A-059 Evaluation of Free Thyroxin Levels in Patients with Low Thyroxin Binding Globulin Concentrations: Is There Interference in the FT4 Indirect Method?

Abstract

Abstract Background Thyroxine-binding globulin (TBG) is the major carrier of thyroid hormones in serum. Variations in the serum concentration of TBG determine proportional variations in the total serum concentrations of T4 and T3, without implying changes in function, as long as the free fraction remains normal. Several common clinical conditions lead to significant alterations in TBG levels, the most important factor behind low levels being genetic defects. As TBG is encoded by a gene located on the X chromosome, the defects are more easily manifested in males. Differences in FT4 concentrations were observed in sera from subjects with TBG abnormalities, when FT4 are measured by different immunoassays, specially at extremes of TBG concentrations. The objective of this study was to evaluate FT4 levels by indirect method in patients with low TBG concentrations. Methods We analyzed blood samples requesting serum TBG, TSH and FT4 levels from patients admitted to a private reference clinical laboratory in Brazil, from January 01, 2017 to December 31, 2022. Anonymized data on laboratory tests was available from a database of the local Laboratory Information System. All the patients included had TSH within the reference range, 0.4–4.3 mUI/mL (ECLIA, Modular, Roche). FT4 was measured by ECLIA (0.7–1.8 ng/dL) and TBG by ICMA, Immulite, Siemens, 13–39 mcg/mL. Results 357 patients with low TBG levels were evaluated, 220 (62%) men, 137 (38%) women; mean age 35 yrs (19 d to 88 yrs). They were divided in six groups according TBG levels (mcg/mL): <3.5–17 (M 15, F2); 3.5 to 6.0 - 26 (M 20, F 6); 6.1 to 8.0 - 30 (M 25, F 5); 8.1 to 9.9 - 59 ( M 32, F 27); 10.0 to 11.9 - 139 (M 78, F 61); 12.0 to 12.9 - 86 (M 50, F 36). Three patients had FT4 levels above 1.8 (1.85; 1.87 and 1.96) ng/mL, all them male, with TBG levels <3.5; 5.79 and 9.53 ng/mL, respectively. Conclusion FT4 Indirect methods are developed taking into account the presence, in the serum, of a thyroxine binding capacity within the normal range, a fact that does not occur in individuals with extreme TBG deficiency. In this study, we found only one slightly elevated FT4 value among 17 patients with TBG values below 3.5 mcg/mL, suggesting little interference of very low TBG levels in this indirect free T4 method.