Thethyroidhormone,triiodothyronine(T3),isproducedbythethyroidgland.Animportantcomponentinthesynthesisisiodine.ThyroxineisconvertedtotheactiveT3(threetofourtimesmorepotentthanT4)withincellsbydeiodinases(5"-iodinase).Thyroxine-bindingglobulin(TGB)isthemajorcarrierproteinforcirculatingthyroidhormone.Onlyaverysmallfractionofthecirculatinghormoneisfree(unbound)0.3%.

hisfractionisBIOLOGicallyactive.Thus,measurementsoffreetriiodothyronineconcentrationscorrelatemorereliablywithclinicalstatusthantotaltriiodothyroninelevels.Forexample,theincreaseintotaltriiodothyroninelevelsassociatedwithpregnancy,oralcontraceptivesandoestrogentherapyresultinhighertotalT3levelswhilethefreeT3(FT3)concentrationremainsbasicallyunchanged.Theconcentrationsofthecarrierproteinsarealteredinmanyclinicalconditions,suchaspregnancy.Innormalthyroidfunctionastheconcentrationsofthecarrierproteinsalters,thetotaltriiodothyroninelevelchangessothatthefreetriiodothyronineconcentrationremainsconstant.ThebindingofT3playsakeyroleinthefeedbackcontrolofthethyroid,withFT3actingonthepituitarytoinhibitthyroidhormonesecretion.Thethyroninesactonthebodytoincreasethebasalmetabolicrate,affectproteinsynthesisandincreasethebody"ssensitivitytocatecholamine(suchasadrenaline).Thethyroidhormonesareessentialtoproperdevelopmentanddifferentiationofallcellsofthehumanbody.Thesehormonesalsoregulateprotein,fat,andcarbohydratemetabolism,affectinghowhumancellsuseenergeticcompounds.Numerousphysiologicalandpathologicalstimuliinfluencethyroidhormonesynthesis.Thyrotoxicosisorhyperthyroidismistheclinicalsyndromecausedbyanexcessofcirculatingfreethyroxine,freetriiodothyronine,orboth.BothT3andT4areusedtotreatthyroidhormonedeficiency(hypothyroidism).Sinceconditionssuchaspregnancy,oestrogentherapyandothernon-thyroidfactorsalterTBGconcentrations,assessmentofthyroidfunctionthroughtotalT3measurementmayresultinanerroneousdiagnosis,becauseFT3levels,areunaffectedbybindingproteinchanges.