The WHO through its ECBS provides an International Standard for TSH diagnostic measurements. The current 4th WHO IS for TSH, coded 81/615, was established in 2023.
The WHO IS has historically shown poor commutability with patient samples, likely due to glycosylation differences between pituitary and serum TSH. This led to C-STFT developing a TSH serum sample panel for harmonisation via recalibration using the APTM approach.
In the early 2020s, both the 3rd WHO IS and the 2nd C-STFT panel needed replacement. These projects were undertaken in collaboration between C-STFT and MHRA/WHO. The study confirmed lack of commutability of WHO IS with patient samples in some assays. Re-calibration using WHO IS would reduce inter-assay harmonisation compared to the C-STFT panel approach.
Despite limitations, pituitary TSH continues for the WHO IS due to ease of availability (~3,000 units lasting 15-20 years). The co-existence of both resources is crucial: WHO IS as long-term widely available material defining the IU, and C-STFT as restricted resource for improving harmonisation. Future collaboration aims to investigate feasibility of a serum-derived WHO IS for TSH (refs 31, 32).