hGHisapolypeptidehormone(molecularweight21500Da)producedbytheacidophilcellsoftheanteriorpituitaryunderthecontroloftwomainsubstancesfromthemedianeminence:Growth-hormoneReleasingFactor(GRF)andaninhibitoryagent,somatostatin.Dopaminergic,adrenergicandSEROtoninergicneuroendocrinepathwaysalsoplayanimportantroleinthecontrolofhGHsecretion.ExcitatorystimuliofhGHsecretionincludehypoglycemia,exercise,fasting,mealswithahighproteincontent,deepsleep,stress,glucagon,LDopa,aminoacids,etc.Inhibitorystimuliincludeglucose,cortisol,hGHandfreefattyacids.Becauseofitsshortplasmahalflife(±25minutes)andofthefrequentexcitatoryorinhibitorystimuli,hGHdisplaysfrequentandlargevariationsofconcentrationinserum.OneofthemainphysiologicalfunctionsofhGHistoactontheliverandothertissuestoproducesomatomedins,whichinturninducegrowthbydirectactionontargettissues.IncontrasttohGH,theconcentrationofsomatomedinsinserumiskeptstablebyvirtueofbeinglargelyboundtocirculatingplasmaproteins.ClinicalapplicationGrowthretardation:hGHhyposecretionisoneofthevariouscausesofsmallstatureinchildren.SerumhGHmeasurementwithahighlysensitiveassay,especiallyfollowingaprovocativestimulus(absenceofresponse),isanimportantwaytoestablishthisdiagnosisbecausethisgroupofpatientscanbetreatedbyadmiNISTrationofhGH.Hypopituitarism:SerumhGHmeasurementisalsoanindexofpituitaryfunctionwhenhypopituitarism(eitheridiopathicorduetotumourandsurgery)issUSPected.Gigantismandacromegaly:SerumhGHmeasurement,especiallyfollowingaprovocativeinhibitorytest(absenceofresponse),isanimportantwaytoestablishthediagnosisofhGHhypersecretionduetoacidophilicpituitarytumour.Thisresultsingigantisminchildrenandacromegalyinadults.BothofthesedisordersmaybetreatedbysurgeryorrADIation.