The elevated antibody level against receptors for TSH assessed by the commercial set TRAK (Henning Berlin) supports the diagnosis of endocrine orbitopathy (EO) which is important in more complicated cases with unilateral EO or EO in euthyroid or hypothyroid patients. During treatment of EO by elimination of the thyroid with radioiodine 131I (with or without previous thyroidectomy) a decline of originally high TRAK values to normal suggests successful elimination. Persistence of high TRAK values is associated with a poorer therapeutic effect of elimination and reasons for this should be sought. In four of 26 such patients the authors found radioiodine accumulation in the residual thyroid tissue, in 8 patients they found a non-accumulating residue by means of sonography. In 14 patients within two years after administration of the radioiodine elimination dose a residue of the thyroid gland was not found by scintigraphy nor by sonography. Improvement of the therapeutic effect occurred in one of these subjects with a drop of the TRAK value after the 30th month following elimination, in another female patient after a high diagnostic radioiodine dose. Improvement of the protrusion after radioiodine treatment with or without previous incomplete thyroidectomy and was recorded in 33 subjects where a drop of the originally high level of TRAK antibodies occurred was significantly greater than in subjects where the TRAK values did not decline.