Medullary thyroid carcinoma accounts for 5-10 per cent of thyroid malignancies and occurs sporadically, and in three familial settings with autosomal dominant inheritance. Calcitonin, a 32 amino acid 3.5-kDa protein, is a biochemical marker of the tumour. Serum levels correlate with disease burden. Treatment initiated by screening allows disease removal at a premalignant phase: C cell hyperplasia. Genetic linkage studies locate the multiple endocrine neoplasia type 2A gene near the centromere of chromosome 10. Flanking genetic markers may allow polymorphic probes to examine DNA from a once-only blood sample to identify affected individuals. At what age thyroidectomy should be carried out in these patients is unclear.