Procalcitonin (PCT) is a peptide precursor of the hormone calcitonin, the latter being involved with calcium homeostasis. It is composed of 116 amino acids and is produced by parafollicular cells (C cells) of the thyroid and by the neuroendocrine cells of the lung and the intestine. The level of procalcitonin in the blood stream of healthy individuals is below the limit of detection (0.01 µg/L) of clinical assays. The level of procalcitonin rises in a response to a pro-inflammatory stimulus, especially of bacterial origin. It is therefore often classed as an acute phase reactant.