A 59-year-old female presented with apathy and 6 kg weight gain. Investigations revealed primary hypothyroidism (TSH >100 μIU/ml). Thyroxine (L-T4) was started and titrated up to 75 μg, o.d., with clinical improvement. Other investigations revealed high titres of anti-thyroid peroxidase and anti-thyroglobulin antibodies. After three months, there was a fall in TSH to 12.74 μIU/ml, however, with unexpectedly high free T4 (FT4) - 6.8 ng/ml and free T3 (FT3) - 6.7 pg/ml concentrations (reference range (rr): 0.81.9 ng/ml and 1.54.1 pg/ml (Siemens) respectively). At this stage L-T4 was stopped, and this was followed by a rapid increase in TSH (to 77.76 μIU/ml), however, FT4 concentration remained elevated (2.1 ng/ml). On admission to our Department, she was clinically euthyroid on L-T4, 88 μg, once daily. Investigations on Roche platform confirmed mildly elevated TSH 5.14 (rr: 0.274.2 μIU/ml) with high FT4 (4.59 (rr: 0.931.7 ng/ml)) and FT3 (4.98 (rr: 2.64.4 pg/ml)). Other tests revealed hypoechogenic ultrasound pattern typical for Hashimoto thyroiditis. There was no discrepancy in calculated TSH value following TSH dilution (101% recovery). Concentrations of FT4 and FT3 were assessed on the day of discontinuation of L-T4 and after four days by the means of Abbott Architect I 1000SR platform. These revealed FT4 and FT3 concentrations within the reference range (e.g. FT4 - 1.08 ng/ml (rr: 0.71.48) vs 4.59 ng/ml (rr: 0.931.7, Roche), FT3 3.70 pg/ml (rr: 1.713.71) vs 4.98 (rr: 2.64.4, Roche)), confirming assay interference. Concentrations of ferritin and SHBG were normal.
Conclusions: Clinicians must be aware of possible assay interference, including the measurements of FT4 and FT3 in the differential diagnosis of abnormal results of thyroid function tests that do not fit the patient clinical presentation.
Declaration of funding: The study was supported by the grant of the Polish Ministry of Science And Higher Education nr NN402 476637 (2009).