High calcitonin levels in the body are most commonly caused by medullary thyroid carcinoma (MTC), a type of thyroid cancer that originates from the C cells of the thyroid gland. In MTC, the tumor cells produce excessive amounts of calcitonin, leading to elevated levels in the bloodstream. Additionally, certain conditions, such as C-cell hyperplasia (an overgrowth of C cells) and familial medullary thyroid carcinoma (inherited form of MTC), can also cause high calcitonin levels. Rarely, non-thyroidal neuroendocrine tumors and certain lung cancers may produce calcitonin, resulting in elevated levels as well. Determining the underlying cause of high calcitonin levels requires further evaluation by a healthcare professional.