ENDOCRINE ORGANS IMAGED
Thyroid
Uses iodine to synthesize:
- T3
- T4
Therefore radioactive iodine and pertechnetate can be used.
Parathyroid
Produces- Parathyroid hormone (PTH)
Controls
- Calcium
- Phosphate
Abnormal glands can be localized using 99mTc-MIBI.
RADIOIODINE UPTAKE TEST (RAIU)
The Radioiodine Uptake Test (RAIU) measures how much radioactive iodine is taken up by the thyroid gland over a specific period.
It is a functional test, not an imaging test.
Principle-
Normal thyroid follicular cells actively transport iodine through the: Sodium-Iodide Symporter (NIS)
The thyroid cannot distinguish between:
- Stable iodine
- Radioactive iodine
Therefore it takes up both.
The amount of radioactive iodine absorbed reflects thyroid function.
Radiopharmaceutical
Usually I-123
Sometimes I-131 (less commonly today because of higher radiation dose).
Procedure
Step 1
Patient receives Oral capsule or liquid containing radioactive iodine.
Step 2
Wait
Usually
- 2–6 hours
- 24 hours
Step 3
A probe is placed over the thyroid.
The amount of radiation emitted is measured.
Step 4
Percentage uptake is calculated.
Normal Uptake
Normal values vary slightly between laboratories.
Typical values:
4- 6-hour uptake: approximately 6-18%
24-hour uptake: approximately 10–30%
Increased Uptake
Occurs when thyroid hormone synthesis is increased.
Examples:
- Graves disease
- Toxic multinodular goiter
- Toxic adenoma
Decreased Uptake
Occurs when thyroid hormone synthesis is reduced or iodine uptake is suppressed.
Examples:
- Thyroiditis
- Exogenous thyroid hormone intake
- Recent iodine contrast administration
- Amiodarone (iodine-rich drug)
Clinical Indications
- Hyperthyroidism
- Differentiate causes of thyrotoxicosis
- Evaluate toxic nodules
- Assess thyroid function before radioiodine therapy
Contraindications
- Pregnancy
- Breastfeeding
- Recent iodinated contrast
- Recent excess iodine intake