We have experienced 2 cases of Graves' disease associated with anti-thyroid hormone auto-antibodies. Case 1: a 24-year-old pregnant woman was found to have exacerbation of Graves' disease of which she had been diagnosed when she was 10 years old. She was treated with propylthiouracil (PTU, 450mg/day) since March 26, 1985 when intra-uterine fetal death was found. Since serum total T
3 (TT
3) levels had been high even long after abortion in the face of normalization of total T
4 (TT
4), free T
4, and thyrotropin (TSH), presence of anti-thyroid hormone antibodies was suspected. Addition of labelled thyroid hormone to her serum followed by precipitation of γ-globulin fraction with 12.5% of polyethylene glycol showed an increased binding of
125I-T
3 or
125I-T
4r being 19.3% or 11.0%, respectively, to her serum γ-globulin fraction. Scatchard plot of anti-T
3 antibodies in her serum was analyzed in two components; one with a higher affinity constant (5.4×10
7 M
-1), and the other with a lower affinity constant (1.0×10
6 M
-1). Case 2: a 16-year-old female had been diagnosed as Graves' disease when she was 15 years old and had been treated with MMI for a year. By the treatment, she became physically euthyroid, and serum total thyroid hormone (TT
3, TT
4) levels declined to the normal level. However, since free thyroid hormone (FT
3, FT
4) levels showed unusually high levels, presence of anti-thyroid hormone antibodies in her serum was suspected. Her serum γ-globulin bound 34.5% and 12.2%, respectively, of
1251-T
3 and
125I-T
4. Scatchard plot of anti-T
3 antibodies in her serum was analyzed into two components; one with a higher affinity constant (2.4×10
10 M
-1), the other with a lower affinity constant (3.1×10
9 M
-1). Although further characterization of anti-T
4 antibodies in both cases has not been done, from the binding data of
125I-T
4 to both sera, in addition to anti-T
3 antibodies, both cases were considered to have anti-T
4 antibodies. Our present cases indicate that it is clinically important to bear the presence of anti-thyroid hormone antibodies in mind to account for the possible error in measuring T
3 or T
4 by RIA. In the case that RIA results give unexpectedly high or low T
3 and/or T
4 value, the presence of autoantibodies should be considered first and a test for them is recommended.
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