2026 ETA guideline on interference in immunoassay measurements used in assessment of thyroid function.

Although measurement of thyroid hormones (THs), TSH and thyroid autoantibodies, is usually straightforward, erroneous results are occasionally generated, raising the possibility of incorrect or delayed diagnoses, unwarranted investigations, and provision of inappropriate treatments. Assay interference can be acquired (e.g. antibodies), genetic (e.g. variant TH-binding proteins), or pharmaceutical (e.g. displacing agents and biotin) in nature. In those with discordant thyroid function, assay interference must be excluded prior to the diagnosis of rare inherited (resistance to thyroid hormone) or tumoural conditions (TSHoma). Liaison between the clinician and clinical biochemistry teams is essential when assay interference is being investigated; appropriate further biochemical testing depends on the clinical picture, initial biochemistry, and local biochemical expertise. In a subset of cases, access to a regional centre where patients can be clinically assessed, in conjunction with performance of specialised clinical laboratory testing (biochemical, +/- genetic), may be required. When assay interference is identified, clear communication with the patient (ideally aided by reliable, accessible patient literature) and documentation to their physician are necessary. Professionals must stay updated on advances in testing and be alert for evolving assay interferences. An accompanying plain language summary is intended to provide an accessible overview of the key messages and recommendations contained in this guideline for non-specialist readers, including patients, carers, healthcare professionals outside the speciality, and the wider public.