Any of the chemical messengers produced by the thyroid gland. For many years it was generally believed that the thyroid gland produced only two closely related hormones, thyroxine and triiodothyronine. However, in 1961 a new hormonal principle, calcitonin, was discovered by Harold Copp. He originally thought that this substance was secreted by the parathyroid glands, but in 1963 it was established that the source of this hormone in mammals was the thyroid gland, and it was renamed thyrocalcitonin. In 1967 it was isolated in pure form and in 1968, synthesized. It is a polypeptide containing a single chain of 33 amino acids, in contrast to thyroxine and triiodothyronine which are iodinated thyronines.