Spuriously elevated plasma calcitonin in a patient with a thyroid nodule not associated with medullary thyroid carcinoma

An increase in plasma calcitonin concentration is widely regarded as a spec ific and sensitive indication of underlying medullary thyroid carcinoma (MT C). We present a case in which the association of increased plasma calciton in concentration and a thyroid nodule was not due to MTC. Subsequent measur ement of plasma calcitonin by a variety of methods highlighted the variabil ity that exists in calcitonin measurement and the potential for clinically misleading results. The rationale for investigation and treatment of MTC, i ncluding a recommendation to screen all patients with thyroid nodules using plasma calcitonin measurement, is based on the use of specific two-site ca lcitonin assays which are not universally used in the UK or USA.