CRP and bacterial infectious disease: However, it has been shown that the diagnosis of individuals with COVID-19, especially measuring CRP and PCT, contributed in some cases to the early detection of TC—the observed high PCT level with a low CRP level was a signal suggesting a cause other than bacterial infection, most often TC (commonly medullary thyroid carcinoma, MTC), as PCT is a precursor of calcitonin, which is produced in excess by the thyroid in TC.