橋本病による若年性甲状腺機能低下症の姉妹例に認められた抗T₃, 抗T₄自己抗体に関する検討

抄録

Anti-thyroid hormone autoantibodies were found in two sisters of short stature (case 1 : 13 years old, case 2 : 10 years old). Physical examination revealed clinical findings of hypothyroidism with diffuse goiter. A diagnosis of Hashimoto's thyroiditis was made by open biopsy of the thyroid gland in both patients. Serum levels of T₃ and T₄ were assayed by double antibody radioimmunoassay. In case 1, serum T₃ level was 16 ng/dl and serum T₄ level was 2.0 μg/dl. In case 2, serum T₃ levels were high (range, 375-4660 ng/dl), while serum T₄ remained at a very low level (0.9μg/dl). On the other hand, the level of T₃ in case 2 as well as the level of T₄ in case 1 was not detectable by a single antibody radio-immunoassay using dextran-coated charcoal or polyethylene glycol separation. In each case, the serum TSH level measured by double antibody radioimmunoassay was extremely high (235 μU/ml in case 1,240 μU/ml in case 2), which was compatible with the clinical features of primary hypothyroidism. Sera from both patients were incubated with ¹²⁵I-T₃ and ¹²⁵I-T₄, followed by precipitation with polyethylene glycol. The binding of ¹²⁵I-T₃ with serum from case 2 and the binding of ¹²⁵I-T₄ with serum from case 1 were markedly high, suggesting the presence of T₃-and T₄-binding substance (s) in the sera. IgG prepared from the sera of both cases showed marked and specific binding with T₃ in case 2 and T₄ in case 1. The association constant for T₄-antibody in case 1 was 5.2×10⁸M^-1 and for ₃-antibody in case 2 was 5.0×10⁹M^-1. The binding capacity for T₄ was 1.2 ng/mg.IgG and for T₃ was 0.3 ng/mg.IgG.