The body maintains a state of balance known as homeostasis, and hormonal regulation is a component of this process. Parathyroid hormone (PTH) is produced by four parathyroid glands located in the neck behind the thyroid. This hormone is central to managing the body’s calcium levels, a mineral needed for nerve signal transmission, muscle function, and bone health.
The Core Feedback Loop
The regulation of parathyroid hormone is a direct response to the concentration of calcium in the blood. This control system operates as a negative feedback loop, meaning the output of the system—in this case, PTH—ultimately reduces the initial stimulus. The cells of the parathyroid glands are equipped with calcium-sensing receptors on their surface that monitor the amount of ionized calcium circulating in the bloodstream.
When these receptors detect that blood calcium levels have fallen below a normal range, a condition known as hypocalcemia, they trigger the parathyroid glands to increase the secretion of PTH. The glands respond to even slight drops in calcium, ensuring a response to restore balance.
How Parathyroid Hormone Restores Calcium Balance
Once released into the bloodstream, PTH travels to three primary target locations: the bones, the kidneys, and the intestines. In the bones, which act as the body’s main reservoir of calcium, PTH stimulates cells called osteoclasts. These cells are responsible for bone resorption, a process where bone tissue is broken down, releasing stored calcium into the blood.
Simultaneously, PTH acts on the kidneys to conserve calcium that would otherwise be lost through urine. It increases the reabsorption of calcium in the kidney tubules, effectively returning it to the bloodstream. The hormone also initiates another process in the kidneys: the conversion of vitamin D into its active form, calcitriol. This active vitamin D then travels to the small intestine, where it enhances the absorption of calcium from digested food.
The ‘Off Switch’ for Hormone Release
The actions of PTH on the bones, kidneys, and intestines cause blood calcium levels to rise. As the concentration of calcium in the blood returns to the normal range, the same calcium-sensing receptors on the parathyroid glands that initiated the response now detect this increase. This elevation in blood calcium serves as an inhibitory signal.
This signal functions as an “off switch” for hormone production. In response to normalized or slightly elevated calcium levels, the parathyroid glands reduce their secretion of PTH. This reduction prevents calcium levels from overshooting the normal range and becoming too high, a condition called hypercalcemia. The entire cycle demonstrates a self-regulating mechanism that maintains calcium within a very narrow and stable range.