Measurements of TSH, total T4 and T3 are widely used diagnostic methods for the evaluation of thyroid function. However, in some cases, there is a nonspecific binding with assay reagents that can interfere with the measurement of these hormones. These interferences will result in abnormal thyroid hormone serum concentrations and discrepancy with the clinical evaluation or other laboratory measurements in these patients. Thyroid hormone auto-antibodies are an important class of interference factor in various immunoassays. We report the case of a 62 yo. female, complaining of anxiety and palpitation but normal on physical examination. Laboratory evaluation showed persistently high total T3 levels, with normal levels of TSH and total T4. Antibodies anti-T3 were identified by radioimmunoprecipitation. Results that appear to be inconsistent or incompatible with the clinical presentation, with or without nonspecific symptoms, should raise the suspicion of the presence of interfering factors in the assay and, therefore, avoid an erroneous diagnosis of thyroid dysfunction and, as a consequence, any unnecessary and potentially harmful treatment.
Thyroid function tests; Auto-antibodies; Anti-T3 antibodies; Anti-T4 antibodies