SUN-313 Lower Than Expected Levothyroxine Doses Achieve Complete TSH Suppression

Abstract Disclosure: N. Gutta: None. Z.Z. Alvi: None. A.G. Gianoukakis: None. Background: Management of differentiated thyroid cancer (DTC) following surgery involves levothyroxine(LT4) administration to suppress TSH levels, thereby reducing recurrence risk. Target TSH ≤ 0.1 mIU/L is recommended in high-risk DTC patients. While the LT4 replacement dose of 1.6 µg/kg in non-cancer patients is well-established, optimal suppressive dosing for thyroid cancer patients is not well studied. Common clinical practice suggests utilizing 2.1 µg/kg but lacks robust supporting data. Objective: To determine the weight-based LT4 dose required to achieve TSH suppression to ≤ 0.1 mIU/L in patients with DTC who have undergone total thyroidectomy, with or without radioactive iodine (RAI) ablation. Methods: An internal RedCap database, was used to record patient demographics, cytological and pathological diagnoses for quality improvement since 2010. A retrospective chart review was conducted using the RedCap database to identify high risk DTC patients who underwent total thyroidectomy between December 2015 and April 2024. Medical records were reviewed to identify patients with a TSH goal ≤0.1 mIU/L, and the LT4 dose at which they achieved their goal was recorded. The LT4 dose was divided by the patient's weight to calculate the weight-based LT4 dose. Eligible patients were adults aged 18–65 years with documented weight, BMI, LT4 therapeutic dose, a defined TSH goal, and at least two TSH measurements ≥4 weeks apart. Exclusion criteria included medullary or anaplastic thyroid cancer, pregnancy, medication non-compliance, interfering medications, systemic therapy with multikinase inhibitors, or significant gastrointestinal or renal disorders. Results: 835 patient charts were reviewed. 100 patients had malignant cytology, of which 55 met the inclusion criteria. The cohort was 86.2% female and 57.3% Hispanic. The median weight was 76.1 kg, the median LT4 dose was 137 mcg, and the median weight-based LT4 dose required for TSH suppression to ≤ 0.1 mIU/L was 1.78 mcg/kg (95% CI: 1.68–1.95). In the same population, the median LT4 dose required to maintain a TSH goal of 0.5–2.0 mIU/L was 1.38 mcg/kg in a cohort of 30 patients which served as an internal control. Conclusion: In our DTC population, a weight-based LT4 dose of 1.78 mcg/kg is sufficient for TSH suppression to ≤ 0.1 mIU/L. The explanation for the lower-than-expected suppressive dose of LT4 identified is unclear, but internally consistent. Further studies are warranted to tailor the LT4 dose in high-risk DTC patients based on the population. Presentation: Sunday, July 13, 2025