Background: Differentiated thyroid cancer is the most common endocrine malignancy and has a high frequency of actionable molecular aberrations including BRAF V600E mutations (45%), RET fusions (10%), and NTRK fusions ( < 2%). FDA approved systemic therapies for metastatic radioiodine refractory differentiated thyroid cancer (RR-DTC) include multikinase inhibitors (Lenvatinib and sorafenib), NTRK inhibitors (larotrectinib and entrectinib for NTRK fusion+ cancers), and RET inhibitors (selpercatinib and pralsetinib for RET fusion+ cancers). Previous phase II clinical trials showed clinical efficacy with first and second generation BRAF inhibitors in patients with BRAF mutant RR-DTC. BRAF inhibitors have not yet been FDA approved for treatment of BRAF mutant RR-DTC. Effective therapeutic options for patients with BRAF mutant RR-DTC remains an important unmet clinical need. BRAF mutant thyroid cancers often show elevated expression of PD-L1. Additionally, BRAF inhibition results in increased expression of PD-L1 in thyroid cancer. This clinical trial seeks to evaluate the safety and efficacy of encorafenib plus binimetinib with or without nivolumab in patients with BRAF mutant metastatic RR-DTC. Encorafenib and binimetinib are highly selective and potent oral inhibitors of BRAF and MEK, respectively. Nivolumab is a potent inhibitor of the immune co-inhibitory receptor programmed cell death protein 1 (PD-1). Methods: This is a phase II, single institution, open-label, randomized clinical trial evaluating the combinations of (Arm 1) encorafenib 450 mg/day + binimetinib 45 mg twice daily and (Arm 2) encorafenib 450 mg/day + binimetinib 45 mg twice daily + nivolumab 480 mg I.V. every 4 weeks in patients with metastatic BRAF mutant RR-DTC. The trial will enroll 20 patients in each arm and treatment will be given in 28 day cycles for up to 2 years. Eligible patients must have metastatic/unresectable BRAF mutant RR-DTC, an ECOG performance status of 0-1 and adequate bone marrow, liver and kidney function. Patients with CNS metastases are included if the metastases have been treated and remained stable or are asymptomatic and ≤10 mm in diameter. Patients may be systemic therapy naïve or have previously been treated with multikinase inhibitors. Prior therapy with BRAF, MEK or immune checkpoint inhibitors is exclusionary. The primary endpoint is confirmed objective response rate (ORR) determined by RECIST v1.1 with restaging imaging every 12 weeks. Secondary endpoints include progression free survival, overall survival, and safety/tolerability (CTCAE v5.0). Arms 1 and 2 will be evaluated independently and are not powered for direct comparison. The trial design includes continuous toxicity monitoring with a Pocock-type stopping boundary. This clinical trial is in progress and 3 patients have been enrolled. Clinical trial information: NCT04061980