Background: \- Medullary thyroid cancer (MTC) is a rare cancer of the thyroid gland. In children and adults, it is often part of a condition called Multiple Endocrine Neoplasia 2 (MEN2). MEN2 is usually caused by a genetic mutation, and it can cause a number of problems in addition to MTC. These problems include adrenal gland tumors, hormone changes, and problems with the bones and other organs. Not much is known about how MTC develops over time, especially in people with MEN2. Researchers want to study MTC in children and adults and see how it affects their growth and development.

Objectives: \- To study how medullary thyroid cancer affects children and adults over time.

Eligibility: \- Children and adults who have medullary thyroid cancer.

Design: * Participants will be screened with a brief physical exam and medical history. Blood and tissue samples will be collected to see whether participants have the MEN2 genetic mutation. * Treatment will not be provided as part of this study. However, participants will be receiving standard care for MTC. They may be eligible for other clinical trials at the National Institutes of Health. * Participants will have regular study visits every 6 to 12 months to evaluate their MTC and any treatment. Blood tests, imaging studies, and other tests may be performed as needed to monitor the disease. * Participants and their parents/guardians will also complete questionnaires about their health and emotions during the study.