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 T
3 and T
4 were assayed by double antibody radioimmunoassay. In case 1, serum T
3 level was 16 ng/dl and serum T
4 level was 2.0 μg/dl. In case 2, serum T
3 levels were high (range, 375-4660 ng/dl), while serum T
4 remained at a very low level (0.9μg/dl). On the other hand, the level of T
3 in case 2 as well as the level of T
4 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-T
3 and
125I-T
4, followed by precipitation with polyethylene glycol. The binding of
125I-T
3 with serum from case 2 and the binding of
125I-T
4 with serum from case 1 were markedly high, suggesting the presence of T
3-and T
4-binding substance (s) in the sera. IgG prepared from the sera of both cases showed marked and specific binding with T
3 in case 2 and T
4 in case 1. The association constant for T
4-antibody in case 1 was 5.2×10
8M
-1 and for
3-antibody in case 2 was 5.0×10
9M
-1. The binding capacity for T
4 was 1.2 ng/mg.IgG and for T
3 was 0.3 ng/mg.IgG.
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