Calcitonin is a neuroendocrine active substance that can be secreted by medullary thyroid carcinoma and is an observable indicator of medullary carcinoma. However, not all medullary carcinomas secrete calcitonin, and high calcitonin does not necessarily mean that the carcinoma is medullary.
It is more important to look at the dynamics of calcitonin levels than the value of a single test. For example, if it’s 100 on one test, 300 on the next, and 500 on the next, that’s something to keep in mind. If it’s 100 consistently, sometimes 90, sometimes 70, in the low range, and there’s no problem with the thyroid, there’s no need to worry too much, and medullary carcinoma is unlikely.