Instability of harmonized thyroid-stimulating hormone immunoassays in clinical practice

Thyroid-stimulating hormone (TSH) harmonization is effective in minimizing differences between the results of immunoassays in healthy subjects. However, the effectiveness of TSH harmonization in clinical practice has not been investigated. The aim of this study was to evaluate the instability of TSH harmonization in clinical practice.

We compared the reactivities of four harmonized TSH immunoassays using combined difference plots of 431 patients. We selected patients with statistically significant deviations in TSH levels and analyzed their thyroid hormone levels and clinical characteristics.

The combined difference plots showed that one harmonized TSH immunoassay exhibited markedly different reactivity even after TSH harmonization compared with the other three immunoassays. Among 109 patients with mild-to-moderate elevation of TSH levels, we selected 15 patients with statistically significant deviations in TSH levels according to the difference plots of three harmonized TSH immunoassays, excluding one immunoassay that showed different reactivity. The thyroid hormone levels of three patients were misclassified as hypothyroidism or normal due to deviating TSH levels. In terms of clinical characteristics, these patients were in poor nutritional status and general condition, possibly due to their severe illness (e.g., advanced metastatic cancer).

We have confirmed that TSH harmonization in clinical practice is relatively stable. However, some patients showed deviating TSH levels in the harmonized TSH immunoassays, indicating the need for caution, particularly in poorly nourished patients. This finding suggests the presence of factors that contribute to the instability of TSH harmonization in such cases. Further investigation is warranted to validate these results.