Nationwide Chinese study for establishing reference intervals for thyroid hormones and related tests

This nationwide study aimed to establish Chinese specific reference intervals (RIs) for thyroid related tests: thyroid stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine (FT3), total thyroxine (TT4), total triiodothyronine (TT3), anti-thyroid peroxidase (TPO-Ab) and anti-thyroglobulin antibodies (TG-Ab).

Apparently healthy individuals (n = 2380) were recruited from Beijing, Guizhou, Urumqi, Dongying, Shenzhen, and Qiqihar in China. All the tests were measured by immunoassay testing. Ultrasonography was performed to exclude thyroid nodules. Multiple regression analysis was performed to identify sources of variation (SVs). Standard deviation ratio (SDR) was calculated by ANOVA for judging the need to partition RI by any given SV. RIs were computed by the parametric method.

TPO-Ab and TG-Ab cutoffs were determined as 5 mIU/L and 2 IU/L, respectively using probability plot analysis and extrapolation of the central linear segment. Individuals with thyroid nodule and/or autoantibodies showed altered levels of thyroid hormones, and were thus excluded (n reduced to 1828). Gender difference was observed for FT3 and TT3 with females having lower levels than males. A significant relationship between age and FT3 was observed in males by Spearman correlation analysis (r = -0.231, p < .05). Although the SDR for gender difference in TSH levels was low, the difference in the upper limits of the RI was beyond allowable bias, and thus RIs (mIU/L) were partitioned by sex: females 0.72-5.50, males 0.70-4.59.

By this nationwide study, RIs for thyroid hormones and cutoff values for anti-thyroid autoantibodies were established as matched to the Chinese population.