Endocrine Abstracts

Introduction: The presence of heterophilic antibodies resulting in assay interference could lead to falsely high or low values in biochemical investigations. We present a case of a 5-year-old girl who had persistently high level of TSH despite Levothyroxine treatment.

History: A five-year girl was found to have an elevated plasma TSH level (24.29 mU/L) with normal Free T4 (both measured using Abbott Alinity immunoassay) while being investigated for short stature. She was clinically euthyroid and her short stature screen was normal. Thyroid peroxidase antibodies were negative, and ultrasonography of the thyroid was normal. Treatment with levothyroxine 25 μg/daily was started. Despite good compliance with the therapy being reported by her parents, plasma TSH remained persistently increased (27–47 mU/L). The dose of thyroxine was therefore gradually increased up to 75 μg/day. She remained clinically euthyroid with normal Free T4 throughout this period. As TSH did not normalize, even after 12 months of therapy, thyroxine was discontinued, and she was re-investigated as assay interference was suspected. TSH when measured by another laboratory using an alternative analytical method (Siemens Atellica) was found to be 1.82 mU/L compared with 39.76 mU/L when measured using the Abbott Alinity method. A similar result (1.28 mU/L) was obtained when TSH was measured following incubation of the plasma to remove heterophilic antibodies, confirming heterophilic antibody interference in the Abbott method. This finding excluded the possibility of thyroid pathology and led to discontinuation of the thyroxine replacement therapy.

Conclusion: Interference in immunoassays due to the presence of heterophilic antibodies can result in falsely increased or decreased analyte concentrations including TSH. Analysis of samples with suspected interference using an alternative analytical method and incubation to remove heterophilic antibodies could enhance the detection of assay interference. Caution must be exercised whilst interpreting laboratory results, especially when the clinical presentation of the patient does not match the assay results.