Tests of Thyroid Function

Thyroid function testing is the most used diagnostic evaluation in endocrine practice and is used as a screening tool, to verify the clinical diagnosis of hyper and hypothyroidism, to assess adequacy of medical treatment, and in the follow up of differentiated thyroid cancer. A small fraction of T4 and T3 are present in the circulation in free or unbound form but only the free thyroid hormone fractions (FT4 and FT3) can enter cells and employ their physiological actions. Most thyroid function tests are performed on serum and are based on automated assays: currently total T4 and T3 (TT4 and TT3) concentrations are measured by competitive immunoassay methods employing immunofluorescence or chemiluminescence, but since several conditions (drugs, pregnancy, nonthyroidal illness, genetic alterations) determine binding protein abnormalities, the measurements of TT4 and TT3 as stand-alone thyroid tests is limited. The assessment of FT4 and FT3 avoids most artifacts due to abnormalities of binding proteins but is technically challenging; direct and indirect methods of measurement have been developed, and their advantages and pitfalls are discussed in this chapter. The sensitivity of the latest generation of TSH assays rarely results in false or artifactual measurements. From a clinical perspective, serum TSH is the first-line test for detecting thyroid dysfunction in ambulatory patients while FT3 measurement is usually unnecessary, since FT4 together with TSH is adequate to diagnose both overt and subclinical hypo- and hyperthyroidism. The measurement of serum thyroglobulin (Tg) in the context of the management of differentiated thyroid carcinoma and other dynamic testing and indicators of thyroid status such as urinary iodine, radioactive iodine uptake, and perchlorate discharge test are also discussed in this chapter.

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Author information

Authors and Affiliations

1. Endocrine Unit, University Hospital of Pisa, Pisa, Italy Giovanni Ceccarini & Ferruccio Santini
2. Department of Endocrinology, University of Pisa, Pisa, Italy Paolo Vitti

1. Giovanni Ceccarini