Changes in serum TSH, T4 and T3 concentrations after complete thyroidectomy were observed for 4 weeks in 9 euthyroid patients with thyroid carcinoma. Completeness of the thyroidectomy was confirmed by a 131 I scintigram performed one or two months after the operation. Serum TSH levels rose progressively during the 4 weeks of observation, and a significant increase was noticed as early as 3 days after the thyroidectomy. Serum T4 and T3 concentrations decreased significantly 3 days and 18 hrs, respectively, after the thyroidectomy. A sharp decrease in serum T3 concentrations within 18 hrs without a significant change in serum T4 levels and possibly without a significant change in the amount of T3 derived from T4 suggests that the amount of T3 secreted from the thyroid is large enough to affect serum T3 concentrations. The rate of decrease of serum T4 (t 1/2: 16 days) or serum T3 (t 1/2: 23 days) after the 3rd day of the thyroidectomy was much slower than the rate of disappearance of labeled T4 or T3 reported previously. The slow decrease rate of serum T3 is probably due to the conversion of T4 to T3 in peripheral tissues, and that of serum T4 may be due to either the decrease in T4 disposal rate in hypothyroidism or due to the release of T4 from peripheral tissues to serum.