Clinical Strategies in the Testing of Thyroid Function

In the past decade, emphasis has shifted from testing of thyroid function in individuals who are likely to have a thyroid disorder to a broader population, an approach that identifies so-called subclinical thyroid dysfunction in up to 10% of women over fifty. The key assays that are used to detect thyroid dysfunction are serum thyroid stimulating hormone (TSH) and the main circulating thyroid hormones thyroxine (T4) and triiodothyronine (T3), either as total or estimated free concentrations. For these key parameters, it is preferable for results to be interpretable in relation to reference intervals that are independent of the particular method that is used. This requirement is satisfied for well standardized assays for serum TSH, total T4 and total T3. However, when free T4 is estimated, assay results often need to be evaluated in relation to method-specific reference intervals or “normal ranges”. This limitation is particularly cogent during pregnancy and in the face of critical illness (see 6.4.2 and 6.7.2 below).