At least two distinct mechanisms appear to be responsible for accumulation of iodine by the thyroid gland. The first, called the "iodide trap" by Vanderlaan and Vanderlaan (1) by abrupt disappearance of the radioiodine from the thyroid within an hour.The foregoing is in marked contrast to the manner of accumulation of radioiodine by the untreated thyroid gland (4, 5). The accumulation of radioiodine in the unblocked thyroid can be expressed by an exponential curve which levels off between 24 and 48 hours and thereafter the quantity present in the thyroid diminishes gradually over a period of several weeks. Hyperfunctioning glands accumulate radioiodine both more rapidly and in larger quantities than normal glands and subsequently lose the radioiodine more rapidly than normal glands. Analyses of thyroid tissue removed 48 or more hours after administration of a dose of radioiodine reveal that more than 90 per cent of the radioiodine present is organically combined (6). Taurog and Chaikoff (7) have shown that the untreated thyroid gland of the rat converts the radioiodine which it accumulates to organic form with astonishing rapidity, 95 per cent of the quantity present in the thyroid being organically bound as early as 15 minutes after the dose has been given. The accumulation curve of the untreated thyroid gland may thus be regarded as reflecting both the iodide-trapping and the synthetic function, but it seems probable that both the maximal point in the curve and the rate at which accumulation occurs reflect hormonal synthesis more than trapping.The accumulation of iodine by the human thyroid as disclosed by radioiodine has been shown to vary significantly in varying states of thyroid activity (4,8,9) and has attracted attention both as an aid in diagnosis of thyroid disease and more importantly as a new approach to the clinical study of normal and disordered thyroid function. It represents a distinctly different aspect of thy-217

No abstract