{"id":1374,"date":"2013-01-01T17:26:34","date_gmt":"2013-01-01T17:26:34","guid":{"rendered":"http:\/\/phosdev.com\/todaysveterinarypractice\/?p=1374"},"modified":"2022-02-18T16:53:19","modified_gmt":"2022-02-18T16:53:19","slug":"feline-hyperthyroidism-diagnosis-therapeutic-modalities","status":"publish","type":"post","link":"https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/endocrinology\/feline-hyperthyroidism-diagnosis-therapeutic-modalities\/","title":{"rendered":"Feline Hyperthyroidism: Diagnosis &amp; Therapeutic Modalities"},"content":{"rendered":"<p class=\"p1\"><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2016\/06\/T1301F02.pdf\"><img decoding=\"async\" class=\"alignnone size-full wp-image-9886\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2011\/07\/pdf_button.png\" alt=\"pdf_button\" width=\"110\" height=\"27\" \/><\/a><\/p>\n<hr \/>\n<p class=\"p1\"><span class=\"s1\"><em>David Bruyette, DVM, Diplomate ACVIM<\/em><\/span><\/p>\n<p class=\"p1\">Hyperthyroidism is recognized as the most common endocrinopathy of older cats. Traditional therapeutic measures include thyroidectomy, radioactive iodine therapy, and antithyroid medications. A new therapeutic option\u2014a limited-iodine diet that helps normalize thyroid hormone concentrations and alleviate clinical signs\u2014is now available.<\/p>\n<hr \/>\n<p class=\"p1\"><span class=\"s1\">Hyperthyroidism is recognized as the most common endocrinopathy of older cats. Despite worldwide occurrence, though, its pathogenesis remains unclear.<\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">Traditional therapeutic measures for managing feline hyperthyroidism include:<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Thyroidectomy<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Antithyroid medications<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Radioactive iodine.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">Recent studies have documented another therapeutic option for hyperthyroid cats\u2014<strong>a limited-iodine diet<\/strong>, which helps normalize thyroid hormone concentrations and alleviate clinical signs.<\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">All therapeutic options are effective, with their own pros and cons. <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\"><strong>Surgery<\/strong> and <strong>radioactive iodine<\/strong> provide permanent treatment for feline hyperthyroidism.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\"><strong>Oral antithyroid drugs<\/strong> and <strong>nutritional management<\/strong> must be administered daily to control hyperthyroidism if a permanent solution is not chosen.<\/span><\/li>\n<\/ul>\n<h2 class=\"p3\"><span class=\"s1\"><b>DIAGNOSIS<\/b><\/span><\/h2>\n<p class=\"p1\"><span class=\"s1\">Diagnosis of feline hyperthyroidism is based on:<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Presence of one or more clinical signs (see <b>Clinical Signs of Feline Hyperthyroidism<\/b>)<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Increased serum total thyroxine (T<sub>4<\/sub>) concentration.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">However, up to 10% of all hyperthyroid cats and 40% of those with mild disease have serum T<\/span><sub>4<\/sub><span class=\"s1\"> values within the reference range.<\/span><span class=\"s2\"><sup>1,2<\/sup><\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Hyperthyroidism should not be excluded based on a single, normal serum T4 value, especially in a cat with typical clinical signs, a palpable thyroid nodule, and serum T<sub><span style=\"font-size: small\">4<\/span><\/sub> in the upper half of the reference range.<\/span><span class=\"s2\"><sup>3<\/sup><\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">In these cases, serum free T<sub><span style=\"font-size: small\">4<\/span><\/sub>, measured by equilibrium dialysis (fT<sub>4<\/sub>ED), may provide an alternative approach to diagnosis. Studies document that up to 20% of sick euthyroid cats can have increased fT<sub>4<\/sub> concentrations.<\/span><span class=\"s2\"><sup>4<\/sup><\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Therefore, it is appropriate and reliable to interpret the 2 values together:<\/span>\n<ul class=\"ul2\">\n<li class=\"li1\"><span class=\"s1\">Mid to high serum total T<sub><span style=\"font-size: small\">4<\/span><\/sub> and increased fT<sub><span style=\"font-size: small\">4<\/span><\/sub> concentration is consistent with hyperthyroidism.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Low serum total T<sub><span style=\"font-size: small\">4<\/span><\/sub> and normal or increased fT<sub><span style=\"font-size: small\">4<\/span><\/sub> values are usually associated with nonthyroidal illness.<\/span><\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<div class=\"orange-box\">\n<h2><span class=\"s1\"><b>Clinical Signs of Feline Hyperthyroidism<\/b><\/span><\/h2>\n<ul>\n<li><span class=\"s1\">Weight loss and poor hair coat<\/span><\/li>\n<li><span class=\"s1\">Aggressive or &#8220;cranky&#8221; behavior<\/span><\/li>\n<li><span class=\"s1\">Periodic vomiting<\/span><\/li>\n<li><span class=\"s1\">Polyuria and polydipsia<\/span><\/li>\n<li><span class=\"s1\">Increased appetite, activity, restlessness, and heart rate<\/span><\/li>\n<li><span class=\"s1\">Occasionally, difficulty breathing, weakness, and depression<\/span><\/li>\n<\/ul>\n<\/div>\n<h2 class=\"p3\"><span class=\"s1\">MANAGEMENT OPTIONS<br \/>\n<\/span><\/h2>\n<p class=\"p3\"><span class=\"s1\">Methods of managing feline hyperthyroidism include:<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><b><\/b><span class=\"s4\"><b>THYROIDECTOMY<\/b><\/span><span class=\"s1\"> provides a permanent solution.<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s4\"><b>RADIOACTIVE IODINE<\/b><\/span><span class=\"s1\"> is considered the gold standard for treatment.<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s4\"><b>ANTITHYROID MEDICATIONS<\/b><\/span><span class=\"s1\"> are often oral and needed daily to achieve and maintain their effect.<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s4\"><b>LIMITED-IODINE DIETS<\/b><\/span><span class=\"s1\"> normalize thyroid hormone concentrations and alleviate clinical signs of hyperthyroidism.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">All management options can be &gt; 90% effective for controlling hyperthyroidism when used appropriately. The selected management option will differ for each cat based on several considerations (<b>Table<\/b>). <\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">Radioactive iodine therapy is considered the gold standard for treatment of hyperthyroidism; however, most pet owners currently chose medical management with oral or transdermal antithyroid drugs. An additional option\u2014nutritional management using a limited-iodine food\u2014is now available for cats with hyperthyroidism.<\/span><\/p>\n<p class=\"p1\"><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/01\/feline-table-2.png\"><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone size-figure_img wp-image-9557\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/01\/feline-table-2-643x300.png\" alt=\"feline table-2\" width=\"643\" height=\"300\" \/><\/a><\/p>\n<h2 class=\"p3\"><span class=\"s1\"><b>RADIOACTIVE IODINE<\/b><\/span><\/h2>\n<p class=\"p1\"><span class=\"s1\">Radioactive iodine therapy is often considered the best option for many hyperthyroid cats because:<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">A single treatment has the potential to eliminate a benign thyroid tumor or abnormal thyroid tissue. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Extra-thyroidal thyroid tissue, which may occur in 10% to 20% of hyperthyroid cats, is treated.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">No general anesthesia is required<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Reported side effects are minimal.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\"><b>The goal of treatment is to restore euthyroidism with the smallest possible single dose of radioactive iodine, while avoiding development of hypothyroidism<\/b>.<\/span><span class=\"s2\"><sup>6<\/sup><\/span><span class=\"s1\"> Controversy exists as to the best method of calculating the optimum dose for individual cats.<\/span><span class=\"s2\"><sup>5,6<\/sup><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">Cats should be in stable condition prior to radioiodine therapy; those with clinically significant cardiovascular, renal, gastrointestinal (GI), or endocrine (eg, diabetes mellitus) disease may not be good candidates, especially due to the required boarding time after treatment.<\/span><span class=\"s2\"><sup>5<\/sup><\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Mechanism of Action<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">After administration, radioactive iodine is actively concentrated by the thyroid gland, with a half-life of 8 days.<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">It emits both beta-particles and gamma-radiation; the beta-particles are responsible for the majority of tissue destruction, but are only locally destructive, traveling a maximum of 2 mm. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Therefore, no significant damage to adjacent parathyroid tissue, atrophic thyroid tissue, or other cervical structures is expected.<\/span><\/li>\n<\/ul>\n<h3 class=\"p4\"><span class=\"s1\"><b>Posttreatment Hypothyroidism<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">Based on the majority of reported cases, posttreatment hypothyroidism is transient and generally uncommon (2%\u20137% of cases); even fewer cats have clinical signs or appear to require thyroid hormone replacement.<\/span><span class=\"s2\"><sup>7-11<\/sup><\/span><span class=\"s1\"> However, in 1 study, up to 30% (50\/165 cats) were hypothyroid 3 months after radioiodine therapy; of these:<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">56% (19\/34 hypothyroid cats with available information) had clinical signs of hypothyroidism.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">52% (23\/44 cats) were given thyroid hormone supplementation.<\/span><span class=\"s2\"><sup>12<\/sup><\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">Since hypothyroidism has been associated with azotemia and decreased survival time in previously hyperthyroid cats, thyroid hormone replacement may be needed in some cats, especially those with concurrent kidney disease.<\/span><span class=\"s2\"><sup>13<\/sup><\/span><span class=\"s1\"> Owners should be advised of this possibility, particularly if their motivation for this particular treatment is to avoid long-term medical therapy.<\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Limitations<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\"><b>The main limitation to widespread use of radioactive iodine is the requirement for special licensing and isolation of the cat after treatment<\/b>, which can range from several days to several weeks depending on state or local radiation regulations and the dose administered.<sup>6<\/sup><\/span><\/p>\n<h2 class=\"p3\"><span class=\"s1\"><b>ANTITHYROID DRUGS<\/b><\/span><\/h2>\n<p class=\"p1\"><span class=\"s1\">Antithyroid drugs are commonly used for treatment of hyperthyroidism in cats.<\/span><span class=\"s2\"><sup>14-21<\/sup><\/span><span class=\"s1\"> They are widely recommended to stabilize hyperthyroid cats prior to surgery and are the only drugs that can be used chronically for management of hyperthyroidism.<\/span><span class=\"s2\"><sup>6<\/sup><\/span><span class=\"s1\"> Almost all cats are potential candidates unless thyroid carcinoma is suspected.<\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Mechanism of Action<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">If administered appropriately, antithyroid drugs reliably inhibit the synthesis of thyroid hormones and, thereby, lower serum thyroid hormone concentrations. These drugs do not affect the thyroid gland&#8217;s ability to trap inorganic iodide or release preformed hormones.<\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Administration<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">The antithyroid drugs used most often in cats are methimazole and carbimazole; both can be given orally or formulated for transdermal application. <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Custom formulation of transdermal products may increase expense of therapy and there is no guarantee of product stability. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">However, results of a recent prospective study showed that once daily treatment for 12 weeks with transdermal methimazole in a novel lipophilic vehicle was as effective as twice-daily carbimazole administered orally.<\/span><span class=\"s2\"><sup>14<\/sup><\/span><\/li>\n<\/ul>\n<h3 class=\"p4\"><span class=\"s1\"><b>Side Effects<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">While many cats are successfully managed long-term with antithyroid drugs, it is important to monitor for side effects associated with their use.<\/span><span class=\"s2\"><sup>15,18,19,21<\/sup><\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">In the study with the largest number of cats, 18% had side effects associated with methimazole; a more recent study revealed that 44% of 39 cats had side effects.<\/span><span class=\"s2\"><sup>15,19<\/sup><\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">In 44 cats receiving carbimazole for 1 year, 44% had associated side effects, with GI signs (decreased appetite, vomiting, diarrhea) being most common. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">In another study, 13% of 39 cats treated with carbimazole experienced side effects.<\/span><span class=\"s2\"><sup>18<\/sup><\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">Determining what percentage of side effects are caused by the drug versus another factor, such as concurrent disease, is difficult.<\/span><span class=\"s2\"><sup>21<\/sup><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">Most adverse reactions occur within the first few weeks to months after beginning therapy and include depression, inappetence, vomiting, and self-induced excoriations of the head and neck (facial pruritus). <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">GI signs are less common with transdermal administration of methimazole.<\/span><span class=\"s2\"><sup>16<\/sup><\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Mild to serious hematologic complications, including agranulocytosis and thrombocytopenia either alone or concurrently and, more rarely, immune-mediated hemolytic anemia, may also occur. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Hepatic toxicity with marked increases in bilirubin concentration and hepatic enzyme activity has been described in less than 2% of cats treated with methimazole.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Serum antinuclear antibodies develop in approximately 50% of cats treated with methimazole for longer than 6 months, usually in cats on high-dose therapy (&gt; 15 mg\/day). Although clinical signs of a lupus-like syndrome have not been reported, decreasing the daily dosage is recommended.<\/span><span class=\"s2\"><sup>6<\/sup><\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\"><b>Cessation of therapy is required if serious hematologic or hepatic reactions develop<\/b>.<\/span><\/p>\n<div class=\"orange-box\">\n<h2 class=\"p3\"><span class=\"s1\"><b>Thyroidectomy<\/b><\/span><\/h2>\n<p class=\"p1\"><span class=\"s1\">Thyroidectomy for treatment of feline hyperthyroidism has become rare due to the availability of other therapeutic options.<\/span><\/p>\n<\/div>\n<h2 class=\"p3\"><span class=\"s1\">NUTRITIONAL MANAGEMENT<\/span><\/h2>\n<p class=\"p3\"><span class=\"s1\">A limited-iodine therapeutic food containing &lt; 0.3 ppm on a dry matter basis (DMB) is now available as an option for managing cats with hyperthyroidism (Prescription Diet y\/d Feline, hillsvet.com).<\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Mechanism of Action<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">Production of thyroid hormone requires sufficient dietary iodine. The only known function for ingested iodine is for thyroid hormone synthesis.<\/span><span class=\"s2\"><sup>5<\/sup><\/span><span class=\"s1\"> This observation led to the hypothesis that limiting dietary iodine intake could be used to control thyroid hormone production and potentially manage hyperthyroidism in cats.<\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Research Studies<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\"><b>Multiple feeding trials<\/b> to determine the safety and effectiveness of limited dietary iodine in the management of feline hyperthyroidism were conducted in a research colony (over 100 cats) with naturally occurring hyperthyroidism. <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Study results showed that a therapeutic food with dietary iodine &lt; 0.3 ppm iodine (DMB) provided a safe and effective management option for cats with naturally occurring hyperthyroidism. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Serum total thyroxine concentrations returned to a normal range within 4 to 12 weeks of initiating nutritional management and 90% of the hyperthyroid cats maintained on the limited-iodine food as the sole source of nutrition became euthyroid.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\"><b>Three studies<\/b> were designed to determine the: <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Magnitude of iodine control necessary to return newly diagnosed cats to a euthyroid state<\/span><span class=\"s2\"><sup>22<\/sup><\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Maximum level of dietary iodine that maintains cats in a euthyroid state<\/span><span class=\"s2\"><sup>23<\/sup><\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Effectiveness of a therapeutic food formulated (based on the previous studies) to control naturally occurring hyperthyroidism in cats.<\/span><span class=\"s2\"><sup>24<\/sup><\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">In summary, results of these studies demonstrated that a food with 0.17 or 0.32 ppm iodine (DMB) maintained normal thyroid hormone concentrations in hyperthyroid cats, helping to further define the amount of iodine that effectively manages hyperthyroidism.<\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">A <b>prospective study<\/b>, which should be completed in 2013, is evaluating the efficacy of a limited-iodine diet in managing feline hyperthyroidism relative to pre- and posttreatment:<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Monitoring of thyroid function (T<sub><span style=\"font-size: small\">4<\/span><\/sub>, fT<sub><span style=\"font-size: small\">4<\/span><\/sub>ED, TSH)<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Clinical signs<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Body weight<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Renal function<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Blood pressure.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">To date, over 150 cats with naturally occurring hyperthyroidism have been managed successfully by feeding a limited-iodine food, most for 2 to 3 years and some cats for as long as 6 years.<\/span><\/p>\n<div class=\"orange-box\">\n<p><span class=\"s1\"><b>Iodine Content of Commercial Cat Foods<\/b><\/span><\/p>\n<p><span class=\"s1\">Iodine occurs naturally in many ingredients typically used when manufacturing commercial pet foods (particularly fish, shellfish, and fresh meats). Unless steps are taken to strictly control iodine content of ingredients, the final iodine concentration in pet foods varies widely.<\/span><span class=\"s2\"><sup><span style=\"font-size: small\">25-28<\/span><\/sup><\/span><\/p>\n<p><span class=\"s1\">Commercial cat foods in New Zealand had iodine amounts ranging from <strong>0.19 to 21.2 ppm<\/strong> in 1 study, whereas in Germany, a range of <strong>0.22 to 6.4 ppm<\/strong> was reported.<\/span><span class=\"s2\"><sup><span style=\"font-size: small\">25,29<\/span><\/sup><\/span><span class=\"s1\"> Evaluation of 28 canned cat foods in the U.S. revealed an iodine content ranging from <strong>1.09 to 52.3 ppm<\/strong>; 14 dry cat foods contained iodine amounts ranging from <strong>1.34 to 5.94 ppm<\/strong>.<\/span><span class=\"s2\"><sup><span style=\"font-size: small\">28<\/span><\/sup><\/span><\/p>\n<p><span class=\"s1\">Based on these studies: <\/span><\/p>\n<ul>\n<li><span class=\"s1\">The amount of iodine is much higher in many canned foods compared to dry foods<\/span><\/li>\n<li><span class=\"s1\">Variability of iodine content is much greater in canned foods.<\/span><span class=\"s2\"><sup><span style=\"font-size: small\">25,28-29<\/span><\/sup><\/span><\/li>\n<\/ul>\n<\/div>\n<h3 class=\"p4\"><span class=\"s1\"><b>Administration<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">If the limited-iodine food is selected as the management option for a client&#8217;s hyperthyroid cat, gradual transition to the food over a minimum of 7 days is recommended. It is very important for owners to understand that success of nutritional management depends on maintaining this food as the sole source of nutrition for their cats.<\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Follow-Up<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">The <b>first recheck evaluation<\/b> should:<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Take place once the cat has been eating the new food exclusively for 4 weeks <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Include, at minimum, a physical examination and measurement of T<sub><span style=\"font-size: small\">4<\/span><\/sub>, blood urea nitrogen, serum creatinine, and urine specific gravity. <\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">All cats should have T<sub><span style=\"font-size: small\">4<\/span><\/sub> concentrations compared with previous baseline results; many will have returned to normal by the 4-week evaluation. Clinical improvements, such as weight gain, improved hair coat, and decreased tachycardia\/cardiac murmur, may also be noted. <\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">The <b>second recheck evaluation<\/b> should take place at 8 weeks.<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Clinical signs should show continued improvement by this evaluation. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Most cats will also be euthyroid at this point; however, some cats require additional time. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">90% of cats should have normal T<sub><span style=\"font-size: small\">4<\/span><\/sub> concentrations if the limited-iodine food is their sole source of nutrition.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\"><b>If euthyroidism is not achieved within 4 to 12 weeks, a thorough history is indicated to confirm that only the limited-iodine food is being fed.<\/b><\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Long-Term Monitoring<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">Once a euthyroid state is achieved, it is appropriate to continue patient monitoring indefinitely. <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>For stable patients without concurrent disease<\/b>, evaluations (including T<sub><span style=\"font-size: small\">4<\/span><\/sub>) can take place every 6 months during the recommended wellness examination. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">However, clinicians may schedule more frequent monitoring based on clinical judgment of individual patients\/owners. <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Hyperthyroid cats with concurrent diseases<\/b> generally should be evaluated at least every 3 to 4 months.<\/span><\/li>\n<\/ul>\n<div class=\"orange-box\">\n<h2 class=\"p3\"><span class=\"s1\"><b>HYPERTHYROID CATS WITH CONCURRENT KIDNEY DISEASE<\/b><\/span><\/h2>\n<p class=\"p1\"><span class=\"s1\">Because chronic kidney disease (CKD) and hyperthyroidism are often diagnosed in older cats, it is not surprising that many hyperthyroid cats have CKD. <\/span><\/p>\n<p class=\"p1\"><span class=\"s1\"><b>Diagnosis of CKD<\/b> is complicated by untreated hyperthyroidism because hyperthyrodism is associated with increased glomerular filtration rate (GFR) and, therefore, often masks biochemical markers of CKD. <\/span><\/p>\n<p class=\"p1\"><span class=\"s1\"><b>Decreased GFR<\/b>, increased serum urea and creatinine concentrations, and overt clinical signs of kidney disease have been reported after successful treatment of hyperthyroidism, regardless of therapeutic modality (methimazole, thyroidectomy, or radioactive iodine).<\/span><span class=\"s2\"><sup><span style=\"font-size: small\">4,30-33<\/span><\/sup><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\"><b>Prognosis<\/b> may be affected by the presence of underlying CKD (see <\/span><span class=\"s5\"><b>Effects of CKD in Cats with Hyperthyroidism<\/b><\/span><span class=\"s1\">). <\/span><\/p>\n<p class=\"p1\"><span class=\"s1\"><b>Treatment options<\/b>, especially those that are irreversible (thyroidectomy, radioactive iodine), should be considered carefully because it is impossible to consistently predict which cats will (1) develop overt CKD after hyperthyroidism treatment or (2) experience progression of kidney disease.<\/span><\/p>\n<p class=\"p1\"><span class=\"s1\"><b>Feeding a therapeutic renal food<\/b> is the only intervention shown to improve quality of life and prolong survival time in cats with naturally occurring CKD, regardless of the option chosen to treat hyperthyroidism.<\/span><span class=\"s2\"><sup><span style=\"font-size: small\">34,35<\/span><\/sup><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\"><b>A limited-iodine food<\/b> is a reasonable choice for managing cats with concurrent hyperthyroidism and CKD, especially those that do not have advanced kidney disease associated with uremic signs. <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">In cats with compromised renal function but no azotemia (IRIS Stage 1) being fed foods <i>high in protein and phosphorus<\/i>, decrease in GFR (associated with normalizing serum T<sub><span style=\"font-size: small\">4<\/span><\/sub> levels) may prevent effective clearing of protein metabolic by-products (blood urea nitrogen and creatinine).<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">These factors could contribute to occurrence of posttreatment azotemia in hyperthyroid cats.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">The limited-iodine food contains <i>controlled amounts of phosphorus<\/i>, <i>sodium<\/i>, and <i>protein<\/i>; therefore, the nutrient profile of this food, which is similar to foods for cats with early CKD, may offset the expected decrease in GFR associated with a normalizing serum T<sub><span style=\"font-size: small\">4<\/span><\/sub>.<\/span><\/li>\n<li><span class=\"s1\"><span class=\"s1\">Additional studies are needed to better understand the effects of using limited-iodine foods in hyperthyroid cats with concurrent kidney disease.<\/span><\/span><\/li>\n<\/ul>\n<hr \/>\n<h2><span class=\"s1\"><b>Effects of CKD in Cats with Hyperthyroidism<\/b><\/span><\/h2>\n<p class=\"p4\">\u2022 A shorter survival time in hyperthyroid cats with azotemia has been documented.<sup><span style=\"font-size: small\">7<\/span><\/sup><\/p>\n<p class=\"p4\">\u2022 The reported occurrence of azotemia after treatment of hyperthyroidism ranges from 15% to 49%.<sup><span style=\"font-size: small\">32-34,36,37<\/span><\/sup><\/p>\n<p class=\"p4\">\u2022 Two recent studies comparing survival of cats (after treatment for hyperthyrodism) that developed azotemia versus those that did not found no significant difference between the groups as long as the cats did not become hypothyroid posttreatment.<sup><span style=\"font-size: small\">38,39<\/span><\/sup><\/p>\n<p class=\"p4\">\u2022 Posttreatment iatrogenic hypothyroidism has been reported in cats after radioiodine therapy and bilateral thyroidectomy, which constituted the predominant therapeutic modalities in previous studies.<sup><span style=\"font-size: small\">37<\/span><\/sup><\/p>\n<p>\u2022 Cats with iatrogenic biochemical hypothyroidism were almost twice as likely to develop azotemia posttreatment compared to euthyroid cats, according to 1 recent study.<sup><span style=\"font-size: small\">38<\/span><\/sup><\/p>\n<ul>\n<li><em>Hypothyroid<\/em> cats with azotemia had shorter survival times than cats without azotemia.<\/li>\n<li>There was no difference in survival times of <em>euthyroid<\/em> cats with or without azotemia.<span class=\"s2\"><sup><br \/>\n<\/sup><\/span><\/li>\n<\/ul>\n<\/div>\n<h2 class=\"p3\"><span class=\"s1\">IN SUMMARY<\/span><\/h2>\n<p class=\"p3\"><span class=\"s1\">Hyperthyroidism is the most common endocrine disease of older cats worldwide. While pathogenesis is unclear, several effective management options are available, including thyroidectomy, radioactive iodine therapy, and antithyroid medications. In addition, feeding a limited-iodine food is now an available option for management of hyperthyroid patients. All options should be discussed with pet owners to allow the best option to be selected for individual patients and their owners.<\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">CKD = chronic kidney disease; DMB = dry matter basis; free thyroxine by equilibrium dialysis = fT<sub><span style=\"font-size: small\">4<\/span><\/sub>ED; GFR = glomerular filtration rate; GI = gastrointestinal; thyroxine = T<sub><span style=\"font-size: small\">4<\/span><\/sub>; TSH = thyroid-stimulating hormone<\/span><\/p>\n<h3 class=\"p5\"><span class=\"s1\"><strong>References<\/strong><br \/>\n<\/span><\/h3>\n<p class=\"p5\"><span class=\"s1\">1. Broussard JD, Peterson ME, Fox PR. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. <em>JAVMA<\/em> 1995; 206:302-305.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">2. Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. <em>JAVMA<\/em> 2001; 218:529-536.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">3. Peterson ME, Graves TK, Cavanagh I. Serum thyroid hormone concentrations fluctuate in cats with hyperthyroidism.<em> J Vet Intern Med<\/em> 1987; 1:142-146.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">4. Wakeling J, Moore K, Elliott J, et al. Diagnosis of hyperthyroidism in cats with mild chronic kidney disease. <em>J Small Anim Pract<\/em> 2008; 49:287-294.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">5. Peterson ME. Radioiodine for feline hyperthyroidism. In Bonagura JD,Twedt DC (eds): <em>Kirk&#8217;s Current Veterinary Therapy (Small Animal Practice).<\/em> Philadelphia: Elsevier, 2008, pp 180-184.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">6. Mooney CT. Hyperthyroidism. In Ettinger SJ, Feldman EC (eds): <em>Textbook of Veterinary Internal Medicine<\/em>. St. Louis: Elsevier, 2010, pp 1761-1779.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">7. van Dijl IC, Hof AJ. Treatment of feline hyperthyroidism with radioactive iodine-131. <em>Tijdschr Diergeneeskd<\/em> 2008; 133:54-62.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">8. Peterson ME, Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. <em>JAVMA<\/em> 1995; 207:1422-1428.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">9. Theon AP, Van Vechten MK, Feldman E. Prospective randomized comparison of intravenous versus subcutaneous administration of radioiodine for treatment of hyperthyroidism in cats. <em>Am J Vet Res<\/em> 1994; 55:1734-1738.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">10. Meric SM, Hawkins EC, Washabau RJ, et al. Serum thyroxine concentrations after radioactive iodine therapy in cats with hyperthyroidism. <em>JAVMA<\/em> 1986; 188:1038-1040.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">11. Mooney CT. Radioactive iodine therapy in feline hyperthyroidism. <em>Vet Rec<\/em> 1990; 127:555.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">12. Nykamp SG, Dykes NL, Zarfoss MK, et al. Association of the risk of development of hypothyroidism after iodine 131 treatment with the pretreatment pattern of sodium pertechnetate Tc 99m uptake in the thyroid gland in cats with hyperthyroidism: 165 cases (1990-2002). <em>JAVMA<\/em> 2005; 226:1671-1675.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">13. Williams TL, Elliott J, Syme HM. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. <em>J Vet Intern Med<\/em> 2010; 24:1086-1092.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">14. Hill KE, Gieseg MA, Kingsbury D, et al. The efficacy and safety of a novel lipophilic formulation of methimazole for the once daily transdermal treatment of cats with hyperthyroidism. <em>J Vet Intern Med<\/em> 2011; 25:1357-1365.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">15. Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. <em>J Vet Intern Med<\/em> 1988; 2:150-157.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">16. Sartor LL, Trepanier LA, Kroll MM, et al. Efficacy and safety of transdermal methimazole in the treatment of cats with hyperthyroidism. <em>J Vet Intern Med<\/em> 2004; 18:651-655.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">17. Trepanier LA. Pharmacologic management of feline hyperthyroidism. <em>Vet Clin North Am Small Anim Pract<\/em> 2007; 37:775-788.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">18. Mooney CT, Thoday KL, Doxey DL. Carbimazole therapy of feline hyperthyroidism. <em>J Small Anim Pract<\/em> 1992; 33:228-235.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">19. Trepanier LA, Hoffman SB, Kroll M, et al. Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism. <em>JAVMA<\/em> 2003; 222:954-958.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">20. Buijtels JJ, Kurvers IA, Galac S, et al. Transdermal carbimazole for the treatment of feline hyperthyroidism. <em>Tijdschr Diergeneeskd<\/em> 2006; 131:478-482.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">21. Frenais R, Rosenberg D, Burgaud S, et al. Clinical efficacy and safety of a once-daily formulation of carbimazole in cats with hyperthyroidism. <em>J Small Anim Pract<\/em> 2009; 50:510-515.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">22. Melendez L, Yamka R, Forrester S, et al. Titration of dietary iodine for reducing serum thyroxine concentrations in newly diagnosed hyperthyroid cats. <em>J Vet Intern Med<\/em> 2011; 25:683 (abstract).<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">23. Melendez L, Yamka R, Burris P. Titration of dietary iodine for maintaining normal serum thyroxine concentrations in hyperthyroid cats. <em>J Vet Intern Med<\/em> 2011; 25:683 (abstract).<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">24. Yu S, Wedekind K, Burris P, et al. Controlled level of dietary iodine normalizes serum total thyroxine in cats with naturally occurring hyperthyroidism. <em>J Vet Intern Med<\/em> 2011; 25:683-684 (abstract).<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">25. Johnson LA, Ford HC, Tarttelin MF, et al. Iodine content of commercially-prepared cat foods. <em>N Z Vet J<\/em> 1992; 40:18-20.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">26. Mumma RO, Rashid KA, Shane BS, et al. Toxic and protective constituents in pet foods. <em>Am J Vet Res<\/em> 1986; 47:1633-1637.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">27. Zicker SC, Nelson RW, Kirk CA, et al. Endocrine disorders. In Hand MS, Thatcher CD, Remillard R, et al (eds): <em>Small Animal Clinical Nutrition<\/em>. Topeka, Kansas: Mark Morris Institute, 2010, pp 559-580.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">28. Wedekind KJ, Kirk CA, Nachreiner R. Methods for preventing feline hyperthyroidism in a cat and composition comprising limited iodine. World Intellectual Property Organization, 2004, pp 1-29. <\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">29. Ranz D, Tetrick M, Opitz B, et al. Estimation of iodine status in cats. <em>J Nutrition<\/em> 2002; 132:S1751-S1753.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">30. Boag AK, Neiger R, Slater L, et al. Changes in the glomerular filtration rate of 27 cats with hyperthyroidism after treatment with radioactive iodine. <em>Vet Rec<\/em> 2007; 161:711-715.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">31. Adams WH, Daniel GB, Legendre AM. Investigation of the effects of hyperthyroidism on renal function in the cat. <em>Can J Vet Res<\/em> 1997; 61:53-56.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">32. Adams WH, Daniel GB, Legendre AM, et al. Changes in renal function in cats following treatment of hyperthyroidism using 131I. <em>Vet Radiol Ultrasound<\/em> 1997; 38:231-238.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">33. Graves TK, Olivier NB, Nachreiner RF, et al. Changes in renal function associated with treatment of hyperthyroidism in cats. <em>Am J Vet Res<\/em> 1994; 55:1745-1749.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">34. Elliott J, Rawlings JM, Markwell PJ, et al. Survival of cats with naturally occurring chronic renal failure: Effect of dietary management. <em>J Small Anim Pract<\/em> 2000; 41:235-242.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">35. Ross SJ, Osborne CA, Kirk CA, et al. Clinical evaluation of dietary modification for treatment of spontaneous chronic kidney disease in cats. <em>JAVMA<\/em> 2006; 229:949-957.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">36. Becker TJ, Graves TK, Kruger JM, et al. Effects of methimazole on renal function in cats with hyperthyroidism. <em>JAVMA<\/em> 2000; 36:215-223.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">37. Williams TL, Peak KJ, Brodbelt D, et al. Survival and the development of azotemia after treatment of hyperthyroid cats. <em>J Vet Intern Med<\/em> 2010; 24:863-869.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">38. Williams TL, Elliott J, Syme HM. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. <em>J Vet Intern Med<\/em> 2010; 24:1086-1092.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">39. Wakeling J, Rob C, Elliott J, et al. Survival of hyperthyroid cats is not affected by post-treatment azotemia. <em>ECVIM-CA Congress Proc<\/em>, 2006, p 1523.<\/span><\/p>\n<p class=\"p5\"><span class=\"author-bio\"><b><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2015\/11\/Author_D-Bruyette.jpg\"><img decoding=\"async\" class=\"alignleft size-full wp-image-6706\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2015\/11\/Author_D-Bruyette.jpg\" alt=\"Author_D-Bruyette\" width=\"100\" height=\"129\" \/><\/a>David Bruyette<\/b>, DVM, Diplomate ACVIM, is the medical director at West Los Angeles Animal Hospital and a clinical professor in the Department of Radiation Oncology at University of California\u2014Los Angeles. Prior to his current positions, he was an assistant professor and head of internal medicine at Kansas State University and director of its Analytical Chemistry Laboratory. Dr. Bruyette received his DVM from University of Missouri and completed an internship at Purdue University and residency in internal medicine at University of California\u2014Davis. He then became a staff internist at West Los Angeles Veterinary Medical Group and member of the Department of Comparative Medicine at Stanford University.<\/span><\/p>\n<p class=\"p5\"><span class=\"s1\">\u00a0<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"<p>David Bruyette, DVM, Diplomate ACVIM Hyperthyroidism is recognized as the most common endocrinopathy of older cats.<\/p>\n","protected":false},"author":1,"featured_media":2694,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"iawp_total_views":3821,"footnotes":""},"categories":[376],"tags":[13],"class_list":["post-1374","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-january-february-2013","tag-peer-reviewed","column-features","clinical_topics-endocrinology"],"acf":{"hide_sidebar":false,"hide_sidebar_ad":false,"hide_all_ads":false},"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v24.7 (Yoast SEO v27.3) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Feline Hyperthyroidism: Diagnosis &amp; 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