Detection of 3,5-diiodothyronine in sera of patients with altered thyroid status using a new monoclonal antibody-based chemiluminescence immunoassay

Abstract

Background: 3,5-Diiodo-L-thyronine (3,5-T 2 ), a potential metabolite of 3,3′,5-triiodothyronine (T 3 ), exerts marked metabolic actions without the undesirable cardiac and central side effects of T 3 . So far the lack of reliable quantification methods for endogenous 3,5-T 2 in human serum has limited further insight into its physiological and pathophysiological roles in endocrine homeostasis and disease status. Methods: Monoclonal anti-3,5-T 2 antibodies (3,5-T 2 mAbs) were produced in mice. We developed a competitive chemiluminescence immunoassay (CLIA) with one selected mAb and optimized it for high sensitivity, linearity, recovery, and low cross-reactivity to structurally related thyroid hormones (THs) and thyronamines. The CLIA was then used to investigate the origin and action of 3,5-T 2 in humans under physiological and pathophysiological conditions in comparison with THs. Patient analysis included individuals with confirmed hypo- or hyperthyroidism and a separate population of thyroidectomized patients on L-thyroxine (T 4 ) replacement therapy. Results: 3,5-T 2 is stable in human serum after storage at 4°C or room temperature as well as several freeze–thaw cycles. The immunoassay did not show any significant cross-reactivity with naturally occurring TH metabolites in physiological and pathophysiological concentrations. The assay shows a lower detection limit of 0.2 nM 3,5-T 2 and an upper detection limit of 10.0 nM. The newly established CLIA generates reliable results after spiking exogenous 3,5-T 2 or by linear dilution of sera. Intra-assay variation is between 4.1% and 9.0%. Overall mean of variation between different assays is 5.6%–12.9%. 3,5-T 2 serum concentrations do not differ in hyperthyroid (0.31±0.02 nM, n =24) compared to hypothyroid (0.43±0.04 nM, n =31) individuals. 3,5-T 2 was detectable and elevated in serum from thyroidectomized and T 4 -substituted patients (0.48±0.03 nM, n =100) in comparison to a sex- and age-matched control group (0.29±0.01 nM, n =99). Conclusion: The established CLIA is highly specific, sensitive, precise and accurate for 3,5-T 2 detection in human serum. Because 3,5-T 2 is not regulated in conditions of an altered thyroid state, it is most likely that serum 3,5-T 2 concentrations are not directly dependent on feedback regulation via the hypothalamic–pituitary axis. In addition 3,5-T 2 is present in thyroidectomized individuals on T 4 substitution, and it is elevated after T 4 substitution compared with healthy controls. We conclude that these data support extrathyroidal production of 3,5-T 2 from T 4 .