Endocrine Abstracts

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): 0.8–1.9 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), 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: 0.27–4.2 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)). 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.

Declaration of funding: The study was supported by the grant of the Polish Ministry of Science And Higher Education nr NN402 476637 (2009).