A-060 Thyroid Laboratory Database Assessment Shows that Total Triiodothyronine Significantly Decreases with Age
Authors: Fontes R , Telles-Dias P , Sousa T C , Araujo P , Santos B S , Lopes A C , Lopes F S , Pinheiro M F , Gomes D M , Sabino C O , Cury A N , Castelo M C , Cavalcante L L , Schrank Y , Veloso T C , Perozo A F 
Published: September 2023

Abstract

Introduction
The effects of aging on human thyroid function are well studied in relationship to thyroid stimulating hormone (TSH) and free tiroxine (FT4); TSH increases, while FT4 decreases with age. Although animal studies show that total triiodotironine (TT3) is lower in aged male rats, these changes are still not clear enough in humans. The National Academy of Clinical Biochemistry (NACB) suggests that TT3 levels would be lower in subjects over 60 years (y.), but the number of elderly assessed was too small for a robust statistical analysis. In search on PubMed, crosschecking the terms elderly/age/aging, with triiodothyronine/T3/total T3, there are a few studies available using current assays with a significant number of patients. Therefore, the demonstration of how the TT3 performs with age is still an opened question.

Objective
The aim of this study was to observe if TT3 changes significantly with age.

Methods
We, retrospectively, analyzed blood samples requesting serum TT3 of unique patients, who had TSH measured on the same day; and thyroid peroxidase antibodies (TPOAb), thyroglobulin antibodies (TGAb), anti TSH receptor antibodies (TRAb) or TSH receptor-stimulating immunoglobulin (TSI) at any time since patient admission. Subjects were adults ≥20 y., admitted to a private reference clinical laboratory in Brazil, from January 01, 2014 to December 31, 2022, using anonymized data. Serum TT3 was measured in Cobas Roche, by electrochemiluminescence assay, reference interval 70–210 ng/dL. Exclusion criteria were (and/or): tests performed in hospitals, use of drugs that could interfere on TT3 or TSH, TSH ≤ 0.1 mIU/L or ≥10.0 mIU/L, and any positive antibody at any time. The level of statistical significance was P &lt; 0.05.

Results
Initially, 959 297 records from the database were surveyed. After applying the exclusion criteria, 141 819 subjects were eligible: mean age = 45 y.; median = 42 y., 62% female (F). Statistically significant differences - P &lt; 0.05 for all comparisons between groups - were found by age and gender in TT3 mean values (in ng/dL). Regarding age: from 20 to 59 y. = 120 ± 26.6; 60 to 79 y. = 109 ± 20.7; and 80 y. and over = 97.2 ± 20.0. By gender: F = 120 ± 28.9; male = 114 ± 20.4. It was also observed that, in elderly, while TT3 decreases, TSH increases (results not shown). Comparing TT3 means by age groups, it is 9% lower in subjects aged 60–79 y. and 19% lower in those aged 80 y. and older, compared to younger (20–59 y.) subjects.

Conclusion
TT3 results is influenced by several factors, including age, gender and laboratory assay. As far as we know, searching in PubMed, this is the largest collection of data on the relationship between TT3 and age. The data obtained showed a statistically significant decrease in TT3 in elderly, and mainly in very elderly, and higher levels in women than in men. This finding draws call attention to the fact that these parameters should taken into account when evaluating TT3 levels.