SUN-392 - Does TSH Concentration after Recombinant Human Thyrotropin Predict Response to Radioiodine Therapy for Differentiated Thyroid Cancer at Two Years?

Background: Recombinant human thyrotropin (rhTSH) facilitates radioiodine (RAI) therapy in patients with differentiated thyroid cancer (DTC). Unlike thyroid hormone withdrawal protocols, TSH levels are not commonly assessed after rhTSH, and optimal TSH thresholds remain unclear. We hypothesized that TSH levels 3 days after rhTSH independently predict excellent response to therapy (ERT) at 2 years.

Methods: We retrospectively reviewed DTC patients receiving rhTSH-stimulated RAI from 2010-2022. Multiple logistic regression was used to identify predictors of ERT 2 years after primary DTC treatment (surgery + RAI). The primary predictor of interest was TSH level measured 3 days after the second rhTSH injection (“day-3 TSH”). Model covariates, all of which were assessed around the time of RAI, included patient age, American Thyroid Association (ATA) risk of recurrence, lymph node metastases, distant metastases, RAI dose, and thyroglobulin concentration 3 days after the second rhTSH injection (“day-3 TG”). Distant metastases, which exhibited near-perfect separation with ERT, was subsequently removed from the model to improve statistical power and model stability. Secondary analyses assessed associations between day 3 TSH and patient factors such as BMI and age.

Results: 212 patients met inclusion criteria and were analyzed. At the time of RAI, mean (SD) age and BMI were 51.6 (13.5) years and 32.9 (9.0) kg/m2, respectively. ATA risk was low, intermediate, and high in 43.9%, 41.5%, and 14.6% of patients, respectively. 52.3% of patients achieved ERT at 2 years. Day-3 TSH was not an independent predictor of ERT at 2 years (adjusted odds ratio [aOR] per 1 mIU/L increase in log-transformed day-3 TSH = 1.05, 95% CI 0.77–1.44, p = 0.75). Each ATA risk designation above “low” was associated with lower odds of achieving ERT at 2 years (aOR = 0.39, 95% CI 0.24–0.63, p < 0.0001), and each ng/mL increase in log-transformed day-3 TG was associated with lower odds of achieving ERT at 2 years (aOR = 0.69, 95% CI 0.53–0.89, p < 0.01). Patient age at time of RAI, lymph node metastases at time of RAI, and RAI dose did not independently predict ERT at 2 years. In secondary analyses, day-3 TSH was not associated with patient age or BMI.

Conclusion: Although both ATA risk category and rhTSH-stimulated TG were independent predictors of ERT at 2 years, this analysis does not support the hypothesis that achieved TSH concentrations after rhTSH administration predict the likelihood of ERT at 2 years among patients receiving rhTSH-facilitated RAI as a component of primary DTC treatment. This finding suggests that TSH assessments shortly after rhTSH-facilitated RAI administration may not provide important prognostic value.

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