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Sodium Iodide I 131

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Sodium Iodide I 131

SODIUM IODIDE I 131
(sodium iodide, i-131) Capsule

DRUG DESCRIPTION

Sodium Iodide 131 (Na 131I) for diagnostic use is supplied for oral administration in opaque white gelatin capsules. The capsules are available in a strength of 3.7 megabecquerels (100 microcuries) iodine I-131 at the time of calibration.

Sodium Iodide I 131 (sodium iodide i 131 (sodium iodide i 131 capsules) capsules) Capsules are prepared by absorbing a solution of carrier-free sodium iodide I-131 into inert fillers. The iodine I-131 utilized in the preparation of the capsules contains not less than 99% iodine I-131 at the time of calibration.

Physical Characteristics

Iodine I-131 decays by beta and associated gamma emissions with a physical half-life of 8.04 days.1 The principle beta emissions and gamma photons are listed in Table 1.

Table 1. Principal Radiation Emission Data

Radiation Mean Percent Per
Disintegration
Energy (keV)
Beta-1 2.12 69.4 Avg.
Beta-3 7.36 96.6 Avg.
Beta-4 89.3 191.6 Avg.
Gamma-7 6.05 284.3
Gamma-14 81.2 364.5
Gamma-17 7.26 637.0

External Radiation

The specific gamma ray constant for iodine I-131 is 2.27 R/hr-mCi at 1 cm. The first half-value thickness of lead (Pb) for iodine I-131 is 0.24 cm. A range of values for the relative attenuation of the radiation emitted by this radionuclide that results from interposition of various thicknesses of Pb is shown in Table 2. For example, the use of 4.6 cm of Pb 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
0.24 0.5
0.95 10-1
2.6 10-2
4.6 10-3
6.5 10-4
*Data supplied by Oak Ridge Associated Universities, Radiopharmaceutical Internal Dose Information Center, Oak Ridge, TN, 1987.

To correct for physical decay of this radionuclide, the fractions that remain at selected time intervals after the date of calibration are shown in Table 3.

Table 3. Physical Decay Chart, Iodine I-131, Half Life 8.04 Days

Days Fraction Remaining Days Fraction Remaining
0* 1.000 16 0.252
1 0.917 17 0.231
2 0.842 18 0.212
3 0.772 19 0.194
4 0.708 20 0.178
5 0.650 21 0.164
6 0.596 22 0.150
7 0.547 23 0.138
8 0.502 24 0.126
9 0.460 25 0.116
10 0.422 26 0.106
11 0.387 27 0.098
12 0.355 28 0.089
13 0.326 29 0.082
14 0.299 30 0.075
15 0.274    
*Calibration Day

1Kocher, David C., “Radioactive Decay Data Tables,” DOE/TIC 11026, page 133 (1981).

Last reviewed on RxList: 5/19/2009
This monograph has been modified to include the generic and brand name in many instances.

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