Endocrine Abstracts

Introduction: Thyroid function tests (thyroid hormones and thyreotropin) are frequently used by physicians all over the world. The interpretation is usually quite simple and straight-forward, however sometimes there is a strange discrepancy between the laboratory findings and clinical presentation. There are several potential explanations to this discrepancy and interference in laboratory assay may be one of them.

Observation: We present a case of a 73-year-old woman without any significant comorbidities except of atrial fibrilation and arterial hypertension. She was presented to endocrinologist by her general practitioner because of elevated estradiol, free thyroxine and triiodothyronine coupled with elevated thyreotropin but she was lacking any signs or symptoms of thyreotoxicosis. Central hyperthyreoidism was mainly considered so we started thyrostatic treatment with thiamazol. Nevertheless the MRI and PET/CT was negative, other pituitary hormones were normal and thiamazol was totally ineffective. At this point we started to think about possible laboratory interference. We suspected presence of antibodies that block the antibody of the assay we used. That was confirmed by precipitation of the sample with polyethylene glycol after which the estradiol, tyroxine, triiodothyronine and thyreotropin normalized. According to the literature IgM rheumatoid factor is the most common cause of such interference so we evaluated it and it was indeed positive. We then immediately withdrawn thiamazol and after short follow up the patient was handed over back to her general practitioner.

Conclusion: Laboratory interference in thyroid function tests is fortunately very rare. However if there is a discrepancy between the laboratory findings and clinical presentation it is good to have it in mind. It is generally quite easy to exclude and it can help you to avoid misdiagnosis and pottentialy harmful unnecessary treatment.