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Utilizing a double antibody radioimmunoassay for human TSH we compared distribution of serum TSH in 167 normal individuals and 51 patients with idiopathic euthyroid goiter. In addition to being clinically euthyroid both groups had normal total thyroxine, and free thyroid index. Forty-two percent of the goiter group had high TSH and the general distribution of TSH values in the goiterous patients was significantly higher than normal (P less than 0.001). Anlysis of subgroups of the normal and goiter populations indicated that the high TSH could not be attributed to age, sex, use of birth control pills or differences between diffuse and multinodular goiter. TSH levels were significantly higher in patients with goiter less than 1 yr compared to goiter greater than 1 yr (P less than 0.025). In patients with goiter greater than 1 yr the TSH levels remained significantly higher than normal (P less than 0.025). These results support the hypothesis that TSH plays a role in the genesis of idiopathic goiter. The elevation may be present only early in the course of goiter development but is also present in a significant number of patients with long standing goiter.