Background:

RET proto-oncogene mutations are responsible for familial thyroid medullary carcinoma and multiple endocrine neoplasia (MEN) type 2A and 2B. These syndromes develop specific biomarkers and, in the case of MEN2B, clinically observable stigmas. However, the diagnosis of patients with MEN2B is usually delayed. Because of the close genotype-phenotype correlation, molecular testing is the final approach for the diagnosis to establish preventive care and therapeutic behaviors. Discussion: pM918T is classified as ''highest risk'' for medullary carcinoma with a 50% of lifetime risk for developing pheochromocytoma. Most cases of MEN2B are due to a de novo mutation. Even with the increased risk of developing pheochromocytoma, our 24-year-old patient does not yet present one. Other factors may be involved in the modulation of the phenotype in different populations.

Case report:

We present the case of a woman diagnosed with a thyroid nodule at the age of nine. She underwent a total thyroidectomy plus radical cervical lymph node dissection, with a diagnosis and initial management of papillary thyroid carcinoma. During the evolution of the disease, she developed pulmonary metastases. At the age of 24, after her first endocrinological evaluation, typical physical manifestations of MEN2B were observed. A re-evaluation of the original thyroidectomy revealed a medullary carcinoma, with positive manifestation CEA and calcitonin. The analysis of RET proto-oncogene identified a de novo mutation in exon 16 (pM918T). Conclusion: The timely diagnosis of MEN2B offers opportunities to make appropriate preventive and therapeutic decisions that may change the natural evolution of the disease and its complications.