Background and Aims The predisposition to the development of tumors in multiple endocrine organs, either simultaneously or sequentially, is inherited as an autosomal dominant. To date, three types of multiple endocrine neoplasia (MEN) syndrome have been described, of which MEN IIa is the most common, with medullary thyroid carcinoma (MTC) (ca. 97%), pheochromocytoma (ca. 30%) and hyperparathyroidism (50%). In 1987, the genetic alteration causing this syndrome was mapped to chromosome 10. The responsible gene was later identified as the RET protooncogene. At one codon, an amino acid is replaced by another.

Patients and Methods The family was mapped over three generations. Genetic testing for the RET mutation was started in 2001. The RET mutation was located in codon 891 in five patients . Total thyroidectomy and associated neck dissection was performed in all five patients. Two patients had a medullary thyroid carcinoma, one had a multifocal medullary microcarcinoma and two had lymph-node metastases. Pathohistologic assessment showed C-cell hyperplasia. Postsurgery basal and peak calcitonin value were normal in four patients. Calcitonin determination did not distinguish between medullary thyroid cancer and C-cell hyperplasia.

Conclusion To the best of our knowledge, a coincidence of tetralogy of Fallot and MEN IIa has never been described, either in the MEN or the Fallot literature. Prophylactic thyroidectomy based on genetic testing allows identification and treatment of patients at an early stage of the disease.