Thyroglobulin is a large protein that is synthesized only by thyroid tissue, including normal thyroid cells, benign thyroid nodules, and well-differentiated thyroid cancers. In the absence of normal thyroid tissue, it is a sensitive and specific marker for the presence of thyroid cancer, since Tg levels should be virtually undetectable after a total thyroidectomy and radioiodine ablation.[14] It is important to use a reliable Tg assay, and to screen for the presence of interfering antithyroglobulin antibodies that may give falsely high or low Tg results; reliable clinical laboratories screen for such antibodies. The release of Tg from thyroid cells is, in part, thyrotropin-dependent. Therefore, a low Tg value while patients are receiving thyrotropin suppressive therapy may be misleadingly reassuring.[15] Thyroglobulin is most sensitive and specific as a tumor marker in the patient who has no remnant of thyroid tissue and who has an elevated thyrotropin level, as occurs in preparation for a radioiodine scan; a low Tg value in this setting is evidence against residual or recurrent disease, and some experts believe such a finding may obviate the need for further radioiodine scanning. Conversely, a normal or elevated Tg concentration in a patient who has undergone previous ablation strongly suggests residual or recurrent disease, whether or not the patient is taking a thyroid hormone.