PTH Intact Test

Intact parathyroid hormone (PTH) is the active version of a hormone made by the parathyroid glands—four tiny glands located behind the thyroid in your neck. This hormone plays a central role in maintaining the balance of calcium and phosphate, two minerals essential for healthy bones, muscles, nerves, and cell signaling.

When calcium levels in your blood drop even slightly—a change as small as 0.1 mg/dL—specialized sensors on parathyroid cells, called calcium-sensing receptors (CaSR), detect the shift. In response, they stimulate the rapid release of PTH. This hormone then acts on three major targets to restore normal calcium levels:

• Bone: PTH pulls calcium from bone stores by stimulating cells called osteoclasts, which break down bone tissue.
• Kidneys: PTH helps the kidneys reabsorb calcium so it’s not lost in urine, while also encouraging phosphate excretion.
• Gut (indirectly): PTH boosts the production of active vitamin D (called calcitriol), which increases calcium absorption from food.

PTH works fast—within minutes—and is quickly cleared from the bloodstream by the kidneys and liver. Its half-life, or how long it stays active in circulation, is just 2 to 4 minutes. After that, the hormone breaks into fragments, especially C-terminal fragments, which are no longer biologically active. In healthy people, only about 5–30% of measurable PTH in blood is the intact, active hormone. The rest consists of these fragments.

Intact PTH is used to evaluate several key health conditions:

• Primary hyperparathyroidism: Caused by overactive parathyroid glands (often from a benign tumor), this leads to excess PTH and high calcium levels, which can weaken bones and increase fracture risk.
• Secondary hyperparathyroidism: Common in CKD or vitamin D deficiency, this is a reactive increase in PTH as the body tries to correct low calcium or high phosphate levels.
• Hypoparathyroidism: A condition where PTH is too low, often after surgery or due to autoimmune disease, leading to low calcium and symptoms like muscle cramps or tingling.
• Pseudohypoparathyroidism: A rare condition in which PTH is high, but the body’s tissues don’t respond properly to it—sort of like insulin resistance, but with PTH.

Ultimately, measuring intact PTH—alongside calcium, phosphate, and vitamin D—offers powerful insights into your bone health, mineral metabolism, and parathyroid function. It helps identify imbalances that, left unchecked, can accelerate aging in bone and kidney systems.