Diarrhoea is a feature in 30 per cent of cases of medullary carcinoma and this may be due to 5-hydroxytryptamine or prostaglandins produced by the tumour cells.

Medullary carcinoma are tumours of the parafollicular (C cells) derived from the neural crest and not from the cells of the thyroid follicle as are other primary thyroid carcinomas. The cells are not unlike those of a carcinoid tumour and there is a characteristic amyloid stroma. High levels of serum calcitonin and carcinoembryonic antigen are produced by many medullary tumours. Calcitonin levels fall after resection and rise again with recurrence making it a valuable tumour marker in the follow up of patients with this disease.

Histology of medullary carcinoma showing characteristic ‘cell balls’ and amyloid.