Case report: When measured free T4 and free T3 may be misleading. Interference with free thyroid hormones measurements on Roche? and Siemens? platforms

摘要

A 59-year old female patient presented with apathy and 6 kg weight gain. Investigations revealed severe primary hypothyroidism (TSH>100 μIU/ml). L-thyroxine (L-T₄) was started and titrated up to 75 μg, once daily, with clinical improvement. Other investigations revealed very high titres of anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (anti-Tg) 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): 0.8-1.9 ng/ml and 1.5-4.1 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) and some decrease in FT₄ and FT₃, however FT₄ concentration remained elevated (2.1 ng/ml). Following this, L-T₄ was restarted. 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: 0.27-4.2 μIU/ml) with high FT₄ [4.59 (rr: 0.93-1.7 ng/ml)] and FT₃ [4.98 (rr: 2.6-4.4 pg/ml)] concentrations. 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: 0.7-1.48)] vs 4.59 ng/ml (rr: 0.93-1.7, Roche?), FT₃ - 3.70 pg/ml (rr: 1.71-3.71) vs 4.98 (rr: 2.6-4.4, 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.