Role of radioactive iodine uptake in management of thyroid disease

Learning Objectives 1. To demonstrate the role of radioactive iodine uptake (RAIU) in diagnosis and therapy of thyroid diseases. 2. To demonstrate different tracers and techniques used for RAIU.

In RAIU I-131 or I-123 administrate orally in a capsule or in liquid form. Medication that might interfere with thyroid uptake should be discontinued for an appropriate length of time and patient should have fasting for 4 hours prior the study. The RAIU at various intervals of time is measured using a gamma scintillation counter or camera based. The percentage of RAIU 24 hours after the administration of RAI is most useful, since in most instances the thyroid gland has reached the plateau of isotope accumulation, and in this time, the best separation between high, normal, and low uptake is obtained. RAIU at 2-6 hours is also done to detect thyroid gland with rapid iodine turnover. Normal values for 24-hour RAIU in most parts of North America are 5 to 30”% percent. In many other parts of the world, normal values range from 15 to 50 %. The main indications of RAIU includes, differential diagnosis of thyrotoxicosis (TOX), estimate the therapeutic dose in Graves’ disease (GD) and in thyroid cancer together with whole body scan estimate residual thyroid post surgery, estimate I-131 therapy effectiveness and follow-up for recurrence. In TOX, RAIU can differentiate between causes with decreased thyroidal RAIU which typically encountered in the syndrome of transient thyrotoxicosis (both de Quervain's and painless thyroiditis), thyrotoxicosis factitia, Amiodarone-induced TOX (type II is near zero and type I may be elevated) and iodide-induced TOX (Jod-Basedow disease) and causes with elevated RAIU as in GD, multinodular toxic goiter or hasitoxicosis. Tc-99m thyroid uptake may be use as alternative to RAIUI with favorable radiation dosimetry and short study length (30 min) however it is not suitable for calculating RAI therapeutic dose. I will demonstrate some example cases for RAIU in different thyroid disease