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Feline Hyperthyroidism
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Feline Hyperthyroidism is the most common endocrine disorder in cats resulting from excessive amounts of thyroid hormones (T4 and T3). Siamese and Himalyans and cats who eat less canned food seem to have a diminished risk for acquiring hyperthyroidism. Feline Hyperthyroidism can show up at four years of age but generally appears around ten years of age. One research study found a cat with the signs at eight months old and was later diagnosed with clinical hypothryoidism. There has been research re pop up cans may be harmful to pet's health. Cats usually become ravenous and drink a lot of water but don't gave weight and later may become anexoric..don't confuse that with wasting (cachexia)from cancer. The hyperthyrodism may start making them looking unkempt. There is usually Tachycardia (rapid heart) and hypertension (high blood pressure).
Some medications are Methimazole (Tapazole®),Propylthiouracil (PTU),Iopate (Orografin ®), and Telepaque. There is also Radioactive Iodine treatment and surgery.

"overproduction of thyroid hormone and a subsequent increase in the metabolic rate...The most consistent finding with this disorder is weight loss secondary to the increased rate of metabolism. The cat tries to compensate for this with an increased appetite. Despite the increased intake of food, most cats lose weight. The weight loss may be so gradual that some owners will not even realize it has occurred or it may be quite rapid. Affected cats often drink a lot of water and urinate a lot. There may be periodic vomiting or diarrhea, and the hair coat may be unkempt. In some cats, anorexia develops as the disease progresses.
methimazole, can control the effects of the overactive thyroid gland..Surgical removal of the affected thyroid lobe(s) is also very affective..A very effective way to treat hyperthyroidism is with radioactive iodine therapy"

Waltham Focus Vol. 11, No.1
*Hyperthyroidism is the most common endocrine disorder in cats and a frequently diagnosed condition in small animal practice. *The aetiology is unknown but the prognosis is good with effective therapy. *Hyperthyroid cats are less symptomatic now than 10-15 years ago. *A diagnosis is readily confirmed with a circulating total T4 measurement alone. *Treatment of hyperthyroidism can result in a decrease in glomerular filtration rate and clinical signs of overt renal dysfunction.

Since first recognised in 1979, there has been a marked increase in the rate of diagnosis of feline hyperthyroidism (1). It is unclear whether this reflects a true increased incidence in the disease or an increased awareness, an improvement in diagnostic acumen, a growing general cat population together with a longer average lifespan for cats or a combination of these. Today, the true incidence of the disease is unknown and this is compounded by apparent geographic differences. Most large case series emanate from the USA, UK and Australasia, but whether this reflects demographic differences in the cat population or other factors is as yet unknown. Despite this, hyperthyroidism is recognised as the most common endocrine disorder of cats and a disease of significance in small animal practice with a reasonable throughput of aged and ageing cats.
Cats represent the only non-human species in which spontaneous thyrotoxicosis develops with frequency. In humans, Graves disease and toxic nodular goitre are the most common forms of hyperthyroidism. In Graves disease, thyroid-stimulating autoantibodies are produced, which bind to the thyroid stimulating hormone (TSH) receptor mimicking its activity. Similar autoantibodies have not been demonstrated in hyperthyroid cats (2). Histopathologically, the disease more closely resembles toxic nodular goitre. This term encompasses both toxic multinodular goitre and toxic adenoma of the thyroid gland. In the former disease the thyroid harbours multiple functional areas of adenomatous hyperplasia and the exact aetiology is unknown. Toxic adenomas are usually solitary and at least some harbour a mutation of the gene for the TSH receptor that results in its constitutive activation. Again, similar mutations have not been identified in hyperthyroid cats (3). Overexpression of the product of the cellular protooncogene c-ras has been demonstrated in affected thyroid tissue from hyperthyroid cats but the significance of this remains unclear (4).
Two large epidemiological studies have investigated the possible risk factors associated with the development of feline hyperthyroidism. In an early study, risk factors identified included exposure to fertilizers, herbicides, flea powders and sprays, consumption of canned food, indoor residence and being non Siamese (5). In a later study, there was a persistent protective effect of breed (Siamese or Himalayan), a two to three fold increased risk of developing the condition in cats eating primarily canned cat food and a three fold increase among those using cat litter (6). The potential role of any or all of these factors in the development or maintenance of hyperthyroidism is unknown.

Veterinarians Guide to Natural Remedies for Cats : Safe and Effective Alternative Treatments and Healing Techniques from the Nations Top Holistic Veterinarians

Evaluation of environmental, nutritional, and host factors in cats with hyperthyroidism
. Kass PH, Peterson ME, Levy J, James K, Becker DV, Cowgill LD
Dept of Population Health and Reprod,School of Vet Med,Univ of Calif,Davis 95616-8746, USA.
J Vet Intern Med 1999 Jul-Aug;13(4):323-9
The pathologic changes associated with hyperthyroidism (adenomatous hyperplasia, adenoma of the thyroid gland) have been well characterized in cats, but the pathogenesis of these changes remains unclear. In this research, we undertook a case-control study to search for potential risk factors for this disease. Owners of 379 hyperthyroid and 351 control cats were questioned about their cats' exposure to potential risk factors including breed, demographic factors, medical history, indoor environment, chemicals applied to the cat and environment, and diet. The association between these hypothesized risk factors and outcome of disease was evaluated by conditional logistic regression. Two genetically related cat breeds (ie, Siamese and Himalayan) were found to have diminished risk of developing hyperthyroidism. Cats that used litter had higher risk of developing hyperthyroidism than those that did not. Use of topical ectoparasite preparations was associated with increased risk of developing hyperthyroidism. Compared with cats that did not eat canned food, those that ate commercially prepared canned food had an approximate 2-fold increase in risk of disease. When these 4 variables (breed, use of cat litter, consumption of canned cat food, and use of topical ectoparasite preparations) from the univariate analysis were selected for further study as candidate risk factors and analyzed by multivariate conditional logistic regression, a persistent protective effect of breed (ie, Siamese or Himalayan) was found. In addition, results suggested a 2- to 3-fold increase in risk of developing hyperthyroidism among cats eating a diet composed mostly of canned cat food and a 3-fold increase in risk among those using cat litter. In contrast, the use of commercial flea products did not retain a strong association. The results of this study indicate that further research into dietary and other potentially important environmental factors (eg, cat litter) is warranted.

excellent overview of Hyperthryoidism
Newman vet's also has an article on thryoid Basics
Transdermal methimazole treatment in cats with hyperthyroidism
G. Hoffmann,S. L. Marks,J. Taboada, G. L. Hosgood, K. J. Wolfsheimer
Dept of Vet Clin Sci, School of Vet Med, Louisiana State Univ, Baton Rouge, LA 708030/8410, USA
Journal of Feline Medicine & Surgery 5, 2 , April 2003, 77-82
The objectives of this study were to assess serum thyroxine concentrations and clinical response in hyperthyroid cats to treatment with transdermal methimazole, and to determine if further investigation is indicated. Clinical and laboratory data from 13 cats with hyperthyroidism were retrospectively evaluated. Methimazole (Tapazole, Eli Lilly) was formulated in a pleuronic lecithin organogel (PLO)-based vehicle and was applied to the inner pinna of the ear at a dosage ranging from 2.5 mg/cat q 24 h to 10.0 mg/cat q 12 h. During the treatment period, cats were re-evaluated at a mean of 4.3 weeks (recheck-1), and again at a mean of 5.4 months (recheck-2). Clinical improvement was observed, and significant decreases in thyroxine concentrations were measured at recheck-1 (mean: 39.57 nmol/L, SEM: 14.4, SD: 41.2) and recheck-2 (mean: 36.71 nmol/L, SEM: 13.9, SD: 45.56) compared to pretreatment concentrations (mean: 97.5 nmol/L, SEM: 11.42, SD: 39.5). No adverse effects were reported

: J Am Anim Hosp Assoc. 2003 Jan-Feb;39(1):67-71. : Juvenile hyperthyroidism in a cat.
Gordon JM, Ehrhart EJ, Sisson DD, Jones MA.
Department of Veterinary Clinical Medicine, University of Illinois, 1008 West Hazelwood Drive, Urbana, Illinois 61802, USA.

An 8-month-old, male domestic shorthaired cat presented for chronic weight loss, intermittent dyspnea, chronic diarrhea, hyperactivity, and weakness. The cat had a palpable thyroid nodule and increased serum total thyroxine and 3,5,3' triiodothyronine levels. The cat was diagnosed with hyperthyroidism, and a unilateral thyroidectomy was performed followed by radioactive iodine at a later date. The clinical signs resolved following radioactive iodine, and the cat subsequently developed clinical hypothyroidism.
R M Arnold
Campbell University,Buies Creek, North Carolina
Compendium on Continuing Education for the Practicing Veterinarian, 2001, Vol 23, Iss 6, pp 558
Amiodipine, a dihydropyridine calcium-channel blocker, is commonly used to treat hypertension. The drug is long acting, vasoselective, and structurally related to nifedipine. Unlike other calcium antagonists, amiodipine has a high volume of distribution, low elimination rate, and prolonged half-life. INDICATIONS Amiodipine is used to treat systemic hypertension in cats (defined as systolic pressure greater than 160 mm Hg and diastolic pressure greater than 100 mm Hg). Systemic hypertension in cats is usually secondary to hyperthyroidism or chronic renal failure. In patients with hyperthyroidism, treatment and maintenance of the hyperthyroid state are essential in the management of the hypertension. Amlodipine has been shown to cause the release of nitric oxide from blood vessels, consequently resulting in vasodilation.The drug also preserves left ventricular volume and function during healing after reperfused myocardial infarction in dogs
Long-term health and predictors of survival for hyperthyroid cats treated with iodine 131
Slater MR, Geller S, Rogers K
Texas A&M Univ; Coll Vet Med; College Stn; TX 77843; USA, Dept Vet Anat & Publ Hlth, College Stn, TX
77843 USA; Texas A&M Univ, Coll Vet Med, Dept Small Anim Med & Surg, College Stn, TX 77843 USA Journal of Veterinary Internal Medicine Jan-Feb 2001; 15 (1) : 47-51.
Two hundred thirty-one cats treated with radioactive iodine at the Texas Veterinary Medical Teaching Hospital were followed for a median of 25 months by means of an ambidirectional (prospective, retrospective) cohort study design. Cox proportional hazards models were used to determine predictors of survival based on data at the time of hyperrhyroid diagnosis (collected retrospectively) and found that only age at diagnosis and sex of the cat were predictors of survival. Increasing age (for each year of age, relative risk [RR] = 1.2, 95% confidence interval [CI] = 1.1-1.3) and being male (RR = 0.68, 95% CI = 0.5-0.9) increased likelihood of death. Tables predicting survival after diagnosis and treatment of hyperthyroidism for various age and sex combinations were created. In addition, Cox proportional hazard models were run with all data available at the end of the study (collected retrospectively and prospectively) including number and type of major health problems reported at the time of death or censoring. In this model, significant factors were age at diagnosis, sex, and either type of major health problem or number of health problems. Cats with renal disease or cancer were more likely not to survive and increasing from none to 2 health problems also decreased survival. Renal problems and cancer were the most common health problems at the time of death or censoring. This study provides estimates of duration of survival for cats successfully treated for hyperthyroidism with radioactive iodine, which can be useful in assisting with client treatment decisions.

Ann Epidemiol 2002 Oct;12(7):509:S dietary and environmental risk factors for clinical hyperthyroidism in pet cats
Edinboro C, Scott-Moncrieff C, Tetrick M, Glickman L.
School of Veterinary Medicine, Purdue University, West Lafayette, IN, USA
PURPOSE: The prevalence of hyperthyroidism in pet cats (benign nodular hyperplasia) has reached epidemic proportions, since it was first described in 1979. Therefore, we tested the hypothesis that consumption of canned foods compared with dry foods increases the risk of hyperthyroidism. Furthermore, the risk associated with lower iodine concentrations in canned foods and chemicals in can linings was evaluated. Secondary hypotheses were that hyperthyroid risk is increased by well water consumption and exposure to environmental endocrine disrupters.METHODS: Case cats (n = 109) had serum T4 concentration >/=5.0 &mgr;g/dl and >/=1 clinical sign of hyperthyroidism; control cats (n = 173) had a serum T4 concentration /=50% canned: OR = 3.45, 95% CI: 1.77, 6.74; <50% canned: OR = 2.84, 95% CI: 1.48, 5.46). Other significant risk factors were use of cans with easy-open lids (vs. dry) (OR = 3.79, 95% CI: 1.68, 8.54), foods without iodine supplementation in any life stage (OR = 4.09, 95% CI: 1.09, 15.40), years of well water consumption (OR = 1.12, 95% CI: 1.02, 1.23), years of exposure to gas fireplaces (OR = 1.16, 95% CI: 1.02, 1.31), age (OR = 1.15, 95% CI: 1.03, 1.29), and female gender (OR = 2.73, 95% CI: 1.38, 5.43).CONCLUSIONS: Endocrine disrupters in food can linings and the environment may explain the recent epidemic of hyperthyroidism in cats, since easy-open cans have gained popularity. This same relationship should also be evaluated in humans.
J Am Anim Hosp Assoc 2003 Jan-Feb;39(1):67-71 : Juvenile hyperthyroidism in a cat.
Gordon JM, Ehrhart EJ, Sisson DD, Jones MA.
Department of Veterinary Clinical Medicine, University of Illinois, 1008 West Hazelwood Drive, Urbana, Illinois 61802, USA.

An 8-month-old, male domestic shorthaired cat presented for chronic weight loss, intermittent dyspnea, chronic diarrhea, hyperactivity, and weakness. The cat had a palpable thyroid nodule and increased serum total thyroxine and 3,5,3' triiodothyronine levels. The cat was diagnosed with hyperthyroidism, and a unilateral thyroidectomy was performed followed by radioactive iodine at a later date. The clinical signs resolved following radioactive iodine, and the cat subsequently developed clinical hypothyroidism.
Nuklearmedizin 2002 Dec;41(6):245-51 : Radioiodine treatment of feline hyperthyroidism in Germany.
Puille M, Knietsch M, Spillmann T, Grunbaum EG, Bauer R.
Klinik fur Nuklearmedizin, Justus-Liebig-Universitat Giessen, Deutschland.

AIM: Establishment of radioiodine treatment of feline hyperthyroidism in veterinary routine in accordance with German radiation protection regulations. PATIENTS AND METHODS: 35 cats with proven hyperthyroidism were treated with 131I in a special ward. Thyroid uptake and effective halflife were determined using gammacamera dosimetry. Patients were released when measured whole body activity was below the limit defined in the German "Strahlenschutzverordnung". RESULTS: 17/20 cats treated with 150 MBq radioiodine and 15/15 cats treated with 250 MBq had normal thyroid function after therapy, normal values for FT3 and FT4 were reached after two and normal TSH levels after three weeks. In 14 cats normal thyroid function was confirmed by controls 3-6 months later. Thyroidal iodine uptake was 24 +/- 10%, effective halflife 2.5 +/- 0.7 days. Whole body activity < 1 MBq was reached 13 +/- 4 days after application of 131I. Radiation exposure of cat owners was estimated as 1.97 microSv/MBq for adults. CONCLUSION: Radioiodine therapy of feline hyperthyroidism is highly effective and safe. It can easily be performed in accordance with German radiation protection regulations, although this requires hospitalisation for approximately two weeks. Practical considerations on radiation exposure of cat owners do not justify this long interval. Regulations for the veterinary use of radioactive substances similar to existing regulations for medical use in humans are highly desirable.
Surgical options for the treatment of hyperthyroidism in the cat
J A Flanders
Journal of Feline Medicine & Surgery 1, 3, September 1999 127-134
Since the first description of feline hyperthyroidism in 1978, numerous treatment options for hyperthyroidism have been reported. Surgical removal of enlarged, autonomously functioning thyroid glands is one of the most commonly used treatment options. Affected cats must have a careful pre-operative evaluation to detect concurrent medical conditions such as renal disease or cardiomyopathy. Since more than 80% of hyperthyroid cats have neoplastic changes in both thyroid glands, bilateral thyroidectomy is necessary for treatment of the majority of hyperthyroid cats. Several different thyroidectomy techniques have been developed in an attempt to minimise potential post-operative complications associated with bilateral thyoidectomy such as hypocalcemia or recurrence of hyperthyroidism. Damage to or removal of all four parathyroid glands during bilateral thyroidectomy causes hypocalcemia, the most common post-operative complication. Recurrence of hyperthyroidism can occur months after initial thyroidectomy if residual adenomatous thyroid tissue is retained in the surgical site. The most effective surgical techniques for bilateral thyroidectomy involve preservation of at least one external parathyroid gland on the surface of the thyroid capsule. Additionally, the majority of the thyroid capsule must be removed to ensure that all neoplastic thyroid tissue is removed. The most recently described feline thyroidectomy techniques involve sequential removal of bilaterally afftected thyroid glands. Staging a bilateral thyroidectomy allows time for ipsilateral parathyroid tissue to revascularise before the second thyroid gland is removed and the blood supply to the contralateral parathyroid glands is potentially interrupted. Thyroidectomy is a very effective treatment option for hyperthyroid cats. Surgical treatment of hyperthyroidism in cats offers permanent cure without chronic medical management. No specialised equipment other than standard surgical instrumentation and facilities are necessary. With practice, feline thyroidectomy can become a routine procedure in most veterinary hospitals.
Feline hyperthyroidism: spectrum of clinical presentions and response to carbimazole therapy.
Bucknell DG
Prahran Veterinary Hospital, Victoria.
Aust Vet J 2000 Jul;78(7):462-5
OBJECTIVE: To determine the spectrum of clinical presentations of hyperthyroidism in cats and response to carbimazole therapy by analysis of historical, clinical and laboratory data. DESIGN: A prospective clinical study involving client-owned cats presenting to a private veterinary practice in Australia. PROCEDURE: Twenty-five cats diagnosed as hyperthyroid during a 23-month period participated in the study with owner consent. Therapy with carbimazole was instituted and revisits were scheduled 2, 6 and 13 weeks after diagnosis. The cats were physically examined and underwent haematological and serum biochemical testing at each revisit. Owners were also asked to assess clinical signs in their cats in the periods between veterinary examinations. Cats with underlying renal disease were managed by alterations or cessation of carbimazole therapy. RESULTS: A high prevalence of lethargic or inappetent cats without detectable underlying nonthyroidal illness was found. There was also a high prevalence of cats less than 10-years-old and cats in good body condition. Fourteen cats treated with carbimazole and monitored for 13 weeks responded favourably to therapy. Side-effects were minor and uncommon. Cats with underlying renal disease that became apparent during the study, responded well to alterations or cessation of carbimazole therapy. CONCLUSION: The trend towards more subtle clinical presentations of hyperthyroid cats reported previously continued in this study. The findings of the current study do not appear to support the traditional view of hyperthyroid cats as being old, hyperactive, hungry and thin. Carbimazole therapy was found to decrease the prevalence of almost all clinical abnormalities in 14 cats and side-effects were minor and uncommon. This study demonstrates the usefulness of medical management of hyperthyroidism in the cat when radioiodine therapy is not possible due to renal compromise or other factors.

Medical therapy of feline hyperthyroidism
EN Behrend
Auburn Univ,Coll Vet Med,Dept Small Anim Surg & Med,Auburn,AL 36849 USA
Compendium on Continuing Education for the Practicing Veterinarian, 1999, 21, 3, 235+
Hyperthyroidism is the most common endocrinopathy of cats. Antithyroid medications and radioiodine constitute the two main medical options. Methimazole and carbimazole are effective in most cats, but monitoring is required to assess efficacy of therapy and development of side effects. Ipodate is an oral a;gent that may be used as an alternative in some cats. Radioiodine therapy is also highly successful, but its use may require prolonged hospitalization (depending on state and local ordinances). Although P-adrenergic blockers do not affect thyroid hormone concentration, they reverse some of the effects of the hyperthyroid state and can be used in combination with antithyroid medications or surgery

Vet Radiol Ultrasound 2002 Nov-Dec;43(6):587-91 : Predictors of response to radioiodine therapy in hyperthyroid cats.
Chun R, Garrett LD, Sargeant J, Sherman A, Hoskinson JJ.
Department of Clinical Sciences, Kansas State University, College of Veterinary Medicine, Manhattan 66506-5606, USA.

The objective of this retrospective study was to evaluate hyperthyroid cats for pretreatment factors that would predict response to radioiodine therapy. Hyperthyroidism was diagnosed in 193 cats based on elevated serum thyroxine levels and/or elevated thyroid to salivary gland ratios on thyroid scintigraphy. All cats were treated with an intravenous bolus of 4 mCi of radioiodine and follow-up serum thyroxine levels were evaluated at 1 week and 1, 3, 6, and 12 months post-therapy. There was a significant relationship between pretreatment thyroxine values and post-treatment thyroxine values at all of the follow-up time points (p < 0.001). There was also a relationship between thyroid to salivary gland technetium scan ratio results and serum thyroxine values at pretreatment and at 1 week post-treatment (p = 0.02, 0.005, respectively). A greater scan ratio was associated with higher thyroxine levels at these time points, but not at 1, 3, 6 or 12 months post-therapy. Ninety-eight cats pretreated with methimazole were analyzed for the effect of this drug on response to therapy. Methimazole was discontinued > or = 5 days before radioiodine therapy in 58 cats and < 5 days in 31 cats, in 9 cats the number of days off methimazole was unknown. There was no difference in response to radioiodine based upon when methimazole was discontinued (p = 0.70).