The Laboratory Approach to Thyroid Disorders
The central role of the thyroid gland in controlling metabolism was recognized in the 19th century, but evaluation of the function of the thyroid remains an evolving science. Initial approaches to the assessment of thyroid function centered on measuring end-organ responses as biological markers of thyroid hormone actions. Development of in vitro competitive binding assay methods allowed the direct quantification of hormone levels in serum, and sensitive immunoassays have demonstrated the subtleties of pituitary and hypothalamic control of the thyroid. Abnormalities of hormone binding by serum proteins necessitated sensitive estimation of free hormone levels. With the detection of serum markers of autoimmune and malignant diseases of the gland has come earlier diagnosis and improved monitoring of these conditions, often with greater sensitivity than may be clinically relevant. Limitations to the measurement methods used exist, however, particularly when underlying assumptions about the comparability of patient and control specimens are invalid. Nonetheless, the clinician can now effectively confirm suspected diagnoses of thyroid dysfunction, cost-effectively screen asymptomatic populations for common diseases, and appropriately monitor the treatment of patients with disorders of the thyroid.