Endocrine Abstracts

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.