Predictors of pre-ablation I-123 neck uptake measurements in post thyroidectomy patients

Objectives Following thyroidectomy, patients with well differentiated thyroid cancer routinely receive I-123 NaI for radiodine neck uptake measurements (RAIU) prior to I-131 ablation. The study's goal was to identify patient specific predictors of RAIU.

Methods 201 patients received 1-3 mCi I-123 NaI po, & imaged at 24 hour for RAIU measurement. Patient prep for RAIU was either Thyrogen injection (n=126) or thyroid hormone withdrawal (n=75). Patient age, weight (wt), operating surgeon, thyroid weight and TSH were recorded. Univariate & mulitiple variate analysis was performed.

Results 201 patients were treated by 6 surgeons. The mean patient age was 51.6 years; with median patient wt 79 kg. The median wt of surgically removed thyroids was 18.4 gm. The mean TSH level was 91.3uU/mL. There was a weak but significant correlation between patient wt & wt of the surgically removed thyroids (p=0.46). The median RAIU was 0.35%, with 25% of patients having RAIU ≤ 0.15% & 25% of patients having RAIU ≥ 1.3%. Patients receiving a Thyrogen injection had significantly lower RAIU values than patients undergoing thyroid hormone withdrawal (p<0.001). Patients who underwent thyroidectomy by the non-endocrine surgeon had significantly higher RAIU values (p=0.017). Patients with higher levels of TSH had higher RAIU values (p=0.030). Patients with greater body wts had lower RAIU values (p=0.088). There was no association between patient age & RAIU (p=0.748) or between the resected thyroid wt & RAIU (p=0.387).

Conclusions TSH levels, patient’s wt and operating surgeon were identified as significant predictors of post thyroidectomy RAIU. Patient’s age and resected thyroid wt were not predictors of post thyroidectomy RAIU.