Procalcitonin and the Calcitonin Gene Family of Peptides in Inflammation, Infection, and Sepsis: A Journey from Calcitonin Back to Its Precursors

Calcitonin (CT) is a hormone that received its name because of its secretion in response to induced hypercalcemia and its hypocalcemic effect (1). It was shown to originate from the thyroid gland (2). More specifically, the hormone was revealed to be located within the thyroidal parafollicular cells, interspersed within and about the follicular epithelium (3–5). Subsequently termed C cells, they occur primarily in the central region of each lobe of the human thyroid gland (6, 7). These cells, which have CT-containing secretion granules, are neuroendocrine. Embryologically, they originate from the neural crest and migrate to the ultimobranchial glands (8). In mammals, the ultimobranchial glands fuse with the thyroid gland. It was the demonstration that medullary thyroid cancer (MTC) was a malignancy of the C cells (5, 9) that eventually led to the isolation of human CT from this tumor and the determination of its structure (10, 11). Simultaneously, the amino acid sequence of porcine CT was determined (12). Later, the development of immunoassays of serum CT in humans led to the observation that the level of this hormone was increased in the serum of patients with MTC (13–15) and to the demonstration that these levels were further augmented after iv calcium and/or pentagastrin administration (13, 16, 17). These findings had a great impact on the clinical diagnosis, the evaluation of efficacy of surgical extirpation, and the follow-up monitoring of MTC. Although RET germline mutation testing has replaced CT for the purpose of determining the presence of carriers of this tumor associated with multiple endocrine neoplasia type 2 (18, 19), the measurement of serum CT has become and has remained the classical clinical marker for MTC. Immature and mature CT