MON-339 - Falsely Elevated FT4 Due to Interference by Anti T4 Antibody

Background
Thyroid hormone immunoassays are subject to interference from endogenous antibodies. While interpreting unusual thyroid function tests (TFTs), the possibility of assay interference should be considered to avoid inappropriate management. We present the case of falsely elevated free T4 (FT4) due to the presence of anti T4 antibody.
Case Presentation
72 year old male with history of dementia, traumatic brain injury presented to the hospital with altered mental status. He had no clinical features of hyperthyroidism or previous history of thyroid disease. He was not on biotin containing supplements. Laboratory evaluation showed TSH 3.6mIU/L (0.27 – 4.2), FT4 >7.77 ng/dL (0.8-1.7), total T3 97.0 ng/dL (72-153). MRI pituitary was normal and alpha subunit was 0.1 ng/mL (0.1-0.5). Ultrasound of the thyroid was unremarkable. Further work up showed elevated TSI. FT4 by equilibrium dialysis was 2.3 ng/dL(1.1-3.5). Given the lack of clinical symptoms of hyperthyroidism and normal FT4 by equilibrium dialysis, to rule out the possibility of assay interference additional testing was performed. Heterophile antibodies, and antibodies to streptavidin and ruthenium were negative however anti T4 antibody was positive.
Conclusion
Anti T4 antibodies are more common in patients with autoimmune thyroid diseases such as our patient. This case highlights the importance of considering assay interference when there is discrepancy between clinical picture and biochemical testing or incongruent FT4/T3 levels in relation to TSH. When in doubt, repeating TFTs with an alternate more specific assay such as FT4 with equilibrium dialysis can prevent unnecessary treatment.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*