Endocrine Abstracts

Objectives: The UK TFT Guidelines (2006) recommend screening for heterophilic antibodies (HA), thyroid hormone resistance (THR) and TSHoma in samples with raised FT₄ and detectable or raised TSH levels. The aim was to determine the frequency and aetiology of such ‘abnormal’ thyroid function tests (TFTs) and to evaluate the use of further analyses for this common biochemical abnormality.

Design: Six-month retrospective study (1/08/06–28/2/07) of TFTs with raised serum FT₄ (>22 pmol/l) and detectable (0.3–4.2 mU/l) or raised (>4.2 mU/l) serum TSH concentrations, followed by a three-week prospective study (21/10–11/11/07) of TFTs that met these criteria, in which all samples were further assayed for FT₃, SHBG and HA.

Results: In the retrospective study 756 samples from 635 patients had raised FT₄ and detectable TSH (2% of total requested), of which 10% (53 single and 11 multiple) were repeated. There were 88 samples from 66 patients with raised FT₄ and raised TSH (0.2% of all TFTs requested), of which 18% (9 single and 3 multiple) were repeated. In the prospective study of the 56 ‘abnormal’ TFTs, 9 were on T₄ therapy (FT₃ between 3.2 and 5.5 pmol/l). Of the remaining 47 samples, no sample was positive for HA or had SHBG >120 mmol/l. Three samples had a raised FT₄ (26.9–40.3 pmol/l), detectable TSH (0.97–1.27 mU/l) and raised FT₃ (6.5–10.2 pmol/l) with SHBG between 9.6 and 72.2 nmol/l. One has been confirmed to have THR and other two are currently being investigated. In the remaining 44 samples of raised FT₄ and detectable TSH, 42 (96%) had FT₃ below and two (4%) above 6.0 pmol/l, 27 (61%) had SHBG below 60 mmol/l and 17 (39%) between 60 and 120 mmol/l. None had a combination of FT₃ above 6.0 pmol/l and SHBG between 60 and 120 mmol/l.

Conclusions: The lack of follow up on ‘abnormal’ TFTs could potentially misclassify a thyroid condition. FT₃ measurements in samples with raised serum FT₄ and detectable serum TSH appears to be of predictive value for detecting THR and for reassurance that either T₄ therapy or antibody interference was likely. Measurement of SHBG and HA appear redundant in further investigation of this TFT abnormality.