The roles of parathyroid hormone (PTH) in regulating the blood calcium level in mammals.

Control of blood calcium

Because calcium ions (Ca²⁺) are essential to the normal functioning of all cells, homeostatic control of the level of calcium in the blood is vital.

If the blood Ca²⁺ level falls substantially, skeletal muscles contract convulsively, a potentially fatal condition.

If the blood Ca²⁺ level rises substantially, calcium phosphate can form precipitates in body tissues, leading to widespread organ damage.

In mammals, the parathyroid glands, a set of four small structures embedded in the posterior surface of the thyroid, play a major role in blood Ca²⁺ regulation.

When the blood Ca²⁺ level falls below a set point of about 10 mg/100 mL, these glands release parathyroid hormone (PTH).

PTH raises the level of blood Ca²⁺ through direct effects in bones and the kidneys and an indirect effect on the intestines.

In bones,

PTH causes the mineralized matrix to break down, releasing Ca²⁺ into the blood.

In the kidneys,

PTH directly stimulates the reabsorption of Ca²⁺ through the renal tubules.

In addition,

PTH indirectly raises the blood Ca²⁺ level by promoting vitamin D production.

A precursor form of vitamin D is obtained from food or synthesized by skin exposed to sunlight.

Conversion of this precursor to active vitamin D begins in the liver.

PTH acts in the kidney to stimulate the completion of the conversion process.

Vitamin D acts on the intestines, stimulating the uptake of Ca²⁺ from food.

As the blood Ca²⁺ level rises, a negative feedback loop inhibits further release of PTH from the parathyroid glands.

The thyroid gland can also contribute to calcium homeostasis.

If the blood Ca²⁺ level rises above the set point, the thyroid gland releases calcitonin, a hormone that inhibits bone breakdown and enhances Ca²⁺ excretion by the kidneys.

In fishes, rodents, and some other animals, calcitonin is required for Ca²⁺ homeostasis.

In humans, however, calcitonin is needed only during the extensive bone growth of childhood.