Interference in free thyroxine immunoassay.
We report two patients who presented with non-specific symptoms of tiredness that prompted their clinician to order a thyroid function test. In both cases, free thyroxine (T4) was reported to be high, with a normal thyroid-stimulating hormone (TSH). Repeated blood tests from the same laboratory using the same assay method yielded similar results. Due to discrepancies between the clinical and laboratory findings, we decided to repeat the blood tests at another laboratory using a different assay method. This revealed normal free T4 and TSH levels, suggesting that heterophile antibodies were interfering with the free T4 assay, leading to clinical confusion.
Ghosh
Sujoy
S
The Ayr Hospital, Dalmellington Road, Ayr KA6 6DX, United Kingdom. drsujoyghosh@rediffmail.com
Howlett
Michael
M
Boag
David
D
Malik
Iqbal
I
Collier
Andrew
A
D000368:Aged; D000369:Aged, 80 and over; D000910:Antibodies, Heterophile / Q000009:adverse effects*; D003937:Diagnosis, Differential; D005189:False Positive Reactions; D005221:Fatigue / Q000175:diagnosis*; D005260:Female; D006801:Humans; D007118:Immunoassay / Q000379:methods; D008297:Male; D013959:Thyroid Diseases / Q000097:blood* / Q000175:diagnosis*; D013960:Thyroid Function Tests; D013974:Thyroxine / Q000097:blood*
D000910:Antibodies, Heterophile; D013974:Thyroxine