Influence of Thyroid Hormone Autoantibodies on 7 Thyroid Hormone Assays

Spurious results of thyroid function tests (TFT) can be recognized when they do not reflect the clinical status of the patient or are not internally consistent [e.g., increased FT4 with nonsuppressed thyroid-stimulating hormone (TSH)]. Potential causes of spurious TFT results include nonspecific binding of endogenous circulating factors, such as heterophilic antibodies, with assay reagents(1), the presence of albumin variants found in familial dysalbuminemic hyperthyroxinaemia(2), and thyroid hormone autoantibodies (THAA)(3). We describe a patient who had discordant TFTs due to circulating THAA and report differences in TFT results obtained on a variety of automated immunoassay platforms. A 19-year-old man presented with tiredness spanning a 12-month period following a prolonged viral illness. A family history of hypothyroidism prompted a request for TFTs. These showed increased FT4 of 60 pmol/L (reference interval 10–25 pmol/L) and FT3 of 8.1 pmol/L (reference interval 2.5–6.5 pmol/L), but an nonsuppressed TSH of 0.9 mU/L (reference interval 0.3–5.5 mU/L). The tests were performed on an Immulite 2500 (Siemens Medical Solutions Diagnostics). The patient …