Testosterone-1,2-3H was administered by vein to 7 subjects (3 with normal thyroid function, 2 with hyperthyroidism, and 2 hypothyroids) in order to measure the t ½ of testosterone in the circulation, to observe the distribution and disappearance of its principal radioactive conjugated 17-ketosteroid metabolites in the plasma, and to calculate the clearance rates of the individual conjugates. It was found that the t ½ of testosterone ranged from 60–80 min in the hyperthyroids as well as in an euthyroid subject. In all instances, the labeled glucosiduronates were removed from the plasma much more rapidly than were the radioactive sulfates, 65–90% of their maximum plasma radioactivity disappearing within 4 hr. In the sulfate fraction, androsterone sulfate persisted longer than etiocholanolone sulfate; only 13–22% of its maximum plasma radioactivity being removed at this time. These findings are consistent with the clearance rates (ml per min) of the individual conjugates: androsterone sulfate (0.5–4.9), etiocholanolone sulfate (8.8–19.8) and the corresponding glucosiduronates (73–350). Myxedema was associated with a lower proportion of circulating labeled androsterone conjugates, particularly sulfate, as compared with the euthyroid and hyperthyroid states which is in accord with the distribution among the urinary conjugates.