Purpose Oral administration of Iodine −131 (I −131) solutions causes high risk of contamination for patients and dispensers. The objective of the study was to adapt hard gelatin capsules (HGCs) for filling with radiopharmaceutical solutions without deformation. Methods Polystyrene (PS) internally lining films with different thicknesses were used to protect HGCs. The insulated HGCs were evaluated for their physicochemical characteristics and rupturing time in different dissolution media. HGCs internally lined with PS were examined for withstand loading with different volumes and radioactivities of I −131 solutions. Radioactivity release was studied in deionized water and acidic media. Quality control of released I −131 was inspected for radiochemical purities. Results There was a directly proportion between PS lining thickness and stability of HGCs after filling with 500 μl aqueous methylene blue solution. HGCs internally lined with PS 100 μm thickness withstand deformation for ˃ two months; however showed fast in-vitro rupturing time in different dissolution media. Internally lined HGCs loaded with different volumes and radioactivities of I −131 solutions resisted for one week without radioactive leakage. Yet, revealed complete release of I −131 after 20 min in dissolution media with great radiochemical purity. Conclusion The study promises safely I −131 aqueous solution delivery via adapted HGCs.