Abstract: In this work, 3,5-L-diiodothyronine as a starting material, 125I-L-triiodothyronine (125I-T3) and 131I-L-triiodothyronine (131I-T3) are prepared, and the specific activity of 125I-T3 is measured by self-displacement and from the standard curve obtained by 131I-T3 for the radioimmunoassay of 125I-T3. The specific activity of Na125I solution can be calculated from that of 125I-T3. This is a new method for the determination of the specific activity of radioiodine. Meanwhile, the total iodide content of the Na125I, Na131I solutions is measured via an iodide selective electrode, and the radioactivity of the Na125I, Na131I solutions via a calibrated 4π-ionazation chamber respectively. Thereby the specific activities of radioiodines can be calculated. The results obtained by different methods agree fairly well. The abundances of iodine-125 and iodine-131 in the Na125I and Na131I solutions are about 50% and 6% respectively. The sources of iodine in the samples are also discussed.