ABSTRACT Studies of iodine metabolism have been carried out in 15 pregnant women, 33 cases with sporadic goitre and 11 with thyrotoxicosis. A low plasma inorganic iodine was common to the three groups. In pregnancy and sporadic goitre the thyroid clearance of iodine was elevated and the absolute iodine uptake normal. A high thyroid clearance of iodine in thyrotoxicosis was associated with a high absolute iodine uptake. The results suggest that both pregnancy and sporadic goitre are physiological responses to an iodine deficiency state while the iodine deficiency state of thyrotoxicosis is secondary to increased thyroid activity.