Objective We undertook an investigation on the frequency and magnitude of elevated serum immunoglobulin G (IgG), and effects of treatment with L-thyroxine (T4) in patients with Hashimoto's thyroiditis. Methods Ninety-seven consecutive cases of Hashimoto's thyroiditis, 104 patients with simple goiter, and 75 normal subjects were analyzed retrospectively. Serum total T4, thyroid stimulating hormone (TSH), microsomal hemagglutination antibody (MHA) titer, and IgG were determined in all subjects.

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... sults IgG was significantly elevated in patients with Hashimoto's thyroiditis as a group, and elevation above the upper limit of the normal range was found mostly in patients with hypothyroidism. There is a significant negative correlation between serum total T4 and IgG, and a significant positive correlation between TSH and IgG in patients with Hashimoto's thyroiditis. In addition, MHA titer was positively correlated with serum IgG. In a longitudinal analysis of hypothyroid patients with Hashimoto's thyroiditis, restoration of euthyroidism by L-T4 administration was associated with a consistent decrease in serum IgG. Conclusion Serum IgG concentration is increased in patients with Hashimoto's thyroiditis, particularly in hypothyroid patients, and treatment with L-T4 in such patients lowers IgG levels.