To evaluate the potential contribution of thyroidal secretion to the relative excess of triiodothyronine (T3) production in hyperthyroidism and to investigate the effects of treatment, iodine (127I), T3 and thyroxine (T4) were measured in digests of thyroid tissue obtained at surgery from 13 patients with Graves' disease. In 11 normal human thyroid glands, 127I content was 630 +/- 60 (all values mean +/- SE in mug/ wet weight) T4, 254 +/- 39 and T3 21 +/-3. The T4I was 26 +/- 3% of the total iodine and the molar ratio T4/T3 was 11 +/- 1. The 13 patients with Graves' disease were divided into three groups. Eleven were clinically euthyroid (Groups I and II) and had received either iodide or iodide plus a thiourea derivative before surgery. Two subjects (Group III) received only propranolol. In Group I (n = 8), mean thyroidal 127I content was 320 +/- 50, T4 was 115 +/- 9 and T3 22 +/- 4. The molar ratio T4/T3 was 5.9 +/- 1 and T4I was 26 +/- 2% of the total. Group II patients (3) had the lowest preoperative serum T4 (less than 2.5 mug/dl) and T3 (less than ng/dl) concentrations with TSH elevated in only one (7 muU/ml). Thyroidal 127I was 100 +/- 26, T49 +/- and T3 1.3 +/- 0.3. The % T4I was 5 +/-2. The two chemically hyperthyroid subjects had a mean tissue 127I of 450; T4, 295 and T3, 56, the T4/T3 ratio was 4.5 and % T4I was 42. There was no correlation between tissue 127I and T4/T3 within either the normal or Graves' disease group. Since adequate clinical and chemical control of hyperthyroidism with antithyroid drugs and iodine was attained in the 8 Group I subjects without a decrease in the % T4I or T3I below that of normal thyroids, it suggests that inhibition of iodotyrosine coupling is not required for this effect. The % T4I was below normal only in patients with marked suppression of serum T4 and T3 concentraions. The lack of correlation between tissue 127I and T4/T3 ratio in the treated patients suggests that the lower T4/T3 ratio in Graves' thyroids is independent of intrathyroidal iodine concentrations. This hypothesis is strengthened by the similarly low T4/T3 ratio in untreated subjects with near normal tissue 127I content. Assuming that the thyroid hormones are secreted in the ratios present in these digests, one can estimate that direct secretion by the thyroid could contribute most, of not all, of the excess T3 production in Graves' disease.