Sources and Production of Procalcitonin
Normal Physiological State
Procalcitonin (PCT) is normally produced exclusively in the C-cells of the thyroid gland as a 116 amino acid polypeptide precursor to calcitonin, with circulating levels in healthy individuals remaining below detectable limits. 1, 2, 3
- PCT has a molecular weight of approximately 13 kDa and serves as the prohormone for calcitonin production 1, 2
- In healthy states, PCT is intracellularly cleaved by proteolytic enzymes into active calcitonin hormone before release 2
- Circulating PCT levels are undetectable in the absence of pathological conditions 2
Extrathyroidal Production During Infection
During severe systemic infections, PCT production shifts dramatically to extrathyroidal tissues throughout the body, with levels rising substantially within 3-5 hours of bacterial exposure and peaking around 10 hours. 1, 4
Primary Extrathyroidal Sources
The most significant PCT-producing organs during infection include:
- Liver and kidney (highest concentrations) 4
- Neuroendocrine cells in the lungs and intestine (probable primary sites) 2
- Adipose tissue, adrenal glands, bladder, and ovaries 4
- Vascular tissue (aorta) 4
Molecular Evidence of Extrathyroidal Production
- RT-PCR studies confirm calcitonin mRNA expression in liver, lung, kidney, adrenal, colon, skin, spleen, brain, and pancreas during sepsis 4
- PCT production occurs even in thyroidectomized patients, definitively proving extrathyroidal synthesis 1
- Tissue concentrations above 0.2 ng/g wet tissue are found in multiple organs following endotoxin exposure 4
Mechanism of Infection-Induced Production
PCT production during inflammation is triggered by bacterial endotoxin (lipopolysaccharide) and inflammatory cytokines, particularly TNF and IL-6. 2
- Serum PCT rises from undetectable to >4 ng/mL within 10 hours of LPS exposure in primate models 4
- The production pathway is distinct from normal thyroidal calcitonin synthesis 2
- PCT produced during sepsis does not bind to cellular calcitonin receptors and does not significantly affect calcium/phosphate metabolism 2
Clinical Caveats
PCT elevation does not always indicate bacterial infection—medullary thyroid carcinoma metastases can produce persistently elevated PCT levels (>100 ng/L) independent of infection status. 5
- In patients with medullary thyroid cancer metastases, PCT is synthesized by tumor cells and remains elevated regardless of infection resolution 5
- This makes PCT an unreliable infection marker in this specific population 5
- Calcitonin levels are also elevated in these cases, confirming tumor-derived production rather than inflammatory response 5