The particle size and morphology of levothyroxine sodium were investigated by GAS precipitation using dense carbon dioxide as the antisolvent and ethanol as the solvent. Experimental runs were carried out at 25 and 40 oC and pressures up to 110 bar. The effects of concentration of the organic solution on particle size and morphology were also investigated. The results show typical spherical nanoparticles and bimodal particle size distribution. Further studies should be done to take advantage of these characteristics in drug formulation and delivery systems.