After thyroidectomy for medullary carcinoma of thyroid, which is impoant for determining recurrence of a tumour?

High levels of serum calcitonin and carcinoembryonic antigen are produced by many medullary tumours, which should be tested for in suspected cases.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.Ref: Bailey and Love, page no: 820