Objective We undertook an investigation on the frequency and magnitude of elevated serum immunoglobulin G (IgG), and effects of treatment with L-thyroxine (T
4) 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 T
4, thyroid stimulating hormone (TSH), microsomal hemagglutination antibody (MHA) titer, and IgG were determined in all subjects.
Results 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 T
4 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-T
4 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-T
4 in such patients lowers IgG levels.
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