FMTC (OMIM#155240) is characterized by a strong
predisposition to MTC in families with a very low incidence
of other endocrinopathies related to MEN2. It is clinically
diagnosed in families with four or more cases of MTC. It is
viewed as a phenotypic variant of MEN2A with decreased
penetrance of pheochromocytoma and primary hyperparathyroidism.
It has been diagnosed more frequently in
recent years and is reported to account for 35–60% of all
MEN2 cases (5,7-9). Regarding MTC in FMTC, penetrance is
lower and the clinical course is more benign than in MEN2A
and MEN2B, with a late onset or no clinically manifesting
disease and the prognosis is relatively good. Therefore, a
family history is often inadequate for establishing familial
disease.