Laboratory diagnosis of thyroid disease

Editorial Perspective: From a dermatologist's vantage point, there are two major reasons to order thyroid function studies: to confirm a clinical suspicion of either hyperthyroidism or hypothyroidism. This is done by checking a thyroid-stimulating hormone (TSH) level with thyroxine (T₄) level (or on occasion, triiodothyronine [T₃]) or by screening for thyroid autoantibodies in an effort to ascertain if a patient is at risk for the development of autoimmune thyroid disease. For example, a 52-year-old woman presents with mild keratoderma and alopecia. A determination of TSH and T ₄ levels will rapidly determine if she is hypothyroid. In another typical case, an 8-year-old boy with alopecia areata is found to have elevated thyroper-oxidase antibodies and a normal TSH. This child may have the potential to develop autoimmune thyroid disease and should be followed periodically with a TSH level to be certain that neither Graves disease (GD) nor Hashimoto thyroiditis ensues. The appropriate use of thyroid function studies depends on a fundamental knowledge of thyroid hormone physiology. Many tests are now considered historical; however, their designs offer insights into thyroid physiology. On occasion, the dermatologist may order imaging studies of the thyroid gland, such as for the evaluation of thry-oglossal duct cysts or for assessing the gland for thyroid adenomas or carcinomas in patients with Cowden syndrome. In this chapter, the clinician reviews the utility and limitations of thyroid function tests and imaging studies so that they may be used appropriately in the diagnosis of thyroid disease, risk assessment for associated disorders, and as tools for monitoring response to treatment for those patients receiving thyroid hormone.