The functional properties of the endothelium of human thyroid arteries remain unexplored. We investigated the intervention of nitric oxide (NO), prostacyclin (PGI(2)) and endothelium-derived hyperpolarizing factor (EDHF) in the responses to acetylcholine and noradrenaline in isolated thyroid arteries obtained from multi-organ donors. Artery rings were suspended in organ baths for isometric recording of tension. The contribution of NO, PGI(2) and EDHF to endothelium-dependent relaxation was determined by the inhibitory effects of N(G)-monomethyl-L-arginine (L-NMMA), indomethacin, and K(+) channel inhibitors respectively. Acetylcholine induced concentration-dependent relaxation; this effect was not modified by indomethacin and was only partly reduced by L-NMMA, but was abolished in endothelium-denuded rings. The relaxation resistant to indomethacin and L-NMMA was abolished by using either apamin combined with charybdotoxin, ouabain plus barium, or a high-K(+) solution. Noradrenaline induced concentration-dependent contractions which were of greater magnitude in arteries denuded of endothelium or in the presence of L-NMMA. In conclusion, the results indicate that in human thyroid arteries the endothelium significantly modulates responses to acetylcholine and noradrenaline through the release of NO and EDHF. EDHF plays a dominant role in acetylcholine-induced relaxation through activation of Ca(2+)-activated K(+) channels, inwardly rectifying K(+) channels and Na(+)-K(+)-ATPase.