Background: This study synthesized alginate-chitosan nanoparticles (ALG/CS NPs) as nanocarriers for the hydrophobic drug levothyroxine and investigated their nanoparticle size. Objectives: Alginate-chitosan nanoparticles were prepared through ionotropic gelation of the chitosan core followed by complexation with alginate. Methods: The nanoparticles were characterized for particle morphology, size, and polydispersity index using transmission electron microscopy (TEM) and photon correlation spectroscopy. Results: Alginate-chitosan nanoparticles were found in a nanoscale spherical shape with an average particle size of 90 - 110 nm. Results indicated that the release mechanism depends on dissolution in the initial hours. For levothyroxine release from ALG/CS NPs, the Higuchi model fits better than the other models. The drug release kinetics were investigated, and the release kinetics data were best fitted with the Higuchi model. Conclusions: Levothyroxine was released mainly in the first 20 hours at an average rate of up to 78% at pH 7.4.