Correlation of the 24-hour radioiodine uptake of the human thyroid gland with the six- and eight-hour up-takes and the accumulation gradient.

Techniques for the determination of thyroid function have become increasingly refined since the demonstration of the relation of basal oxygen consumption to thyroid activity 50 years ago. Newer methods include the determination of serum protein-bound iodine and various procedures using radioactive iodine. The latter include direct measurements of the accumulation in the thyroid gland of an administered dose of radioactive iodine (1), studies of urinary excretion (2), and observations of the rate of formation of serum protein-bound radioiodine (3). All observers agree that there is an elevated rate of accumulation of radioactivity in the thyroid gland in hyperthyroid, a depressed rate in myxedematous, and an intermediate rate in euthyroid subjects. This is true whether the uptake is measured directly over the gland or indirectly by excretion curves. There is no standardized procedure at present among various institutions using radioactive iodine, but the general preference seems to be for techniques measuring the accumulation of radioiodine directly over the thyroid rather than by excretion ctirves. Probably the measurement of radioactivity in the thyroid gland 24 hours following the administration of I131 (4) is most commonly used. Astwood and Stanley (1) found that if the radioactivity in the thyroid is measured at frequent intervals following the administration of a tracer dose of I131, the concentration in the thyroid is a linear function of the square root of time. The slope of the line expressing this linear relationship is constant for the first eight to 12 hours in euthyroid subjects and has been called the "accumulation gradient." By means of this technique it is pos-