In Vivo Non Imaging Procedures:

Radioactive Iodine Uptake Test (RAIU)

RADIOPHARMACEUTICAL

 I-123 sodium iodide (200 mCi) or  I-131 sodium iodide (5 mCi)

PROCEDURE

• We want to answer the question “What percent of the I-123 NaI in the capsule accumulates in the thyroid gland at 24 hr post administration of the dose?"
   
• Patient is NPO from midnight
   
• Capsule(s) administered with cup of water.
   
• Patient is NPO until 1 hour after capsule administration
   
• Patient’s thyroid is counted 24 hr post dose administration.
   
• Calculation of % RAIU
   

CALCULATIONS OF THE RESULTS

FORMULA:

% UPTAKE =     (NET COUNTS / MIN IN THYROID)  X 100%

                    DECAY-CORRECTED NET COUNTS/MIN IN CAPSULE

SAMPLE PROBLEM:

Given: 400 µCi capsule of I-123 NaI was counted At T₀. Patient then swallowed capsule and returned at 24 hr for counting.

       (t_1/2 = 13.3 hr; 24 hr decay factor: 0.2863)

Patient background:               1,000 counts/5 min

Capsule background:             1,000 counts/5 min

Thyroid counts At T₂₄:            50,000 counts/2 min

Capsule counts At T₀:            300,000 counts/2 min

CALCULATION:

% UPTAKE =            (NET COUNTS/MIN IN THYROID)  X 100%

                         DECAY-CORRECTED NET COUNTS/MIN IN CAPSULE

 
% UPTAKE        =              (25,000-200)          X 100%

                                    (150,000-200) X .2863

 
% UPTAKE        =     58.7%

 

INTERPRETATION

• Normal: 8-30%
   
• Hypothyroid: <8%
   
• Hyperthyroid: >30% (Upper limit of normal range varies from 30-35% at different hospitals)

INTERFERENCES

• Contrast media containing iodine
   
• Certain vitamin preparations, cough drops
   
• Thyroid extract, synthroid, PTU, amiodarone
   
• Dietary supplements, e.g., kelp and seaweed based foods