Abstract. The aim of this work was to estimate the daily urinary excretion of free and conjugated thyroxine using a direct radioimmunoassay and enzyme hydrolysis. The renal clearance of free T4 was also determined. The mean urinary values of free and total T4 (mean ± 1 sd) in 112 euthyroid controls were 1353 ± 496 and 1855 ± 651 pmol/24 h, respectively. Urinary excretion of free hormone in 13 hyperthyroid patients was 5552 ± 4320 pmol/24 h and total T4 was 8122 ± 7219 pmol/24 h. Urinary free T4 excretion was 223 ± 223 pmol/24 h in hypothyroid patients and total T4 was 542 ± 490 pmol/24 h. These results indicate that daily urinary T4 excretion is a good indicator of thyroid function. The mean renal clearance of free T4 was 52 ± 19 ml/min (mean ± 1 sd) in euthyroid patients, 53.7 ± 12.3 ml/min in hyperthyroid patients, and 67.6 ± 13.1 ml/min in hypothyroid patients. We estimated the endogenous creatinine renal clearance as a control of the renal filtration rate. The data suggest that there is T4 filtration of unbound T4 and partial tubular reabsorption. Further experimental studies will be necessary to clarify the renal handling of thyroxine as well as the fate of reabsorbed T4.