Optimal laboratory testing for diagnosis and monitoring of thyroid nodules, goiter, and thyroid cancer.

Optimal use of laboratory tests to diagnose and monitor patients with goiter, thyroid nodules, or thyroid cancer requires an appreciation of the pathophysiologic factors implicated in thyroid hyperplasia and neoplasia: growth factors (especially thyrotropin, TSH), growth-stimulating immunoglobulins, activating mutations of the TSH receptor, and other oncogenic transformations. In patients with diffuse goiter and thyroid nodules, serum TSH measurement in a highly sensitive assay excludes both primary hypothyroidism and common causes of thyrotoxicosis. In selected patients, screening for anti-thyroid peroxidase with or without anti-thyroglobulin antibodies can confirm the diagnosis of autoimmune thyroiditis. Serum calcitonin measurement is appropriate only when medullary thyroid carcinoma (MTC) is clinically suspected. Laboratory testing is essential in management of thyroid carcinoma patients after primary surgical therapy. Serum TSH measurement is vital to ensure that thyroxine replacement and TSH suppression are adequate in treatment of epithelial cancers. Serial monitoring of serum thyroglobulin (Tg) can detect tumor recurrence and quantify tumor burden. Interpretation of serum Tg results requires an appreciation of certain technical considerations (e.g., anti-Tg antibody interference) and the patient's concurrent TSH status. Periodic serum Tg measurements and 131I scans are complementary monitoring techniques. Serum calcitonin measurement and screening for ret protooncogene mutations are both valuable for identifying individuals with MTC.