Parathyroid Hormone

• Parathyroid Hormone (PTH) is secreted by the parathyroid glands, small glands located in the neck.
• Its primary function is to regulate calcium and phosphate levels in the blood.
• PTH increases blood calcium by acting on bones (releasing calcium), kidneys (reabsorbing calcium and excreting phosphate), and indirectly on the intestines (via vitamin D activation).
• Proper PTH function is essential for strong bones, healthy nerve transmission, and muscle contraction.
• Imbalances in PTH can lead to significant health issues, including bone disorders and kidney problems.

What is Parathyroid Hormone (PTH)?

Parathyroid Hormone (PTH) is a polypeptide hormone secreted by the parathyroid glands, which are typically four small glands situated behind the thyroid gland in the neck. The primary role of Parathyroid Hormone is to maintain calcium homeostasis within the body, ensuring that blood calcium levels remain within a very narrow and healthy range. This intricate process is essential because calcium is not only a major component of bones and teeth but also plays critical roles in nerve impulse transmission, muscle contraction, blood clotting, and cellular signaling. The mechanism by which Parathyroid Hormone is explained involves a feedback loop where low blood calcium levels stimulate its release, and high calcium levels inhibit it.

When calcium levels in the blood drop, the parathyroid glands detect this change and respond by increasing the secretion of PTH. Conversely, when calcium levels rise, PTH secretion is suppressed. This continuous monitoring and adjustment highlight why PTH is a cornerstone of mineral metabolism, directly impacting the health and function of multiple organ systems. Its actions are complex and coordinated, involving several target organs to achieve its regulatory goals.

Parathyroid Hormone’s Role in Calcium and Phosphate Balance

The intricate parathyroid hormone function is primarily centered on regulating the concentrations of calcium and phosphate in the blood. This hormone acts on three main target organs: the bones, the kidneys, and indirectly, the intestines, to achieve its effects. The overall parathyroid hormone role in body is to elevate blood calcium levels while simultaneously decreasing blood phosphate levels, thereby maintaining a delicate balance critical for physiological health.

Here’s how Parathyroid Hormone achieves this balance:

• On Bones: PTH stimulates osteoclasts, which are cells responsible for breaking down bone tissue. This process, known as bone resorption, releases calcium and phosphate from the bone matrix into the bloodstream, directly increasing blood calcium levels.
• On Kidneys: In the kidneys, PTH has a dual effect. It enhances the reabsorption of calcium from the filtered fluid back into the blood, preventing its loss in urine. Simultaneously, it promotes the excretion of phosphate into the urine, which helps to lower blood phosphate levels. This is important because high phosphate levels can bind with calcium, reducing the amount of free, active calcium available in the blood.
• On Intestines: PTH indirectly influences calcium absorption from the diet. It stimulates the kidneys to convert inactive vitamin D into its active form, calcitriol (1,25-dihydroxyvitamin D). Calcitriol then acts on the intestines, significantly increasing the absorption of dietary calcium into the bloodstream.

Through these coordinated actions, Parathyroid Hormone ensures that the body maintains optimal calcium and phosphate levels, which are essential for skeletal integrity, proper nerve and muscle function, and overall metabolic health. Dysregulation of PTH can lead to conditions such as hyperparathyroidism (excess PTH) or hypoparathyroidism (insufficient PTH), both of which can have significant health consequences due to calcium and phosphate imbalances.