Studies of the distribution and rate of peripheral degradation of exogenous labeled thyroid hormones have been carried out in goitrous patients from the Uele endemic. The biological half-life of radioiodinated L-triiodothyronine (T3*) is shortened (4 patients). Investigations with I131-labeled thyroxine (T4*), carried out in 15 patients, show a shortening of the biological half-life, an increased fractional rate of turnover of T4* and a reduction of the extrathyroidal organic iodine pool (ETT). The increased turnover rate of T4* and the reduction in ETT compensate each other to some extent and appear to account for the fact that, despite the often substantial reduction of circulating hormonal iodine, these patients catabolize an amount of thyroxine greater than would be expected by the level of protein-bound iodine. The total amount of thyroxine degraded per day gives, on the average, a value at the lower limit of normal, and in some cases a hypothyroid value.