ABSTRACT. Individuals diagnosed with intermediate risk differentiated thyroid cancer (DTC) encounter significant uncertainty regarding the risks and benefits of radioactive iodine (RAI). RAI is routine treatment for patients with high risk DTC based on the documented increased rate of survival. In contrast, RAI is generally not used for patients with low risk DTC, as it does not decrease recurrence nor increase survival. Due to absence of data from randomized controlled trials, controversy remains regarding the balance of benefits and risks for RAI treatment for patients with intermediate risk DTC. Clinical practice guidelines now suggest consideration of RAI in patients with intermediate risk disease. Thus, most patients with intermediate risk DTC choose between surgery alone vs surgery plus RAI. Data from our team and others indicate patients report a lack of information about the potential side effects of RAI including salivary gland damage, nasal and lacrimal symptoms, reproductive impact, secondary malignancies, and effects on quality of life. Salivary gland damage occurs among 21-77% of patients after RAI in a dose-dependent manner. Other RAI-dose dependent effects include lacrimal and nasal symptoms. Patients also report increased fatigue and decreased quality of life, particularly with complications from RAI. Patients with intermediate risk DTC have a need to balance what is known about the risks and benefits of RAI. Using established data concerning RAI benefits and compelling new data about the frequency of side effects due to RAI, we seek to develop and evaluate a decision aid (DA) to promote informed decisions about RAI treatment among these patients. Guided by the Ottawa Decision Support Framework, we will first develop and pilot test the DA (UG3 Aim 1). We will use preliminary data, team input and published guidelines to create an outline of the DA. We will develop a web-based prototype and conduct usability testing with 16 patients (8 with, 8 without RAI). We will revise the DA and recruit 30 patients with newly diagnosed intermediate risk DTC for a pilot randomized controlled trial of the DA to examine the feasibility, usability and satisfaction with the DA. We will compare the DA (n=20) to a usual care (UC) educational control website (n=10) to explore the effect of the DA on informed choice about RAI, knowledge, self-efficacy, and decisional conflict and regret at 4 weeks post-randomization. We will make iterative refinements to the DA. In the UH3 phase (years 3-7; UH3 Aims 2, 3), we will conduct a multi-site randomized controlled trial of the finalized DA at three clinical sites. We will screen 800 patients with newly diagnosed intermediate risk DTC and enroll and randomize 400 patients (n=200 DA arm; n=200 UC arm). We will conduct assessments at baseline and 1 week, 4 weeks and 6 months post-randomization. Our primary outcome is informed choice at 4 weeks (Aim 2). Secondary outcomes include knowledge, decisional conflict and r...