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There are a number of connections between calcitonin and the parathyroid hormone (PTH). The two substances are both proteins that serve as hormones, and cause changes throughout the body by binding to cellular receptors. Both are made in similar areas of the body. Additionally, both of these hormones play an important role in maintaining the correct levels of calcium and phosphate within the blood. Calcitonin and PTH are also both regulated by their own levels in the blood, as well as by the blood calcium concentration.
One of the first connections between calcitonin and PTH is that the two substances are produced in the same general area of the body. Calcitonin is made by the parafollicular cells, also known as the C-cells, of the thyroid gland. An endocrine organ located in the anterior neck region, the thyroid gland's main function is to produce thyroid hormone, a substance important in regulating the body's metabolism. Four glands called the parathyroid glands are located symmetrically in the posterior aspect of the thyroid gland. Their main function is to produce PTH, which is made by the chief cells of these glands.
Another link between calcitonin and PTH is that both molecules are hormones, which are chemical mediators that help the body maintain a suitable internal environment. Both of these substances are polypeptides, which means that they are composed of amino acids linked together in a specific order. These substances cause changes in the operation of the body by binding to cellular receptors.
Calcitonin and PTH are further linked because they both help to regulate the levels of calcium and phosphate throughout the body. In general, the effect of PTH is to increase the levels of calcium and phosphate in the blood; it does this by increasing the release of these minerals from the bone, producing an activated form of vitamin D that helps the intestines absorb more calcium, and increasing the reabsorption of calcium in the kidneys. Calcitonin has an opposite effect, as it decreases the levels of calcium in the blood. Overall, PTH has a strong effect on the blood concentration of calcium, whereas calcitonin has a more moderate effect and is not critical in helping the body maintain normal blood calcium levels.
Similarities in calcitonin and PTH also exist in how the production of these hormones is regulated. Low blood calcium levels stimulate the production of PTH, whereas high levels increase calcitonin secretion. Both hormones are also regulated by negative feedback loops, meaning that having high blood levels of these hormones shuts off further production. For example, the chief cells of the parathyroid glands can sense when there are high levels of PTH in the blood, and cease production of PTH when they detect this state.