[en] An improved, simplified radioimmunoassay method for the in vitro determination of L-triiodothyronine in unextracted blood serum involves the use of a combination reagent constituted by a buffered solution containing radioactive L-triiodothyronine and an inhibitor for inhibiting binding of L-triiodothyronine to thyroxine-binding globulin. Optionally the reagent may also include an antiserum containing antibody capable of immunoreactivity with L-triiodothyronine. Packaged test kits for use in conveniently carrying out the radioimmunoassay are also provided. Certain salts of 8-anilino-1-naphthalene sulfonic acid, which may be regarded as purified forms of the acid, which may be regarded as purified forms of the acid, are preferably employed as inhibitors for inhibiting binding of L-triiodothyronine to thyroxine-binding globulin. An improved radioimmunoassay method for the in vitro determination of thyroxine in unextracted blood serum is characterized by the use of an antiserum containing antibody capable of immunoreactivity with thyroxine and prepared from an immunogen comprising a conjugate of the N-acetyl derivative of thyroxine coupled to bovine serum albumin with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide