Procalcitonin
Procalcitonin (PCT) is a protein whose levels rise during severe bacterial, fungal, parasitic infections, as well as sepsis and multiple organ dysfunction syndrome. PCT does not increase in cases of autoimmune diseases, allergies, or viral infections. Localized limited bacterial infections, minor infections, and chronic inflammation do not cause its elevation. Procalcitonin is tested during infections.
Calcitonin
Calcitonin (CT) is a 32-amino acid peptide secreted by the parafollicular cells (also known as C cells) of the thyroid gland. Its main physiological function is to lower blood calcium and phosphate levels and antagonize parathyroid hormone. Elevated blood calcium stimulates calcitonin secretion. Clinically, calcitonin can be injected to treat patients with osteoporosis. Calcitonin is a tumor marker for medullary thyroid carcinoma. It can be tested when thyroid nodules are suspected of malignant transformation.
Effects of calcitonin on bone: inhibits the formation and activity of osteoclasts, suppresses bone matrix degradation and bone salt dissolution; promotes the transformation of osteoclasts and stromal cells into osteoblasts, enhances their activity, promotes bone formation, and reduces blood calcium and phosphate.
Effects of calcitonin on the kidneys: directly inhibits renal tubular reabsorption of calcium and phosphate, increasing urinary calcium and phosphate excretion.
Effects of calcitonin on the small intestine: inhibits renal 1α-hydroxylase, thereby suppressing calcitriol production, which in turn inhibits intestinal absorption of calcium and phosphate.