Endocrine Abstracts

Objective: Thyroid function tests are common biochemical analyses and agreement between the routinely used immunoassays is important for diagnosis and monitoring of thyroid disease. Efforts are continuously made to align the biochemical assays, and we aimed to evaluate agreement between immunoassays used in a clinical laboratory setting among non-pregnant and pregnant adults.

Methods: We performed a cross-sectional study including serum samples from 192 Danish blood donors (non-pregnant adults) and from 86 pregnant women in the North Denmark Region. The samples were used for measurement of thyroid-stimulating hormone (TSH), total thyroxine (TT4), and total triiodothyronine (TT3) with three routinely used automatic immunoassays (Alinity, Abbott Laboratories, Cobas, Roche Diagnostics, and Atellica, Siemens Healthineers). Assay agreement was evaluated by comparison of the medians using Kruskal-Wallis test or Mann-Whitney U test as appropriate. Furthermore, reference intervals for TSH, TT4 and TT3 were established in non-pregnant adults (n = 162) as the 2.5 and 97.5 percentiles among thyroid autoantibody-negative (thyroid-peroxidase antibodies < 60 U/mL and thyroglobulin antibodies < 33 U/mL.

Results: In non-pregnant adults, levels of all thyroid function tests were higher with Cobas and Atellica than with Alinity as reflected by the median TSH (Alinity: 1.39 mIU/l; Cobas: 1.57 mIU/l; Atellica: 1.74 mIU/l, P < 0.001), TT4 (Alinity: 88.8 nmol/l; Cobas: 98.1 nmol/l; Atellica: 97.2 nmol/l, P < 0.001), and TT3 (Alinity: 1.52 nmol/l; Cobas: 1.81 nmol/l; Atellica: 1.69 nmol/l, P < 0.001). Similarly, levels were higher with Cobas than with Alinity among pregnant women as seen for median TT4 (Alinity: 106 nmol/l; Cobas: 127 nmol/l, P < 0.001) and TT3 (Alinity: 1.77 nmol/l; Cobas: 2.47 nmol/l, P < 0.001), and a parallel non-significant trend was seen for TSH (Alinity: 1.90 mIU/l; Cobas: 2.33 mIU/l, P = 0.2), Established reference intervals in non-pregnant adults with each of the assays showed higher upper reference limits with Cobas and Atellica than with Alinity, and the reference intervals were for TSH (Alinity: 0.46-3.44 mIU/l, Cobas: 0.57-3.96 mIU/l, Atellica: 0.57-4.23 mIU/l), TT4 (Alinity: 63-121 nmol/l, Cobas: 68-138 nmol/l, Atellica: 64-135 nmol/l), and TT3 (Alinity: 1.17-2.02 nmol/l, Cobas: 1.35-2.58 nmol/l, Atellica: 1.25-2.27 nmol/l,).

Conclusions: Results of thyroid function tests obtained with different immunoassays were not interchangeable when evaluated among pregnant and non-pregnant adults. The distinct differences are relevant for clinical decision making and emphasize the necessity of clinical laboratory information when different assays are used for diagnosis and monitoring of patients with thyroid disease.