December 6, 2005

Anthony Butch, PhD

This passage is excerpted from the National Academy of Biochemistry’s Laboratory Medicine Practice Guideline titled “LABORATORY SUPPORT FOR THE DIAGNOSIS AND MONITORING OF THYROID DISEASE.” To view the complete document—in English, Spanish, or French—go to the NACB web site

Physicians need quality laboratory testing support for the accurate diagnosis and cost-effective management of thyroid disorders. On occasion, when the clinical suspicion is strong, as in clinically overt hyperthyroidism in a young adult or with the presence of a rapidly growing thyroid mass laboratory thyroid hormone testing simply confirms the clinical suspicion. However in the majority of patients, thyroid disease symptoms are subtle in presentation so that only biochemical testing or cytopathologic evaluation can detect the disorder. However overt or obscure a patient's thyroid problem may be, an open collaboration between the physicians and clinical laboratory scientists is essential for optimal, cost-effective management of the patient with thyroid disease.

Over the past forty years, improvements in the sensitivity and specificity of biochemical thyroid tests, as well as the development of fine needle aspiration biopsy and improved cytological techniques, have dramatically impacted clinical strategies for detecting and treating thyroid disorders. In the 1950s, only one serum-based thyroid test was available—an indirect estimate of the total (free + protein-bound) thyroxine (T4) concentration, using the protein bound iodide technique.

Today, urine iodide concentrations are measured directly by dry or wet-ash techniques and are used to estimate dietary iodide intake. The development of competitive immunoassays in the early 1970s, and more recently non-competitive immunometric assay methods, have progressively improved the specificity and sensitivity of thyroid hormone testing. Currently, serum-based tests are available for measuring the concentration of both the total (TT4 and TT3) and free (FT4 and FT3) thyroid hormones in the circulation. In addition, measurements of the thyroid hormone binding plasma proteins, thyroxine binding globulin (TBG), transthyretin/prealbumin (TBPA), and albumin are available.

Improvements in the sensitivity of assays to measure the pituitary thyroid stimulating hormone thyrotropin (TSH) now allow TSH to be used for detecting both hyper- and hypothyroidism. The recognition that autoimmunity is a major cause of thyroid dysfunction has led to the development of more sensitive and specific tests for autoantibodies to thyroid peroxidase (TPOAb), thyroglobulin (TgAb) and the TSH receptor (TRAb). Current thyroid tests are usually performed on serum by either manual or automated methods that employ specific antibodies. Methodology continues to evolve as performance standards are established and new technology and instrumentation are developed.