There are many causes of interference in immunoassays causing erratic patient results. A method-specific interference due to antiruthenium antibodies in Roche free thyroxine (fT4) and free triiodothyronine (fT3) assays has been described previously. As a result, a new generation fT4 assay has been introduced by Roche. We describe six cases of interference due to antiruthenium antibodies, where in four cases interference in the Roche thyroid-stimulating hormone (TSH) assay was found as well. This raised the question as to whether other assays on this platform would also give incorrect results in patients with antiruthenium antibodies. Interference due to antiruthenium antibodies was suspected because of discrepancies between clinical presentation and/or TSH, fT4 and fT3 results. Samples of these six patients were re-analysed in Roche Diagnostics Laboratory, where it was demonstrated that the found discrepancies were indeed caused by interfering antiruthenium antibodies. Subsequently, these patients were asked to donate some blood once more for further evaluation, and three subjects agreed to participate. Their plasma was used to assay 18 analytes on Modular E and on a ruthenium-independent platform. The results were compared taking into account the known differences between distinct methods. As expected, significant interference was found in TSH. Also, in the new generation fT4 assay, ruthenium-induced interference was still present. However, the other assays, both competitive and immunometric, did not show clear interference. We therefore conclude that although antiruthenium antibodies theoretically can interfere in all assays on the Modular E platform, this kind of interference is found in the thyroid hormone assays, without marked interference in the other assays.