Context: Calcitonin is a well established tumor marker for medullary thyroid carcinoma (MTC). Since surgery is the only effective treatment for patients with MTC, the post operative level of serum calcitonin will dictate if residual disease was left behind and if a re-intervention is necessary.

Results: We describe here the case of 41 yr-old man with metastatic MTC. Despite extensive disease in the neck as well as metastatic lesions in the liver, his serum calcitonin, measured with a commercial one-step immunoradiometric assay, was only minimally elevated (244 ng/L). After serial dilutions, a non-linear relationship became evident, suggesting the presence of a "hook effect". Treatment of the serum with heterophilic blocking reagent revealed no change. Calcitonin was then measured with a different immunoradiometric assay and revealed a much higher level. Similar discrepancies were found in different samples from various patients when analyzed with different calcitonin immunoassays.

Conclusion: To our knowledge, this is the first reported case of a phenomenon like the "hook effect" in a calcitonin IRMA assay in patients with MTC. Being aware of this phenomenon is important as a low calcitonin result could give false reassurance to both the patient and the clinician, and can dramatically change the prognosis of the patient.