Hyperthyroidism:InformationforCatOwners
Whatishyperthyroidism?
Yourcat,likeothermammalsincludinghumans,hasapairofthyroidglandslocatedin
theneckarea.Theseglandscontrolyourcat’smetabolicratebyproducingthehormone
thyroxine(T4).Hyperthyroidismoccursinmanyoldercatswhenbenign(notcancerous)
growthsinthethyroidglandsstartproducingexcessiveamountsofthyroxine.Theexcessive
levelsofthyroxinecanbedamagingtomanybodysystems,includingtheheart.
Themostcommonsignsofhyperthyroidismareweightloss,increasedappetite,
vomiting,diarrhea,increasedthirst,increasedurination,increasedactivity,sheddingorchanges
inhaircoat,vocalization,agitation,andpanting.Somecatsmaybecomelethargicandhave
poorappetites.Onphysicalexam,yourveterinarianmaynoticeincreasedheartrateoran
irregularheartrhythm,oranenlargedthyroidnodule.Hyperthyroidismismostoften
diagnosedbyelevatedT4levelinbloodwork.Bothmaleandfemalecatscanbeaffected,and
theaverageageattimeofdiagnosisis12years.
Howcanhyperthyroidismbetreated?
Ifhyperthyroidismgoesuntreated,itcanbeafataldisease.Youhave4mainoptionsfor
treatmentofyourhyperthyroidcat;itisimportanttoconsideralltheoptionsandchoosewhat
isbestforyourowncat.However,expertsagreethatradioactiveiodinetherapyisthebest
optionforalmostallcats.
1. Medicaltreatment:AntithyroidDrugs(Methimazole,Tapazole)
Advantages:relativelyinexpensiveshortterm,nohospitalization,noriskof
hypothyroidism,nohypoparathyroidism
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Disadvantages:Priceofmedicationoftenincreases,andwhencalculatedoverlifetime
ofanimal,inadditiontodrugmonitoringbloodwork,costsaccumulate.Inaddition,
medicationsmustbegiventwicedaily.Relapsesmayoccurduringtreatment,andsome
catsdonotrespondtomedication.Approximately25%ofcatswillexperienceside
effectsrelatedtothemedication,includinglethargy,anorexia,orvomiting.Asmall
numberofcatsmayalsoexperiencemoreserioussideeffects,includingfacialitching,
liverfailure,andsuppressionofbonemarrow.Bloodworkisrequiredevery3-6months,
andregardlessoftreatmentsuccess,thediseaseisnevercured.Medicaltreatmentwill
belifelong.
2. Dietarytherapy–Hillsy/d–Thisisaspecialdietdevelopedwitharestrictedlevelof
iodine.
Advantages:Nomedicationnecessary.Goodsuccessratesreported.
Disadvantages:Fairlynew,longtermeffectsofrestrictediodineunknown.Canbe
difficulttomanageinmulti-cathouseholds.Mustbeverycarefulthatdietisonlyfood
forcat–noextratreats,evensomemedicationscancontainiodineifflavored.Requires
lifelongtherapy,notacure.
3. SurgicalTreatment:Thyroidectomy
Advantages:Usuallysuccessful,persistenthyperthyroidismrare,correctshyperthyroid
statein1-2days,andusuallyresultsinpermanentcure
Disadvantages:Thiscanbeexpensive,hospitalizationisrequiredforafewdays,andrisk
ofanesthesiacanbesignificantwithseniorcats,duetohighpotentialforcardiovascular
andrenaldisease.Itisadifficultsurgery,asallaffectedthyroidtissuemustbe
identified,andtheparathyroidglands,whichregulatecalciumbalance,caneasilybe
damagedorremoved.Otherpotentialcomplicationsincludenerveinjury,whichcan
resultinHorner’ssyndromeorvocalfoldparalysis.
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4. RadioactiveIodine–I-131
Advantages:Thismethodhasthehighestcurerateforhyperthyroidism,usuallyupwards
of95-98%.Regardlessoftumorlocation,thismethodwilldestroyallthyroidtumor
tissue.Thisisusuallyapermanentcurerate,withaverylowrecurrencerate.
Treatmentissimple–asinglesubcutaneousinjection,nopilling,noanesthesia,no
seriouscomplicationsorsideeffects.Limitedmonitoringwillberequiredafter
treatment,andinthemajorityofcases,noongoingtherapywillbeneeded.
Disadvantages:Althoughthisisaverysimpleandsafemethodoftreatment,therearea
fewthingstoconsider.Radioiodinetherapyismoreexpensiveshorttermthan
medicationssuchasTapazole,althoughyoumustremembertoaccountforongoing
expenseswithmedications.Hospitalizationofyourcatisarequirement,usually
between4daysand1week,andyouwillnotbeabletovisitforthedurationof
hospitalizationduetofederalregulations.Forashorttimeafterdischarge,yourcatwill
stillbeemittingradiation;youwillbegivenspecificinstructiontofollowtoensureyour
safety,butyourcatwillhavetoremainindoorsforaperiodoftimeafterdischarge.If
yourcatdoesnotreliablyusethelitterbox,theymaynotbeagoodcandidatefor
radioiodine.
Whenconsideringalloftheadvantagesanddisadvantages,itisclearthatradioiodineis
thebestsolutionfortreatinghyperthyroidism.Withasingleinjection,agreaterthan
95%cureratecanbeachieved.Althoughtheideaofbeingseparatedfromyourcatfor
anentireweekmaybedifficult,imaginetheyearsofgoodqualityoflifeyoucangive
yourfelinefriend.Aftertreatment,thyroidfunctionwillreturntonormalover1-4
weeks,andtherearenolong-termsideeffectsfromtheradioiodine.Yourcatisnever
toooldtoundergotreatmentforhyperthyroidism,andunlesssufferingfromserious
heartdisease,kidneydiseaseorotherlife-threateningsystemicdisease,anycatisa
goodcandidateforthistherapy.
HowshouldIpreparemycatfortreatment?
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Youandyourveterinarianwillworktogethertoensureallrequiredinformationisavailable
priortotreatment.Bloodworkandurinetestsshouldbecompletedwithin30daysof
treatment.IfyourcathasbeenstartedonTapazole(methimazole)priortoradioiodine
therapy,youshouldstopmedications2weeksbeforetreatmentandaT4levelshouldbetaken
5-7daysafterstoppingtreatment.Forcatsthathaveheartdiseaseorotherseriousillness
withoutmedication,orhavebeentreatedwithTapazoleforseveralyears,wewillworkwith
youandyourregularveterinariantoformulateanappropriatetreatmentplan.Other
supplementsshouldalsobediscontinued2weekspriortotreatment.Ifyoucatdoesrequire
dailymedications,therewillbeadditionalcostsfordailycare.
Howlongwillmycathavetoremaininhospital?
Bylaw,yourcatmustremaininhospitaluntilheorsheisatasafeandlegalradiationlevel.The
averagelengthofstayis7days,whichmaybeextendedasrequired.TheRadioiodinetherapy
roomisfoundinaquietcornerofournew,stateoftheartfacility,andwewilldoourbestto
keepyourcathappyduringhisorherstay.Youcanbringtoysandblanketstomakethemfeel
athome;however,nothingwillbereturnableaftertreatmentduetoradiationlegislation.We
suggestyoubringyourpet’sfavoritefoodtoencourageagoodappetitewhileinhospital.We
havearadiotoplaymusictokeepthecatsentertained,anddailyexerciseoutsideofthekennel
willbeprovided.Yourcatwillbecheckedontwicedailybytrainedtechniciansand
veterinarians.Rememberthatthetimeawaywillbeeasierforyourcatthanforyou,andthe
long-termbenefitsshouldoutweighanyconcernsyouhave.Wewillcalloremailyoudailywith
updates,butnovisitingispermittedduringthehospitalizationperiod.
Whatkindofpost-treatmentcarewillIhavetoprovide?
Afterreturninghome,yourcatwillhaveasmallamountofradiationremaininginthethyroid
gland.Wewillprovidedetailedinstructionsonhomecareforthefirst10daysafterdischargeto
ensuresafetyforyourselfandotherfamilymembers.Yourcatwillnotbedangeroustoother
petsinthehousehold,butmustremaininsideforaspecifiedtimetopreventenvironmental
contamination.Youwillbeinstructedonspecialhandlingforlitter.Youwillhavetolimitclose
contactsuchassleepingwithyourpetforthefirst10days,andpregnantwomenorchildren
under12yearsshouldnothavecontactwithyourpetforthisperiod.Theseprecautionsarefor
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yoursafety,andyoushouldrememberthatthedoseofradiationemittedfromyourpetatthe
timeofdischargeismuchlessthantheradiationreceivedfromadentalxray,adayatthe
beach,oralongflight.Ifyouareuncomfortablewiththisaftercare,boardingcanbearranged
foryourpet.
Aftertreatment,youshouldschedulefollow-upswithyourregularveterinarianat1and3
monthstomonitorT4levels.
Whattreatmentcomplicationscanoccur?
Lessthan3%oftreatedcatswillbecomehypothyroidaftertreatmentwithradioiodine.This
tendstooccurincatsonantithyroidmedicationsatthetimeoftreatment,orifthethyroid
tumorisinvolvingsomuchofthethyroidglandthatnotenoughnormaltissueremainsto
producesufficientlevelsofT4.Hypothyroidism,unlikehyperthyroidism,isnotalifethreateningdisease.Clinicalsignsincludeweightgain,lethargyandpoorhaircoat.Ifclinical
signsandalowT4occurinyourcat,heorshecanbegivenadailythyroidsupplement.This
supplementismoreeasilytoleratedbycatsthanTapazole,isinexpensive,hasfewerside
effects,andrequireslessmonitoring.
Hyperthyroidismcan,inaverysmallnumberofcases,recurincats.IfT4levelsdonotreturnto
normalwithin90daysoftreatment,wewillretreatyourcatatnocharge–thiswillrequirea
lowerdosethantheinitialtreatmentandashorterhospitalizationtime.Iftherecurrenceof
hyperthyroidismisnottheresultofatreatmentfailure,butaregrowthoftumortissueonthe
thyroidgland,treatmentcanbedoneagainwithnoadditionalrisktothecat.Thisusually
happenswhencatsarediagnosedandtreatedforthefirsttimeatanearlyageandisa
reflectionofincreasedsurvivaltimeinthesecats–tumorregrowthismostcommon3ormore
yearsafterinitialtreatment.Evenintheseveryrarecases(0.5%),treatmentiscosteffective
comparedtoprolongedtreatmentwithmedications.
Pleasecontactusshouldyouhaveadditionalquestionsaboutradio-iodinetherapyfor
yourcat.
550PineGlenRoad,Riverview,NB.E1B4X2-(506)387-4015(506)387-7656fax
www.riverviewanimalhospital.ca,
[email protected]