Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells of the thyroid gland in humans, and in many other animals in the ultimopharyngeal body. It acts to reduce blood calcium (Ca2+), opposing the effects of parathyroid hormone (PTH). In its skeleton-preserving actions, calcitonin protects against calcium loss from skeleton during periods of calcium mobilization, such as pregnancy and, especially, lactation.