OBJECTIVES:　Patients　with　hyperthyroidism　frequently　have　neuropsychiatric　complaints　such　as　lack　of　concentration,　poor　performance　in　memory,　depression,　anxiety　and　mania.　These　symptoms　suggest　the　dysfunction　of　brain.　However,　the　underlying　process　of　this　dysfunction　is　not　well　understood.　At　the　same　time,　glutamatergic　system　has　been　considered　important　in　neuropsychiatric　process　by　recent　studies.　Thus,　this　study　is　to　investigate　the　change　of　glutamate　concentration　in　patients　with　hyperthyroidism　using　proton　magnetic　resonance　spectroscopy.　METHODS:　Fifteen　untreated　patients　with　hyperthyroidism　and　fifteen　age-　and　gender-　matched　controls　participated　in　the　study.　The　region　of　the　posterior　cingulate　cortex　was　examined　by　magnetic　resonance　spectroscopy　with　a　technique　referred　as　TE-averaged　PRESS　at　3T　field　strength.　The　concentrations　of　N-Acetylaspartate,　creatine,　choline　and　glutamate　were　assessed　using　jMRUI　v4.0　software.　RESULTS:　Hyperthyroid　patients,　compared　with　controls,　showed　a　decrease　of　glutamate　concentration　(P＜0.047)　and　glutamate/creatine　ratios　(P＜0.009)　in　the　posterior　cingulate　cortex.　The　decrease　of　choline　concentration　(P＜0.004)　and　choline/creatine　ratios　(P＜0.012)　were　also　discovered.　No　significant　difference　was　found　in　the　concentrations　of　N-Acetylaspartate　or　creatine　between　patients　and　controls.　CONCLUSION:　Concentration　of　glutamate　decreased　in　the　region　of　posterior　cingulate　cortex　in　patients　with　hyperthyroidism.　This　reduction　indicated　a　possible　involvement　of　glutamate　in　the　brain　dysfunction　in　hyperthyroidism.