Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells (also known as C 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).Calcitonin has been found in fish, reptiles, birds, and mammals. [Source: Wikipedia 
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