet. Surg. 35 (3), 2006, pp. 287-293. PMID 16635010 .
  20. ^ ME Peterson: Radioiodine treatment of hyperthyroidism. In: Clin. Tech. Small Anim. Pract. 21 (1), 2006, pp. 34-39. PMID 16584029 .
  21. M. Puille et al.: Radioiodine therapy in cats: Radiation protection of contact persons. In: Tierärztl Prax. 33 (K), 2005, pp. 291-295. ISSN  1434-1239

Web links

This article was added to the list of excellent articles on September 8, 2006 in this version .