Indirect reference intervals estimated from hospitalized population for thyrotropin and free thyroxine

To establish indirect reference intervals from patient results obtained during routine laboratory work as an alternative to laborious and expensive producing of their own reference range values according to international instructions.

All results for thyrotropin (TSH) and free thyroxine (T4) that were stored in our laboratory information system between 2004 and 2008 were included in this study. After a logarithmic transformation of the raw data, outliers were excluded. Non-parametric reference intervals were estimated statistically after visual observation of the distribution using stem-and-leaf plots and histograms. A standard normal deviation test was performed to test the significance of differences between sub-groups.

There was no significant difference in serum TSH or free T4 concentrations between male and female participants. Because no differences were found within the time span of the study, combined reference intervals were calculated. Indirect reference values were 0.43-3.93 mU/L for TSH and 11.98-21.33 pmol/L for free T4.

Using patient laboratory data values is a relatively easy and cheap method of establishing laboratory-specific reference values if skewness and kurtosis of the distribution are not too large.