Studies of the effect of L-thyroxine administration (0.3 mg daily for 7-9 wk) on the peripheral metabolism of (131)I-labeled triiodothyronine (T(3)) and (125)I-labeled thyroxine (T(4)) and on the concentration and binding of T(4) and T(3) in serum were carried out in 11 euthyroid female subjects. Administration of L-thyroxine led to consistent increases in serum T(3) concentration (137 vs. 197 ng/100 ml), T(3) distribution space (39.3 vs. 51.7 liters), T(3) clearance rate (22.9 vs. 30.6 liters/day) and absolute T(3) disposal rate (30 vs. 58 mug/day), but no change in apparent fractional turnover rate (60.3 vs. 60.6%/day). The proportion and absolute concentration of free T(3) also increased during L-thyroxine administration. Increases in serum total T(4) concentration (7.3 vs. 12.8 mug/100 ml) and in both the proportion and absolute concentration of free thyroxine also occurred. In five of the subjects, the kinetics of peripheral T(4) turnover were simultaneously determined and a consistent increase in fractional turnover rate (9.7 vs. 14.2%/day), clearance rate (0.84 vs. 1.37 liters/day), and absolute disposal rate (64.2 vs. 185.0 mug/day) occurred during L-thyroxine administration. Despite these increases in the serum concentration and daily disposal rate of both T(4) and T(3), the patients were not clinically thyrotoxic. However, basal metabolic rate (BMR) values were marginally elevated and, as in frank thyrotoxicosis, T(4)-binding capacities of thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA) reduced, suggesting that subclinical thyrotoxicosis was present. Thus, the often recommended replacement dose of 0.3 mg L-thyroxine daily may be greater than that required to achieve the euthyroid state. The studies have also provided additional evidence of the peripheral conversion of T(4) to T(3) in man and have permitted the calculation that approximately one-third of exogenously administered T(4) underwent deiodination to form T(3). To the extent that a similar fractional conversion occurs in the normal state, it can be calculated that a major fraction of the T(3) in serum derives from the peripheral deiodination of T(4) and that only a lesser fraction derives from direct secretion by the thyroid gland.