PurposeMethicillin-resistant Staphylococcus aureus (MRSA) infections are emerging as a cause of suppurative thyroiditis. The resistance to several antibiotics and finding treatment has grown more difficult. Hence, the purpose of this study was to examine biosynthesized iron oxide nanostructures’ antimicrobial activity as well as their cytotoxicity on normal and thyroid cancer cells. MethodsBy using the sugar alcohols of Cystoseira sp., the authors synthesized the advanced biosynthesis of iron oxide nanoparticles (COINs). An agar-well diffusion method and a broth dilution method were used to investigate COINs’ antimicrobial activity against MRSA isolates. However, their cytotoxicity against normal and thyroid cancer cell lines was determined by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. ResultsA GC–MS analysis of the dried Cystoseira sp. extract revealed a high percentage of D-mannitol, which is essential for the synthesis of spherical nanoparticles with a mean average size of 5.8 ± 0.8 nm. The COINs showed a higher inhibition zone (9–15 mm) for 66.6 % of MRSA isolates, and the MIC of COINs was 256 μg/ml. The cell wall of these bacteria inhabits the absorption of COINs. As a result, a large concentration of COINs is required to restrict bacterial growth. Moreover, COINs have cytotoxicity on thyroid cancer cell proliferation and normal thyroid cell lines at IC50s of 1.71 ± 0.1 and 25.9 ± 1.6 μg/ml, respectively. This can be due to the negative charge of sugar alcohols, which affect protein absorption and subsequent biological behaviors. ConclusionThe promising findings of this study are in favor of the creation of very small iron oxide nanostructures for therapy.