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1、EUTHYROIDSICKSYNDROMEMichaelT.McDermottCHAPTER391.Whatiseuthyroidsicksyndrome?Euthyroidsicksyndromereferstochangesinserumthyroid-stimulatinghormone(TSH),serumthyroidhormone,andtissuethyroidhormonelevelsthatoccurinpatientswithvariousnonthyroidalillnessesandstarvation.Itisnotaprimarythyroiddisorderbu
2、tinsteadresultsfromchangesinthyroidhormonesecretion,transport,andmetabolisminducedbythenonthyroidalillness.2.Whathormonechangesoccurinpatientswithmildtomoderatenonthyroidalillnesses?Serumtotaltriiodothyronine(T3)andfreeT3andtissueT3levelsdecreaseasaresultofreducedconversionofthyroxine(T4)toT3inperi
3、pheraltissues,predominantlytheliver,wherehepaticdeiodinasetypeI(D1)activityisdecreased.SerumfreeT4andTSHlevelsusuallyremainwithinthereferencerangeinthemildestformofthiscondition.3.Describethehormonechangesinpatientswithmoderatetoseverenonthyroidalillnesses.SerumtotalT3andfreeT3levelsdecreasefurther
4、;totalT4alsodecreases,andtheT3resinuptake(T3RU)increases.Thelatterchangesresultfromreducedbindingofthyroidhormonestotheirtransportproteinsbecauseofbothimpairedproteinsynthesisandthepresenceofcirculatinginhibitorsofproteinbinding.SerumTSHlevelsremainnormalorbecomedecreasedatthisstage.FreeT4maybenorm
5、al,decreased,orincreased.4.Describethehormonechangesassociatedwithrecoveryfromnonthyroidalillnesses.FreeT4decreases,andTSHincreases.Ashepaticproteinsynthesisimprovesandcirculatinginhibitorsofproteinbindingdisappear,serumfreeT4levelsdroptransientlywithacompensatoryincreaseinserumTSHlevelsbeforecompl
6、etenormalizationoccurs.SerumT3levelsalsoeventuallynormalize.5.Howcaneuthyroidsicksyndromebedistinguishedfromhypothyroidism?Ineuthyroidsicksyndrome,serumT3isdecreasedproportionatelymorethanT4,theT3RUishigh,andTSHisnormalormildlydecreasedandthenmildlyincreasedduringrecovery.Inprimaryhypothyroidism,se
7、rumT4isreducedproportionatelymorethanT3,theT3RUislow,andTSHisincreased.Othertestsmayalsobehelpful.Ineuthyroidsicksyndrome,freeT4isusuallynormalandreverseT3(rT3)isincreased;inhypothyroidism,bothfreeT4andrT3a
