Inappropriate TSH secretion, as defined by an elevated free thyroxine (fT4) and non-suppressed thyrotropin (TSH) result, can be caused by acute illness, medications, TSH secreting tumours, thyroid hormone resistance or immunoassay interference including thyroid hormone autoantibody interference. T4 autoantibody (T4AAb) prevalence has not been determined. We determined the prevalence of inappropriate TSH secretion using the Immulite 2000 assay and the prevalence of T4AAb within this subgroup.MethodsSamples were stored over 13 months and thawed once for batch analysis. T4AAb was detected using radioimmunoprecipitation with > 5% of the radiolabel within the immunoprecipitate indicating true positivity. All case notes and medication charts were reviewed.ResultsOf 13,286 thyroid profiles reviewed, 85 (0.64%) samples demonstrated inappropriate TSH secretion. One of these 85 samples (1.2%) was positive for T4AAb with a radioimmunoprecipitate of 21%. 46/85 (54%) samples were collected on hospitalised patients, 7 patients were prescribed amiodarone, 12 patients were taking beta blockers, 30 patients were on thyroxine replacement and 7/85 (8%) were collected on outpatients not taking medication known to affect thyroid function.ConclusionT4AAb interference is an extremely rare explanation for inappropriate TSH secretion with the Immulite 2000 assay but should be excluded before investigating for rarer causes of this biochemical picture.