After adjusting for age, gender, radioiodine treatment, vitiligo and myxedema, neither isotypes nor polymorphisms of C4 was significantly associated with GO, although GD patients with <2 copies (0 or 1) of the C4A gene were less susceptible to GO (p = 0.014, OR = 0.549, 95% CI: 0.303-0.998) as compared to those with 2 copies of C4A, and those with A3B1 polymorphism were less susceptible to GO (p = 0.001, OR = 0.374, 95% CI: 0.146-0.960) as compared to those with A2B2 polymorphism. The gene discussed is C4A; the disease is myxedema.