Parathyroid hormone

Parathyroid hormone (PTH), also known by its older names parathormone or parathyrin, is a polypeptide hormone secreted by the chief cells of the parathyroid glands. These small endocrine glands, typically four in number and located on the posterior surface of the thyroid gland, are the primary sensors and effectors of calcium homeostasis. PTH is synthesized as a precursor prohormone and then processed into its active form, a molecule composed of 84 amino acids.

Its gene resides on chromosome 11. The secretion of PTH is exquisitely sensitive to extracellular calcium concentrations. When serum calcium levels fall below a critical threshold, PTH release is stimulated; conversely, elevated calcium levels suppress PTH secretion.

This feedback loop is fundamental to maintaining physiological calcium levels, which are essential not only for skeletal integrity but also for neuromuscular excitability, blood coagulation, and enzymatic functions.

PTH exerts its profound effects through specific receptors, primarily the PTH 1 receptor (PTH1R), which is highly expressed in bone and kidney cells. This receptor is activated by the N-terminal portion of the PTH molecule. In bone, PTH has a dual effect: it promotes bone resorption by indirectly stimulating osteoclasts, the cells responsible for breaking down bone matrix, thereby releasing calcium and phosphate into the circulation.

This action is critical for rapidly increasing serum calcium during periods of deficiency. Simultaneously, PTH also influences osteoblasts, the bone-building cells, though its net effect on bone remodeling is complex and depends on the frequency and level of PTH exposure. In the kidneys, PTH acts on the distal tubules to increase calcium reabsorption, reducing urinary calcium excretion and conserving this vital mineral.

It also inhibits phosphate reabsorption, leading to increased phosphate excretion in the urine, helping to lower serum phosphate levels. Furthermore, PTH stimulates the enzyme 1-alpha-hydroxylase in the kidneys, which converts calcidiol (25-hydroxyvitamin D) into calcitriol (1,25-dihydroxyvitamin D), the active form of vitamin D. Calcitriol then enhances calcium and phosphate absorption from the small intestine, providing another crucial pathway for mineral acquisition.

The interplay between PTH, calcium, and phosphate is a delicate balancing act. While PTH increases serum calcium, its effect on phosphate is generally to lower it by increasing urinary excretion. This is important because high phosphate levels can complex with calcium, reducing the amount of free, physiologically active calcium.

By promoting calcitriol synthesis, PTH indirectly facilitates phosphate absorption from the gut, but the direct renal effect of increasing phosphate excretion often dominates, leading to a net decrease in serum phosphate. This coordinated regulation ensures that both calcium and phosphate remain within their narrow physiological ranges, a state known as homeostasis. The bones serve as a vast reservoir, buffering fluctuations in mineral metabolism, and PTH acts as the key messenger to mobilize these resources when needed, ensuring that vital cellular functions are not compromised by mineral imbalances.

Dysregulation of PTH secretion or action has significant clinical consequences. Hypoparathyroidism, characterized by insufficient PTH production, leads to hypocalcemia (low blood calcium) and hyperphosphatemia (high blood phosphate). Symptoms can range from mild paresthesias and muscle cramps to severe tetany and cardiac arrhythmias.

Conversely, hyperparathyroidism, often caused by adenomas of the parathyroid glands, results in excessive PTH secretion, leading to hypercalcemia and hypophosphatemia. Chronic hypercalcemia can cause kidney stones, bone disease (osteitis fibrosa cystica), and neurological symptoms. PTH levels can also be affected by other conditions, such as certain cancers (paraneoplastic syndromes) or vitamin D deficiency.

Understanding the molecular mechanisms and physiological roles of PTH is crucial for diagnosing and managing these complex endocrine disorders, highlighting its central importance in human health.