Endocrine Abstracts

JOINT1936

Introduction: Interference in thyroid function tests is a relative frequent condition that can lead to improper diagnosis and treatment. Interference can be multiple and involve certain drugs or patient-specific factors such as antibody production. These interferences can produce both false-positive and false-negative Results.

Case Report: We report a case of a 65-year-old man suffering from Waldenstrom macroglobulinemia (WM). During a hospitalisation for subacute neurological impairment, thyroid function tests were performed as a screening. Blood tests showed TSH of 0.79 mUI/l [0.27-4.2 mUI/l], total T4 >320 nmol/l [66-181 nmol/l], free T4 >100 pmol/l [11.2-24.1 pmol/l], total T3 >10 nmol/l [1.3-3.1 nmol/l], free T3 15.5 pmol/l [3.1-6.8 pmol/l]. Further workup revealed high titers of anti-TPO antibody (1231 kUI/l [<34 kUI/l]), anti-thyroglobulin (21.4 kUI/l [<33 kUI/l]) and TRAb (143.2 UI/l [<1.75 UI/l]). The patient was clinically euthyroid and without previous thyroid history or treatment known to cause thyroid dysfunction. This biochemical pattern in an asymptomatic patient raised suspicion of interference in thyroid function tests. This suspicion was supported when another laboratory platform was used to re-analyse thyroid function and showed values in the normal range. Biotin levels were normal. Tests repeated after serum pretreatment in heterophilic blocking tubes (HBT) showed normal values. The interference in this case is due to heterophilic antibodies. We hypothesised that the IgM production in WM can interfere with the immunoassay with the same mechanism of endogenous antibodies.

Discussion: This case report demonstrates a rare cause of heterophilic antibodies interference due to excessive monoclonal immunoglobulin M (IgM) production. It highlights the importance of considering the patient’s history, clinical examination and collaboration with the laboratory. When an interference is suspected, changing the manufacturer assay or re-measuring the analyte after dilution, precipitation or adding of blocking agents, can suggest an interference. However, a lack of effect from these methods does not rule out interference.

Conclusion: Heterophilic antibody-like behavior of WM-associated IgM can complicate diagnosis and interpretation of various laboratory tests. It must be considered when interpreting results in known WM patients, to avoid misdiagnosis and mistreatment. In our patient, it affected thyroid function tests, but many interferences have been reported for other endocrine tests (cortisol, testosterone, prolactin), tumoral markers (AFP, PSA) and cardiac markers (troponin).