If calcitonin does not itself cause cancer, a possible alternative hypothesis to support a putative relationship between calcitonin and increased cancer risk is to suggest that calcitonin elicits a nonspecific tumor-promoting effect. An experiment of nature is medullary thyroid cancer (MTC), an unusual but not rare tumor of the parafollicular (C) cells of the thyroid. It may occur sporadically or as a component of several multiple endocrine neoplasia (MEN) constellations. Individuals with MTC may elaborate extraordinarily high levels of hCT, and serum hCT is used as both a diagnostic and prognostic factor in the approach to treatment [26]. If the hypothesis that calcitonin is a nonspecific oncogen is correct, it could be expected that individuals with MTC would have a high likelihood of second primary cancers, as many tumor types are associated with bidirectional risk of a second primary malignancy. The US Surveillance, Epidemiology and End Results (SEER) program collects data on all persons diagnosed with cancer in specific geographic regions. Using nine SEER registries, Ronckers et al. examined the likelihood of a second primary malignancy after the occurrence of thyroid cancer(s) [27]. The authors were able to identify 884 cases of MTC which were followed for a median of 8 years. These individuals did not show evidence of an increased risk of secondary malignancies, as opposed to individuals with papillary and follicular thyroid carcinoma, who did show evidence of increased risk. These data do not support an association between hCT and increased risk of malignancy.