Comparison of the Early Thyroidal Iodide Clearance With Estimates Obtained at Later Intervals

Abstract

SUMMARY The rate of thyroidal iodide clearance has been measured repeatedly during the periods 2–20 min., 20 min.-2 hr., 2–10 hr. and 10–24 hr after i.v. administration of 131I Early measurements (after 2–20 min.) gave higher values than the estimates obtained at later, more conventional, periods. Treatment with carbimazole produced a slight decrease in the clearance when measured between 2 and 20 min., a more pronounced decrease when measured between 20 min. and 2 hr., and a decrease to virtually zero after 2 hr. It is suggested that the 2–20 min. measurements may correspond to the theoretical 'unidirectional' clearance, whereas later measurements, and especially those obtained after 2 hr., may correspond to the theoretical 'net' clearance. The 'net' clearances, expressed as a percentage of the 'unidirectional' one, may probably be used as an index of the proportion of iodide which, after trapping, remains organically bound in the thyroid. In normal persons the mean 2–10 hr. clearance was 55·4 ± 3·09 (s.e.)% of the value obtained between 2 and 20 min., suggesting that normally only about 55 % of the iodide trapped remains in the thyroid. In thyrotoxicosis and simple goitre the clearance values were high in absolute terms, but when expressed as a percentage of the 2–20 min. values, results obtained during the periods 20 min.-2 hr. and 2–10 hr. did not differ significantly from normal. In a patient with Pendred's syndrome with defective organic binding of iodide, the 2–10 hr. clearance was 7·4% of the 2–20 min. value.

The Thyroid

“…The 1-dose was lower when the gland was hyperfunctional ; and correlation was observed with PIl, and hence presumably intracellular iodide concentration (210). Koutras and co-workers (212), following the earlier suggestion of Ingbar, subdivided thyroidal clearance into "unidirectional clearance" at two to twenty min utes after intravenous inj ection of 1311, and "net clearance" at two hours or more when loss of unbound 1* was complete (212). Koutras and co-workers (212), following the earlier suggestion of Ingbar, subdivided thyroidal clearance into "unidirectional clearance" at two to twenty min utes after intravenous inj ection of 1311, and "net clearance" at two hours or more when loss of unbound 1* was complete (212).…”

Measurements of the Radioactive Iodide and Pertechnetate Uptake for the Study of the Trapping and Binding Functions of the Human Thyroid

“…However, Thomas et al (30) were probably the first who deliberately introduced the 10 min radioiodine uptake for studying thyroid function in patients receiving antithyroid drugs. This subject has been investigated more intensively by Koutras and Sfontouris (21), who showed that the early thyroidal iodide clearance rate (calculated between 2 and 20 min) resulted in systematically higher values than estimates obtained at later time intervals. These workers suggestend that the earlier the estimate, the more it corresponded to the theoretical "unidirectional" clearance, i.e.…”

“…Papadopoulos et al (26) have detected a small degree of intrathyroidal metabolism of pertechnetate, but this appears to be of little significance for early uptake measurements. The iodide trapped is also bound organically only in part (8,21), and so it seems that the difference between iodine and pertechnetate is quantitative rather than qualitative, but nevertheless considerable. Since the degree of organic binding of these isotopes is so different, it should be of interest to compare their thyroidal uptake.…”

“…Since the degree of organic binding of these isotopes is so different, it should be of interest to compare their thyroidal uptake. Because early uptake measurements reflect the trapping process to a greater extent than later estimates (21), in the present work several measurements have been obtained, 5, 30 and 60 min. after the i.v.…”

Use of"Tcm for the Routine Assessment of Thyroid Function

“…It has been claimed that early thyroid uptake measurements are unaffected by such drugs and that they can be used to measure the thyroid iodide concentrating mechanism or trap (Thomas et al, 1960;Koutras and Sfontouris, 1966). Whether or not this is so will depend on the rate at which trapped iodide becomes bound, on the time at which the measurement is made, and on the extent to which organic binding of iodine is impaired by the antithyroid drug.…”