Progress in standardizing and harmonizing thyroid function tests.

Iodine is an essential component of thyroid hormone. Because thyroid hormone synthesis is affected by iodine deficiency on the one hand and by excess iodine intake on the other, thyroid function biomarkers may be useful for assessing iodine status and studying the effects of iodine supplementation. However, reference intervals for some of the most useful thyroid function biomarkers, including serum concentrations of thyroid-stimulating hormone (TSH), free thyroxine (FT4), and thyroglobulin, vary widely due to variability in the commercially available immunoassays for these tests. Recognizing the need for standardization of thyroid function testing, the International Federation of Clinical Chemistry and Laboratory Medicine established a working group, later restructured as the Committee for Standardization of Thyroid Function Tests, to examine its feasibility. The committee has established a conventional reference measurement procedure for FT4 and an approach to harmonization of results for TSH. Panels of single-donation human blood specimens that span the measuring interval of the immunoassays were used to assess the performance of commercially available immunoassays and form the basis for their recalibration. Recalibration of the manufacturers' methods for both FT4 and TSH has shown that the variability among immunoassays can be successfully eliminated for euthyroid individuals as well as for patients with thyroid disease. The committee is not investigating the standardization of thyroglobulin at the present time.