Thyroid hormone excess increases basal and insulin-stimulated recruitment of GLUT3 glucose transporters on cell surface.

BACKGROUND: In hyperthyroidism, tissue glucose disposal is increased to adapt to high energy demand. Our aim was to examine the glucose transporter isoforms involved in this process and their regulation through insulin in monocytes from subjects with hyperthyroidism.
METHODS: Blood (20 ml) was withdrawn from 12 healthy and 12 hyperthyroid subjects. The abundance of glucose transporter isoforms (GLUT) on the monocyte surface membrane was determined in the absence and presence of insulin (10-100 mU/l) using flow cytometry. Anti-CD14-PE monoclonal antibody was used for monocyte gating. GLUT isoforms were determined after staining the cells with specific antisera to GLUT1, GLUT3 and GLUT4.
RESULTS: Hyperthyroidism increased basal monocyte-surface GLUT1, GLUT3 and GLUT4 transporters. In these cells, insulin had a marginal effect on GLUT4 translocation (25 %, p < 0.02) and a more significant effect on GLUT3 translocation (45 %, p < 0.001) on plasma membrane.
CONCLUSIONS: In the hyperthyroid state, (1) basal abundance of GLUT1, GLUT3 and GLUT4 transporters on the cell surface is increased; (2) insulin mainly increases the recruitment of GLUT3 and, to a lesser extent, GLUT4 glucose transporters on the plasma membrane. These findings may provide a mechanism to explain the increment of glucose disposal in peripheral tissues in hyperthyroidism.