A sensitive and specific double antibody radioimmunoassay for the measurement of serum thyroglobulin (Tg) has been developed. The minimum detectable concentration of Tg was 5.0 ng/ml. Coefficients of within and between assay variation were 2.4% and 12.0%, respectively. The mean recovery of Tg added to normal serum was 102.9%; and T4, T3, DIT and MIT did not crossreact in this assay system. Dilution curves of normal thyroid extract, tumor extract and patient's serum were shown to be parallel with the standard Tg preparation. The mean serum Tg level in normal males and females was 42.8 +- 5.3 ng/ml (mean +- SE) (N=29) and 117.1 +- 20.9 ng/ml (N=20), respectively. There was a significant difference between male and female groups (p less than 0.001). The mean serum Tg level was 365.0 +- 69 ng/ml in 19 hyperthyroid patients with Graves' disease and 248.1 +- 35.8 ng/ml in 21 patients who were in a euthyroid state from the treatment with antithyroid drugs, showing statistically no significant difference. However, 8 patients in permanent remission showed definitely low Tg values, 83.6 +- 16.2 ng/ml. The mean serum Tg level was 2101.1 +- 57.6 ng/ml in 6 patients with chronic thyroiditis without auto-anti Tg and 525.1 +- 207.5 ng/ml in 5 patients with thyroid adenoma. The effect of total thyroidectomy on the serum TSH and Tg was studied in a patient (M.T.) with pulmonary metastases from thyroid follicular adenocarcinoma. The serum TSH level rose progressively to hypothyroid levels during nine days after thyroidectomy; the value was 45.7 muU/ml on the 9th day after the thyroidectomy. The serum Tg level in this case was 4 925 ng/ml before surgery. After a transient fluctuation caused by the operation, the serum Tg level in the patient increased progressively during 39 days after surgery with a concomitant increase in serum TSH; the levels at the 3rd, 6th and 9th day after surgery were 5,825 ng/ml, 7,910 ng/ml and 11,190 ng/ml, respectively. The suppression of endogenous TSH secretion with treatment of T3 60 MICROGRAMS/DAY WAS FOLLOWED BY A GRADUAL FALL In serum Tg levels, decreasing to 630 ng/ml at the 114th day. Bovine TSH was administered to this patient at the 114th day, so as to study the effect of exogeneous TSH on serum Tg. Serum Tg reached a maximal peak at the 24 hr. after bovine TSH injection. The maximal increase of serum Tg above baseline was 221%. Despite complete removal of the thyroid gland, the increase in serum Tg after thyroidal stimulation with endogenous and exogenous TSH was observed in the patient. In addition, the increase in serum Tg after bovine TSH injection was also observed in two patients with differentiated thyroid carcinoma who underwent a total thyroidectomy and had only metastatic tissue. These results indicate that the elevated serum Tg was released from metastatic tissue by TSH. The present study demonstrates direct evidence that metastatic tissue from thyroid carcinoma is responsive to TSH...