Elevated serum immunoglobulin G4 levels in patients with Graves' disease and their clinical implications

Immunoglobulin G4-related disease (IgG4-RD) is a new clinical entity that affects various organs with increased IgG4 positive plasmacytes and progressive fibrosis. While IgG4-RDs in association with Hashimoto's thyroiditis or Riedel's thyroiditis have been reported, the relationship between IgG4-RD and Graves' disease (GD) is yet unknown. To elucidate the relation of GD to IgG4-RD, serum IgG4 levels and their clinical implications in patients with GD were investigated.

In this prospective study, serum IgG4 levels were measured in 109 patients with GD and classified into two groups according to the comprehensive diagnostic criteria of IgG4-RD previously established: (i) GD with elevated-IgG4 levels (≥ 135 mg/dL), and (ii) GD with nonelevated IgG4 (<135 mg/dL).

Seven out of 109 patients with GD (6.4%) had elevated serum IgG4 levels [mean ± standard deviation (range): 175.0 ± 44.5 (136-266) mg/dL] and elevated ratios of IgG4/IgG [12.7 ± 4.5% (7.6%-21.2%)]. The remaining patients with GD had serum IgG4 levels and IgG4/IgG ratios of 39.6 ± 27.6 (3-132) mg/dL and 3.2 ± 2.2% (0.3%-11.5%), respectively. Ages in the elevated IgG4 group were significantly higher than those of the nonelevated IgG4 group: 54.7 ± 6.2 versus 43.4 ± 15.4 years, respectively. Ultrasound examinations revealed that the elevated IgG4 group had significantly increased hypoechogenic areas in the thyroid in comparison to the nonelevated IgG4 group (low echo scoring: 1.66 ± 0.81 vs. 0.61 ± 0.89, respectively). In the correlation analysis, TSAb (rs=0.385, n=42) titers were significantly correlated with se rum IgG4 levels, while they were not significantly different between the two groups. In the elevated IgG4 group, symptoms were controllable with a small dose of antithyroidal drug (ATD; n=4), a combination treatment with ATD and L-T4 (n=1), or L-T4 administration only one year after the first visit (n=2).

A small portion of GD patients harbored elevated serum IgG4 levels. They were older, had increased hypoechoic areas in the thyroid, and appeared to be responsive or prone to be hypothyroid after ATD treatment. Thus, the present study suggests the presence of a novel subtype of GD. Measuring serum IgG4 levels may help to distinguish this new entity and provide potential therapeutic options for GD.