The most specific autoantibody test for autoimmune thyroiditis is an ELISA test for anti-TPO antibody. Anti-TPO antibodies are diagnostic for chronic autoimmune thyroiditis, such as Hashimoto thyroiditis. Some patients with Graves disease may also have these antibodies, which are often absent in toxic adenoma. The diagnostic antibody for Graves disease is the thyrotropin receptor antibody (TRab). Although measurement of the TSH level is the most reliable screening method for assessing thyroid function, the degree of thyrotoxicosis cannot be estimated easily in this way. Instead, thyrotoxicosis must be measured using an assay of thyroid hormone levels in the plasma.
Toxic multinodular goiters are characterized by irregular areas of relatively diminished and occasionally increased uptake. Overall radioactive iodine uptake (RAIU) is mildly to moderately increased. In Graves disease, scintigraphy shows diffuse enlargement of both thyroid lobes, with uniform uptake of isotope. Overall RAIU is elevated.
For more on the workup of hyperthyroidism and thyrotoxicosis, read here.
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Cite this: Romesh Khardori. Fast Five Quiz: How Much Do You Know About Hyperthyroidism? - Medscape - May 08, 2017.