Clinical Information

The test cannot be used in patients receiving treatment with lipid-lowering agents containing dextrothyroxine (D-T4). If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4 to 6 weeks to allow the physiological state to become re-established. Binding protein anomalies seen with familial dysalbuminemic hyperthyroxinemia, for example, may cause values which, while characteristic of the condition, deviate from the expected results. Performance of this assay has not been established with neonatal specimens. The anticonvulsant drug phenytoin may interfere with total and free T4 levels due to competition for TBG binding sites. Free T4 values may be decreased in patients with non-thyroidal conditions and in patients taking carbamazepine. Thyroid autoantibodies in human serum may interfere and cause falsely elevated free T4 results. For assays employing antibodies, the possibility exists for interference by human anti-animal antibodies (i.e., heterophile antibodies) in the patient sample. Patients who have been regularly exposed to animals or have received immunotherapy or diagnostic procedures utilizing immunoglobulins or immunoglobulin fragments may produce antibodies, (eg, HAMA), that interfere with immunoassays. This may falsely elevate or falsely decrease the results.