To establish reverse triiodothyronine (rT₃) biological reference interval suitable for laboratory by indirect method.

From April to September 2019, 797 cases (332 males, 465 females, age: 12-95 years) underwent thyroid function, thyroid related antibody and rT₃ tests from hospitalized population in Dongzhimen Hospital Affiliated to Beijing University of Chinese Medicine were retrospectively analyzed. The reference individuals with normal thyroid hormone, antibody and without thyroid nodule or goiter were screened as inclusion criteria, and the factors such as acute and chronic diseases or drugs that might affect the values of rT₃ were excluded. Independent sample t test, one-way analysis of variance and least significant difference t test were used to analyze data. The rT₃ reference interval was established by non-parametric sequencing method, and 2.5% and 97.5% percentile values of data distribution were selected as the upper and the lower reference limits. In order to verify the rT₃ reference interval, 20 healthy individuals and 20 inpatients who met the inclusion and exclusion criteria were selected to test rT₃ with a simple random sampling method.

A total of 159 reference individuals (66 males, 93 females, age: 23-87 years) were enrolled. The rT₃ values of 23-29(n=4), 30-39(n=18), 40-49(n=29), 50-59(n=43), 60-69(n=40), 70-79(n=19) and over 80(n=6) years old groups were (0.62±0.16), (0.63±0.12), (0.64±0.11), (0.61±0.11), (0.65±0.14), (0.65±0.11) and (0.79±0.10) μg/L, respectively. There was a statistically significant difference in the rT₃ test results among different age groups (F=2.17, P=0.049). There were statistically significant differences of rT₃ between the individuals over 80 years old and other age groups (all P<0.05), while there were no statistically significant differences among the other groups (all P>0.05). The rT₃ of males and females under 80 years old were (0.62±0.11) and (0.64±0.12) μg/L, respectively, with no significant difference between them (t=-0.81, P=0.420). The newly established rT₃ reference interval suitable for people above 20 years old and below 80 years old was 0.47-0.92 μg/L, and the lower limit was significantly higher than that of the reference interval in the reagent specification (0.20-0.95 μg/L). The rT₃ range of 20 healthy individuals was 0.57-0.82 μg/L and that of 20 inpatients was 0.48-0.77 μg/L, which were all within the new reference interval.

The rT₃ biological reference interval established here has clinical application value, but its applicable range of age still needs to be further improved.

Triiodothyronine, reverse; Chemiluminescent measurements; Reference values