Standardization of FT4 and harmonization of TSH measurements - a request for input from endocrinologists and other physicians.

Given the prevalence of thyroid disorders and the subtle signs and symptoms that may accompany subclinical disease, reliable laboratory testing for serum TSH and free thyroid hormones is important for both primary care physicians and endocrinologists. The laboratory community has recognized the need for standardization of thyroid function tests to achieve comparability of measurement results between methods. This applies in particular to tests for free T4 (FT4), which may be considered controversial in terms of clinical and analytical validity. However, variability is also observed with TSH testing - a fact which has not been emphasized in ongoing discussions regarding lowering the upper limit of normal and/or common decision limits to start treatment for hypothyroidism. In response to this need, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) Committee for Standardization of Thyroid Function Tests worked over the years towards the goal of standardization of FT4 and TSH testing.