Endocrine Abstracts

Introduction: Biotin-streptavidin detection method is used in a majority of commercial immunoassay. Biotinylated antibodies (capture antibodies) bind strongly to streptavidin that anchors those antibodies to the solid phase of the assay. Thus, high biotin concentration in the serum interferes with this bond and alters the expected results. Clinical history of patient and additional laboratory test is important for troubleshooting of discordant thyroid function test result.

Case presentation: A 52 years male was referred by an endocrinologist to our laboratory to evaluate clinic-biochemical discordance for thyroid function test. The resultsshowed a thyroid-stimulating hormone (TSH) level of 0.02 µIU/ml (0.35–5.5 µIU/ml) and a free thyroxine (fT4) level of 5.48 ng/dl (0.89–1.76 ng/dl). Similar finding was found in the report issued by second laboratory. TPO antibody, anti-thyroglobulin and TRAb were negative. Patient had normal thyroid ultrasound and low normal uptake in technitium-99 thyroid scan. This discordant TFT results prompted us to investigate for the possible interference in the TSH and fT4 assay. Patient did not have history of previous thyroid disease. There was no family history of similar TFT findings. He was recently diagnosed with X-linked adrenoleukodystrophy based on findings of long chain fatty acid test after his complaints of progressive weakness and stiffness of lower limb. For his illness, he was taking low dose hydrocortisone and was under trial of high dose biotin. Thus, biotin interference as a cause of discordant laboratory finding was suspected. We reviewed the results of previous TFT and consulted with the clinical laboratories about the design of their immunoassay. Initial results of TSH and fT4 were obtained from Vitros 3600 immunoassay. The second result was from Roche Diagnostics Modular E170. These both immunoassay systems used biotin to label the reaction. We repeated TFT in Siemens, Advia Centaur which used acridinium ester to label the reaction. The results obtained were normal. Hyperthyroidism was ruled out and high dose biotin interference was confirmed.

Discussion: The excess free biotin in sample saturates the streptavidin binding sites preventing adherence of the signal bound antibody–antigencomplex in a non competitive assay (e.g. TSH). This creates a falsely lowered TSH result by decreasingthe signal detected. For fT4 level, a competitive immunoassay principal is used where high biotin concentration causes reduction in light generated and has positive interference with fT4 assay. When an immunoassay with no biotin is used (e.g.Siemens Advia Centaur), the interference disappears.