Establishment of Reference Intervals for Serum Thyroid-related Hormones in the Chinese Oldest-old

Background: Thyroid function differs between the oldest-old and younger adults, however, there is a lack of appropriate RIs for the serum FT4, T4, FT3, T3, and TSH concentrations for the oldest-old. Methods: A total of 349 people was selected from the natural longevity cohort of Guangxi residents over 90 years old. The distributions of serum FT4, T4, FT3, T3, and TSH concentrations and the FT3/FT4 ratio were also analyzed. Regression curves and centile curves were constructed. Correlations between body indices and blood pressure and serum thyroid-related hormone levels were analyzed. The RIs were determined using a nonparametric approach (2.5th-97.5th percentiles) following the CLSI guidelines. Results: The RIs for Chinese oldest-old are different from the current standard RIs for younger adults. A notable correlation was found between FT4 levels and age. BMI and WHtR were positively correlated with FT4, FT3, T3 and the FT3/FT4 ratio. The T3 level is correlated with SBP and DBP, and the FT3/FT4 ratio is correlated with SBP. The RIs established for the healthy oldest-old were as follows: T4, 81-193 nmol/L; FT4, 10.39-20.46 pmol/L; T3, 0.94-2.05 nmol/L; FT3, 3.56-6.43 pmol/L; TSH, 0.29-5.28 μIU/ml; and FT3/FT4, 0.197-0.496. Conclusions: In this study, we established RIs of thyroid-related hormone levels for the oldest-old in China and evaluated the associations between thyroid hormone levels and body indices. These findings may provide evidence for the diagnosis and treatment of thyroid-related diseases in the oldest-old.