Antithyroid drugs (ATD) are known to reduce 131I efficacy in thyrotoxicosis, though the underlying mechanism remains misunderstood. To study the impact of long term administration of carbimazole on both iodine stores (127I, secondary ion mass spectrometry microscopy) and targeting (125I, radioautography) at the intraglandular level in a woman who underwent surgery for Graves' disease. 125I distribution was dramatically heterogenous and large areas of the sample appeared poorly or no stained at all. This may correspond to flat follicles, hypofunctioning or ATD blocked ones and to the various histological changes related to the thyroiditis. SIMS counting showed huge variations of the interfollicular iodine stores (0 to 1.18 microg/mg) and lower mean values than those observed in nodular goiters. SIMS imaging depicted iodine free areas and others with preserved thyroglobulin synthesis, as assessed via 32S- mapping, but low to undetectable 127I, suggesting focal organification defects. Since ATD reduce iodine storage and uptake capabilities and enhance the iodine heterogeneity of interfollicular targeting, a related enhancement of the spatial 131I dose distribution is unavoidable. ATD may reduce 131I efficacy by variably reducing the number of follicles which can be actually or significantly targeted, e.g. irradiated (antirecruitement effect).