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Background: Antimicrosomal (anti-M) and antithyroglobulin (anti-Tg) antibodies are commonly measured together to detect Hashimoto's thyroiditis. Since this nearly doubles the cost of testing for one antibody, we wished to determine whether significant diagnostic loss would occur if the two tests were replaced by anti-M alone., Methods: Both tests were performed in 2030 consecutive patients referred by general internists and endocrinologists., Results: With a positive result defined as either test being positive at a 1:100 dilution, anti-M was much more sensitive than anti-Tg. Anti-M was positive in 99% (823/831) of all patients with positive tests, while anti-Tg was positive in 36% (302/831). Anti-M was the only positive test in 64% of all patients with positive tests, while anti-Tg was the only positive test in 1%. With a cutoff point of 1:400 dilution, the results were similar., Conclusions: Anti-M alone appears sufficient to detect autoimmune thyroid disease at about one half the cost of routinely performing both anti-M and anti-Tg studies. The widespread practice of performing both tests increases the cost without an offsetting diagnostic gain.