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Parathyroid hormone is released in response to low extracellular concentrations of free calcium. When calcium concentrations fall below the normal range, there is a steep increase in secretion of parathyroid hormone. Low levels of the hormone are secreted even when blood calcium levels are high. The figure to the right depicts parathyroid hormone release from cells cultured in vitro in differing concentrations of calcium.

Parathyroid hormone (PTH) has a significant role in calcium homeostasis in humans. A rise in PTH levels stimulates all the following except
   
A,the deposition of calcium into bones and teeth making them strong and healthy

   
B,the increase in absorption of calcium ions by kidney nephrons

   
C,the breakdown of bones and teeth to release calcium into the blood

   
D,the increase in absorption of calcium from the intestines to raise calcium levels

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Calcitonin and PTH are an example of two antagonistic hormones in regards to calcium levels.
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