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Iodide I-123 (Sodium Iodide I-123) - Description and Clinical Pharmacology

 
 



Sodium Iodide I-123 Capsules

Rx Only.
Diagnostic-For Oral Administration

DESCRIPTION

Sodium Iodide I-123 (Na123I) for diagnostic use is supplied in capsules for oral administration. The capsules are available in strengths of 3.7 and 7.4 megabecquerels (MBq) (100 and 200 μCi) I-123 at time of calibration.

The radionuclidic composition at calibration is not less than 97.0 percent I-123, not more than 2.9 percent I-125 and not more than 0.1 percent Te-121. The radionuclidic composition at expiration time is not less than 87.2 percent I-123, not more than 12.4 percent I-125 and not more than 0.4 percent Te-121. The ratio of the concentration of I-123 and I-125 changes with time. Graph 1 shows the minimum concentration of I-123 as a function of time and Graph 2 shows the maximum concentration of I-125 as a function of time.

Graph 1. Radionuclidic Concentration of I-123 PERCENT OF TOTAL RADIOACTIVITY: IODINE-123

Graph 1. Radionuclidic Concentration of I-123 PERCENT OF TOTAL RADIOACTIVITY: IODINE-123


Graph 2. Radionuclidic Concentration of I-125 PERCENT OF TOTAL RADIOACTIVITY: IODINE-125

Graph 2. Radionuclidic Concentration of I-125 PERCENT OF TOTAL RADIOACTIVITY: IODINE-125

Physical Characteristics

Iodine-123 decays by electron capture with a physical half-life of 13.2 hours [Kocher, David C., Radioactive Decay Data Tables. DOE/TIC-11026, 122 (1981).] . The photon that is useful for detection and imaging studies is listed in Table 1.

Table 1. Principal Radiation Emission Data
RadiationMean %
Disintegration
Energy
(keV)
Gamma-283.4159

External Radiation

The specific gamma ray constant for I-123 is 1.6 R/hr-mCi at 1 cm. The first half-value thickness of lead (Pb) for I-123 is 0.005 cm. A range of values for the relative attenuation of the radiation emitted by this radionuclide that results from the interposition of various thicknesses of Pb is shown in Table 2. For example, the use of 1.63 cm of lead will decrease the external radiation exposure by a factor of about 1,000.

Table 2. Radiation Attenuation by Lead Shielding
Shield
Thickness (Pb), cm
Coefficient of
Attenuation
0.005
0.10
0.88
1.63
2.48
0.5
10-1
10-2
10-3
10-4

Note that these estimates of attenuation do not take into consideration the presence of radionuclidic contaminants.

To correct for physical decay of I-123, the fractions that remain at selected intervals after the time of calibration are shown in Table 3.

Table 3. Iodine I-123 Decay Chart: Half-Life 13.2 Hours

Hours
Fraction
Remaining

Hours
Fraction Remaining

*Time of Calibration

0*
3
6
9
12
15
1.000
0.854
0.730
0.623
0.533
0.455
18
21
24
27
30
0.389
0.332
0.284
0.242
0.207

CLINICAL PHARMACOLOGY

Sodium Iodide I-123 is readily absorbed from the upper gastrointestinal tract. Following absorption, the iodide is distributed primarily within the extracellular fluid of the body. It is trapped and organically bound by the thyroid and concentrated by the stomach, choroid plexus and salivary glands. It is excreted by the kidneys.

The fraction of the administered dose which is accumulated in the thyroid gland may be a measure of thyroid function in the absence of unusually high or low iodine intake or administration of certain drugs which influence iodine accumulation by the thyroid gland. Accordingly, the patient should be questioned carefully regarding previous medications and/or procedures involving radiographic media. Normal subjects can accumulate approximately 10 to 50% of the administered iodine dose in the thyroid gland, however, the normal and abnormal ranges are established by individual physician's criteria. The mapping (imaging) of Sodium Iodide I-123 distribution in the thyroid gland may provide useful information concerning thyroid anatomy and definition of normal and/or abnormal functioning of tissue within the gland.

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