Pembrolizumab in High-risk Thyroid Cancer

This study is testing whether adding Pembrolizumab — an immunotherapy drug that helps the immune system fight cancer — before and after surgery improves outcomes for people with high-risk thyroid cancer. **You may be eligible if...** - You are 18 or older with a confirmed diagnosis of differentiated thyroid cancer (a type of thyroid cancer) that has not yet been treated - Your cancer is considered high risk: large tumor (over 4 cm), spread outside the thyroid, spread to lymph nodes, or has distant spread to other parts of the body - You have not yet received any cancer treatment - You are otherwise healthy enough to undergo surgery and treatment **You may NOT be eligible if...** - You are pregnant or breastfeeding - You have previously received immunotherapy (anti-PD-1 or similar drugs) - You have received other cancer treatments within the past 4 weeks - You have an active autoimmune disease or conditions that suppress your immune system - You have had severe immune-related reactions to prior therapies Talk to your doctor to see if this trial is right for you.