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Endocrine Abstracts (2016) 44 EP110 | DOI: 10.1530/endoabs.44.EP110

1Maidstone and Tunbridge Wells Hospitals NHS Trust, Maidstone, UK; 2Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK.

Positive interference in free thyroxine (fT4) and free triiodothyronine (fT3) immunoassays is well known, however we report a rare case of negative interference in the Roche fT4 and fT3 immunoassays in a 3-year-old girl with undetectable fT4 and fT3.

She presented with increasing tiredness, abdominal pain, pain in her lower limbs and constipation. Examination was unremarkable. Thyroid function tests (TFTs) of TSH 1.5 mU/l (0.85–6.50), fT4 <3.0 pmol/l (12.1–22.0) and fT3 <1.5 pmol/l (3.0–9.1) were confirmed on a repeat specimen using the Roche method. Prolactin, Insulin like growth factor 1 (IGF1) and synacthen test were within respective reference intervals. An MRI of her brain was normal.

Idiopathic isolated central hypothyroidism is rare. In view of this, her TFTs were re-analysed by three different methods, all of which reported the following normal results: Centaur Method TSH, 2.25 mU/l; fT4, 14.8 pmol/l (10.3–22.7); fT3, 5.8 pmol/l (3.5–6.5), Abbott method: fT4, 18 pmol/l (9–19), Perkin Elmer Auto-DELFIA method, TSH, 2.68 mU/l (0.40–4.0); fT4, 12.7 pmol/l (9.0–20.0). FT4 was also estimated from a total T4 measurement of 159 nmol/l (69.0–141.0) and a TBG concentration of 28.5 μg/ml (14.0–31.0) using published values for the binding constant of T4 to TBG; this returned a value for fT4 of 15.0 pmol/l.

This case emphasizes the need for vigilance in interpreting extremely unusual immunoassay results, even if clinically plausible.

Volume 44

Society for Endocrinology BES 2016

Brighton, UK
07 Nov 2016 - 09 Nov 2016

Society for Endocrinology 

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