Hyperthyroidism in Cats

by E-NewsCast Team
June 21st, 2010

DiseaseHyperthyroidism is caused by a tumor, usually benign, of thethyroid gland that secretes excessive amounts of thyroidhormone. This has many effects on the body since the thyroidcontrols many basic body functions such as heart rate andcontraction, appetite, weight control, heat tolerance, waterconsumption, to name a few. When a cat is hyperthyroid, thesefunctions become exaggerated. Clinical signsIn most cases, they eat with exceptionally good appetites butlose weight. They drink and urinate more. Frequent vomiting(several times a month) and/or diarrhea often occur. Their heartbeats faster, and the heart muscle can become thickened(hypertrophied), a state that can lead to heart failure. Highblood pressure can occur, and hypertension can lead to furtherstrain on the heart and eye problems (such as detached retinas).Behavior changes such as nervousness, restlessness or evenaggression can result. If left untreated, this condition willresult in a progressive weight loss and overly stressed organs.DiagnosisCats with these signs are suspected of being hyperthyroid andsometimes an enlarged thyroid gland can be felt along the bottompart of the neck alongside the windpipe. However, the diagnosisis made on the basis of a blood test. A complete panel and urinalysis are helpful to rule outconcurrent diseases, and it is specifically a T4 level orsometimes, a free T4 (done by a different laboratory technique)that diagnoses this condition. Assessment of the cat’s blood pressure is recommended sincehypertension is a common complication of hyperthyroidism.Sometimes, this resolves with the treatment of the hyperthyroidstate. However, sometimes, it necessitates specific medicationto control. As with humans, high blood pressure has severalnegative side-effects including additional stress on the heartand kidneys and the risk for sudden blindness due to retinaldetachment. Once your cat’s condition has been diagnosed, your veterinarianwill discuss treatment options. There are 3 treatments, eachwith advantages and disadvantages.MedicationThe most commonly used medication is methimazole (Tapazole).This can be prescribed through a human pharmacy as a tablet or aveterinary compounding pharmacy as a flavored chewable tablet ora flavored liquid, given orally, or as a transdermal form (apaste applied topically to inside the ear). Depending on thecat’s starting T4 levels and the cat’s response to themedication, it is given 1-2 times a day. Side effectsNegative reactions to the medication can occasionally occur:decreased appetite (some leveling off from a ravenous appetiteis normal after starting treatment), vomiting, diarrhea anditchiness of the head and neck. Adverse reactions, if theyoccur, usually happen in the first 3 months of treatment. MonitoringAfter starting medication, at minimum, a blood test of T4 levelswill need to be rechecked every 3-4 weeks until control isachieved. This is not a time tested and can be done at any timein relation to when the medication was given. It is ideal todo a complete blood panel that includes a T4 level since thisallows for evaluation of the cat’s other organs. Liver enzymesare often increased with hyperthyroidism; they should go backinto the normal range with treatment of hyperthyroidism. Whenthey remain increased or continue to go up, there is concern forconcurrent liver disease. The kidney values are evaluated alsosince some cats will also have mild kidney failure that is”masked” by the hyperthyroid state. In these cases, cats willonly “show” their kidney problems once the hyperthyroidism istreated. It also measures platelets and red blood cell countsthat are sometimes decreased in numbers by the medication. ControlControl of hyperthyroidism is achieved when the cat is doingwell – no longer vomiting or having diarrhea, is no longerlosing weight and is showing no ill effects from the medicationand when the lab values are within normal. For most cats, thiscan include blood tests every 3-4 weeks until the proper dosageis attained. Long term maintenanceFor long term maintenance, a complete physical exam and completeblood panel including T4 and urinalysis and blood pressuremeasurement are recommended for every 6-12 months thereafter. Pros of medicationIt is relatively inexpensive. Depending on what formulation, howoften the medication must be given and which pharmacy is used,costs can vary from $35-$60/month. Cons of medicationThe cost of the medication will accumulate over the cat’slifetime since this is a long term treatment. A cat will notbecome “unhyperthyroid” and thus, is dependent on the Tapazoleto suppress the overly active thyroid gland. If the medicationis stopped, the cat becomes hyperthyroid again, and the diseaseprocess continues. Blood tests need to be done at regular intervals to ensurecontinued control of the hyperthyroidism, and these laboratorycosts can accumulate (depending on which lab tests are done, thecost can vary from $50-$100 each time). Frequency of testingdepends on how the cat responds to the medication and presenceof concurrent diseases.Treatment with radioactive I 131This treatment permanently treats the cat’s condition byeliminating the hyperactive thyroid tissue. This radioisotopeworks specifically on the thyroid gland and doesn’t affect theother organs. This treatment can be done at any time during ahyperthyroid cat’s life, even after initiation of treatment withtapazole. Recent blood work and urinalysis is required beforereferral for this treatment. If a patient has been on tapazolefor more than 90 days, stopping the drug for 7 days prior toI131 treatment is required. Long term maintenance For long term maintenance, treated catsare recommended to have their T4 levels monitored the first andthird months post treatment. A complete physical exam with acomplete blood panel including T4 and urinalysis and bloodpressure measurement are recommended for every 6-12 monthsthereafter.Pros of I131It is a permanent treatment, and it avoids the need for dailymedication with Tapazole and eliminates the need for frequentlab testing. It is the treatment of choice for cats that areexperiencing adverse effects to Tapazole.Cons of I131The treatment is done at only a few facilities with thenecessary equipment (a referral can be made for hospitals in thearea providing this service). It can be costly. Total treatment costs are usually around$850-1000. This includes a scan using technetium (not done atall facilities), the I131 injection, general health andradiation monitoring and the stay at the hospital. After treatment, since the cat and the cat’s wastes (urine andfeces) are radioactive, they need to be hospitalized for aperiod of time until their levels drop to an acceptable level.The usual stay in the hospital is 4-10 days during which the catcannot be visited. Even after discharge, for a period oftime-usually 2 weeks, there are certain precautions to befollowed such as limiting close contact and extra care withremoval of urine and feces from the litter box. While usually only one treatment with I 131 is needed,occasionally, a second treatment is needed. This is needed inless than 2% of cats. Almost all cats are cured after a secondtreatment. Also, while most of cats that have received I 131 will havenormal thyroid function after treatment, there are a few catsthat have too low thyroid levels (hypothyroid) after treatment.This can occur in less than 5% of cats. If these cats show signsof low energy levels and dull coats with dandruff, these catswill need thyroid supplementation. This medication, thyroxine(Soloxine), is a long term treatment. However, it is lessexpensive than Tapazole, and fewer medical problems arise fromhypothyroidism compared to hyperthyroidism.SurgerySurgery can be done to remove the hyperthyroid gland(thyroidectomy). This approach is usually recommended in caseswhere the thyroid tumor is malignant. Long term maintenanceFor long term maintenance, treated cats are recommended to havetheir calcium levels monitored daily the first 5-7 days postsurgery. A complete physical exam with a complete blood panelincluding T4 and urinalysis and blood pressure measurement arerecommended for every 6-12 months thereafter.Pros of SurgeryIt is usually a permanent treatment. Cons of SurgeryThese cats are often at risk with anesthesia. Occasionally, thehyperthyroid tissue can re-grow, resulting in recurrence of thehyperthyroidism, months to years after a successfulthyroidectomy. Occasionally, hypothyroidism can result aftersurgical removal, thus necessitating thyroid supplementation. Occasionally, inadvertently, the parathyroid glands, responsiblefor control of calcium and phosphorus levels of the body, can beremoved during surgery to result in low blood calcium levels, alife-threatening problem that can result in seizures and musclespasms. This condition is usually temporary, and calciumsupplementation can help treat this, but calcium levels must beclosely monitored post-operatively.Ablation This relatively new technique is done only at UC Davis School ofVeterinary Medicine where ultrasound-guided ablation of thehyperthyroid tissue is done by an injection of ethanol. Thetechnique is considered safe with relatively few side-effects.PrognosisLike most other diseases, hyperthyroidism is best diagnosed andtreated in its early stages. The prognosis depends on the cat’scondition at the time of diagnosis, its response to treatmentand the presence of concurrent diseases such as kidney failure,diabetes, liver disease and cancer.Whatever mode of treatment that is chosen to treat your cat’shyperthyroidism, it is important that you consistently givemedication, follow up for necessary tests for monitoring yourcat and notice changes in your cat’s overall demeanor, energylevels, appetite, water consumption and urine output. It is important you work closely with your veterinarian infollowing up with the appropriate tests and advice to ensurecontrol of your cat’s disease. Together, you can provide a goodquality of life for your cat.If you have any questions regarding this disease or itstreatment, don’t hesitate to contact your veterinarian.The above is general veterinary information. Do not beginany course of treatment without consulting your regularveterinarian. All animals should be examined at least once every12 months.

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