A 70-yr-old woman with simple goiter showed normal serum levels of T4, T3, free T4, TSH receptor antibody (TRAb) and increased TBG. Discrepancy in serum hTSH level was observed by different assay methods. Coexistence of both autoantibodies for hTSH and for anti-hTSH antibody were demonstrated by the reaction of the patient's antibody with both 125I-hTSH and 125I-anti-hTSH (monoclonal antibody; mAb). These two autoantibodies belong to the polyclonal immunoglobulin G (IgG). The autoantibody for hTSH recognized only beta-subunit of hTSH. Neither stimulating type of TRAb in Graves' disease nor blocking type of TRAb in primary hypothyroidism interfered with the binding of the patient's antibody to 125I-hTSH or 125I-anti-hTSH. Anti-idiotypic antibody (anti-ID antibody) for anti-hTSH antibody was purified by anti-hTSH antibody affinity chromatography. The binding reaction of 125I-anti-hTSH (mAb) with this anti-ID antibody could be inhibited by the unlabeled hTSH. This anti-ID antibody might represent the internal image of the nonbiological active site of TSH molecule, because of absence of thyroid stimulating activity. Goiter in this patient may have occurred by the unbound TSH with IgG (free TSH) and the bound TSH with IgG, because TSH levels in both the whole serum and the IgG free serum (the unbound TSH with IgG) were decreased significantly by T4 treatment. Coexistence of these antibodies may participate in the autoimmune mechanism of an idiotype-anti-idiotype network.