Endocrine Abstracts

Background: Measurements of thyrotropin (TSH), free thyroxine (FT4) and triiodothyronine (FT3) are widely used diagnostic methods for thyroid function evaluation. However, some serum samples will demonstrate a nonspecific binding with assay reagents that can interfere with the measurement of these hormones. Few case reports of TSH interfering antibodies have described the presence of such interferences resulting in reported abnormal concentrations of thyroid hormones. Unusual combination of TSH and thyroid hormone results may have a pathological source, poor compliance or interfering antibodies. Interference from anti-thyroid hormone antibodies is method dependent. We report a case of interfering T4 antibodies giving falsely elevated FT4 result.

Case report: A 39-year-old lady presented with symptoms of feeling tired, and lethargic. She had family history of hypothyroidism. On examination she was clinically hypothyroid and had diffuse smooth goitre. Thyroid function test (TFT) using Siemens competitive immunoassay revealed TSH of (92.35 mu/l), FT3 (2.2 pmol/l), FT4 (17.1 pmol/l) and a positive thyroid microsomal antibodies >1300. There is discrepancy between free FT4 and TSH values leads to suspicion of T4 interfering antibodies. Repeated of TFT using roche elecsys 2010 assay showed TSH (74.56 mu/l), FT4 (4.4 pmol/l), and FT3 (2.5 pmol/l) supporting the presence of T4 interfering antibodies. Symptoms resolved completely with levothyroxine replacement and here TFT were normalized.

Conclusion: This case highlights the importance of considering interfering T4 antibodies when interpreting unusual TFT and importance of repeating TFT using different assay for confirmation.

Reference

1. A patient showing increased T3 and T4 levels due to ALP-interfering substances and heterophilic antibody. Rinsho Byori 1996 44 (9) 899–902.