The tiny, rice-sized glands controlling your body's calcium balance

Health · Published 18 October 2026 Swipe more like this →

In short: Hidden behind your thyroid are four glands, each no larger than a grain of rice. They secrete parathyroid hormone (PTH), a powerful chemical messenger that regulates calcium and phosphorus. When this delicate balance shifts, the consequences for your bones, kidneys, and even your mood can be profound.

Parathyroid hormone (PTH) is a peptide hormone that acts as a master regulator of calcium homeostasis. Its primary role is to increase serum calcium levels by acting on the bones, kidneys, and small intestine. In the bones, PTH does not bind directly to bone-destroying osteoclasts. Instead, it binds to osteoblasts, triggering a complex signaling cascade involving RANKL and the inhibition of osteoprotegerin (OPG). This process stimulates osteoclast precursors to fuse into mature cells that resorb bone matrix, releasing calcium into the bloodstream. This hormone is a polypeptide prohormone composed of 84 amino acids with a molecular mass of roughly 9500 Da. It also promotes the synthesis of calcitriol, which is the active form of vitamin D.

Dysregulation of this hormone leads to two distinct conditions: hyperparathyroidism and hypoparathyroidism. Hyperparathyroidism occurs when excess PTH is produced, often due to a non-cancerous growth called an adenoma (primary hyperparathyroidism) or as a response to low calcium or vitamin D levels (secondary hyperparathyroidism). This excess hormone can lead to serious complications, including osteoporosis, kidney stones, and high blood pressure, which increases the risk of stroke and heart disease. Secondary hyperparathyroidism is frequently observed in patients suffering from kidney disease or intestinal issues. High phosphate levels can also trigger this secondary overproduction.

The symptoms of excess PTH can be subtle or easily mistaken for other ailments. Patients may experience fatigue, muscle weakness, bone and joint pain, constipation, and even depression. Because many people show no or only mild symptoms, the condition is frequently discovered incidentally during blood or urine tests for unrelated issues. In rare, complex medical cases, even certain adrenal tumors have been found to secrete PTH, further complicating the clinical picture of calcium imbalance. The hormone has a remarkably short half-life of about 4 minutes. Beyond bone and kidney cells, a second receptor for PTH is found in the central nervous system, pancreas, and placenta.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

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