Laboratory support for the diagnosis and monitoring of thyroid disease

Abstract

Over the last 40 years, thyroid testing has evolved from manual isotopic tests performed in specialised laboratories to automated non-isotopic measurements made in routine clinical chemistry laboratories. Despite improvements in methodological sensitivity and specificity, a number of technical issues still negatively impact the accuracy and interpretation of thyroid testing in clinical practice. The diagnostic sensitivity of TSH measurement has been maximised by sensitivity improvements (third generation functional sensitivity 0.01 mlU/L). However, controversy remains regarding the setting of the TSH reference range and the clinical significance of mild subclinical hypothyroidism. Heterophilic antibodies (HAMA) can still interfere with a number of the tests, especially when patients have received monoclonal antibody therapies. Free hormones measured by equilibrium dialysis/tandem mass spectrometry or the older two-test index approach (FT4I and FT3I) appear to be more reliable than the newer free hormone immunoassays (FT4 and FT3) when binding proteins are grossly abnormal such as during pregnancy or non-thyroidal illness. Expanded clinical utility and new physiological insights have resulted from technical improvements in the thyroid autoantibody tests (TPOAb, TgAb and TRAb). This presentation will discuss the strengths and weaknesses of current thyroid tests relative to their use and interpretation in clinical practice.