RF23 | PSAT300 The Curious Case of Abnormal Thyroid Function Tests: Think Streptavidin Interference

Interpretation of abnormal thyroid function tests (TFTs) is routine in endocrine practice. However, when there is a discrepancy between biochemical parameters and the clinical picture, sleuthing is necessary to provide appropriate care.A 34-year-old woman was evaluated by an outside endocrine clinic for abnormal TFTs in the setting of worsening anxiety, night sweats, and menstrual irregularities. These showed elevated free thyroxine (FT4) levels (7.3; ref: 0.9-1.7 ng/dL), free triiodothyronine (FT3) levels (13.9; ref: 2.3-4.1 pg/mL) and total triiodothyronine (T3) (186; ref: 94-170 ng/dL) with normal TSH (0.65; ref: 0.27-4.20 mIU/L), negative microsomal and thyroid receptor antibodies. She denied use of contraceptives, biotin, lithium, glucocorticoids, prior radiation, or recent intravenous contrast. Mother and maternal aunt had unspecified thyroid disease. No physical signs of overt hyperthyroidism/hypothyroidism were noted. Our workup also showed elevated FT4 (1.8 ng/dL), FT3 (13.3 pg/mL), total thyroxine (T4) (24.8; ref: 5.5-10.2 µg/dL), T3 (>651; ref: 79-165 ng/dL), normal TSH (0.71; ref: 0.4-5.5 mIU/L), and normal gonadotropin alpha subunit; ultrasound showed no nodules nor evidence of thyroiditis. FT4 by equilibrium dialysis and liquid chromatography-mass spectrometry was normal (1.3; ref: 0.8-2.0 ng/dL). Human anti-mouse antibodies (HAMA) treated TSH was within normal limits (1.4; ref: 0.4-4.5 mIU/L; Siemens). Leading differential diagnosis was initially thyroid hormone resistance; however subsequent labs, done by PCP for non-specific symptoms, revealed a suppressed parathyroid hormone (PTH) (<6; ref 15-65 pg/mL) with normal serum calcium levels. Given no clinical correlate, lab interference was considered. All the testing, with the exception of the FT4 by mass spectrometry and HAMA TSH, were analyzed utilizing Roche Diagnostics (electrochemiluminescent immunoassays). To evaluate vendor-specific interferences, labs were analyzed utilizing Siemens (chemiluminescent immunoassay) which showed normal T4, T3, FT3, and PTH levels. Unlike the Siemens assays, Roche depends on the formation of a biotin-streptavidin complex during measurement and a ruthenium complex to produce a signal. HAMA and polyethylene glycol treatment also returned differing results from untreated samples possibly indicating the interference was a heterophilic antibody to an unknown target. An investigation by Roche Diagnostics showed evidence of streptavidin antibodies. Streptavidin is a common reagent in endocrine assays. The endogenous streptavidin antibody produces an unintended competition between streptavidin-biotin and streptavidin-antibody in the assay; resulting in falsely-low results for sandwich assays (e.g. PTH) and falsely-high results for competitive assays (e.g. FT3 and T4). The impact to the individual analyte is variable due to assay formulation (e.g. TSH).Interference from anti-streptavidin antibodies is a rare phenomenon and may cause unnecessary diagnostics or treatment. Analytical interference should be considered when clinically suspicious of results, and lab consultation is recommended. The patient was advised to obtain a Medic-Alert bracelet. She was also provided a letter detailing the findings so that providers may consult the laboratory prior to testing. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m., Sunday, June 12, 2022 12:54 p.m. - 12:59 p.m.