Abstract: A radioimmunoassay method for the determination of triiodothyronine in human and animal fluids and tissues, particularly in serum. The method involves the separation of triiodothyronine from its binding proteins using a dextran gel column followed by competitive binding between the freed triiodothyronine and radioactive labeled triiodothyronine which remain bound to the gel column for triiodothyronine-specific anti-serum. Bound and free labeled triiodothyronine are separated by washing the column and their ratio is determined based on the amount of radioactivity remaining in the column to that originally added thereto.