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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-T 4 ) 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 T 4  (FT 4 ) - 6.8 ng/ml and free T 3  (FT 3 ) - 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-T 4  was stopped, and this was followed by a rapid increase in TSH (to 77.76 μIU/ml) and some decrease in FT 4  and FT 3 , however FT 4  concentration remained elevated (2.1 ng/ml). Following this, L-T 4  was restarted. On admission to our Department, she was clinically euthyroid on L-T 4 , 88 μg, once daily. Investigations on Roche® platform confirmed mildly elevated TSH - 5.14 (rr: 0.27-4.2 μIU/ml) with high FT 4  [4.59 (rr: 0.93-1.7 ng/ml)] and FT 3  [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 FT 4  and FT 3  were assessed on the day of discontinuation of L-T 4  and after four days by the means of Abbott® Architect I 1000SR platform. These revealed FT 4  and FT 3  concentrations within the reference range [e.g., FT 4  - 1.08 ng/ml (rr: 0.7-1.48)]  vs  4.59 ng/ml (rr: 0.93-1.7, Roche®), FT 3  - 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 FT 4  and FT 3  in the differential diagnosis of abnormal results of thyroid function tests that do not fit the patient clinical presentation.

thyroid research

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