Parathyroid Hormone

Parathyroid Hormone (PTH) is an 84-amino acid peptide hormone secreted by the chief cells of the parathyroid glands. It plays a critical role in calcium and phosphate homeostasis by regulating serum calcium levels through its actions on bone, kidneys, and indirectly on the gastrointestinal tract. PTH secretion is tightly regulated by circulating ionized calcium concentrations via the calcium-sensing receptor (CaSR) on parathyroid cells.

Biosynthesis and Secretion of Parathyroid Hormone

Synthesis and Processing

PTH is initially synthesized as a preprohormone (preproPTH) with 115 amino acids. The prepro sequence directs the nascent peptide into the endoplasmic reticulum, where the signal peptide is cleaved, producing proPTH. ProPTH (90 amino acids) is then further processed in the Golgi apparatus to yield the mature, biologically active 84-amino acid PTH molecule, which is stored in secretory granules for release upon stimulation.

Regulation of Secretion

The primary regulator of PTH secretion is the extracellular ionized calcium concentration. The parathyroid chief cells express the CaSR, a G-protein coupled receptor that senses changes in serum calcium. When calcium levels are high, CaSR activation suppresses PTH secretion. Conversely, hypocalcemia reduces CaSR activation, promoting PTH release. Additional modulators include serum phosphate concentrations, vitamin D metabolites (especially 1,25-dihydroxyvitamin D), and fibroblast growth factor 23 (FGF23), which generally inhibit PTH secretion.

Secretion Dynamics

PTH is secreted in a pulsatile fashion and has a short half-life in circulation (~2-4 minutes). The hormone circulates mostly as intact PTH(1-84), but fragments resulting from peripheral metabolism also exist. Measurement of intact PTH is essential for clinical assessment of parathyroid gland function.

Mechanisms of PTH Action and Target Organs

PTH Receptors and Signal Transduction

PTH exerts its biological effects by binding to the parathyroid hormone 1 receptor (PTH1R), a G-protein coupled receptor expressed primarily in bone and kidney. Binding activates multiple intracellular signaling cascades, predominantly the adenylate cyclase/cyclic AMP (cAMP)/protein kinase A (PKA) pathway and the phospholipase C (PLC)/protein kinase C (PKC) pathway. These signaling pathways mediate various cellular responses depending on the target tissue.

Actions on Bone

In bone, PTH has dual effects depending on the exposure pattern:

• Intermittent PTH exposure promotes osteoblastic activity, leading to bone formation.
• Continuous PTH elevation stimulates osteoclast-mediated bone resorption indirectly by increasing expression of receptor activator of nuclear factor kappa-B ligand (RANKL) on osteoblasts and decreasing osteoprotegerin (OPG), favoring osteoclast differentiation and activity.

The net effect of sustained PTH is increased bone resorption, releasing calcium and phosphate into circulation.

Actions on the Kidney

PTH acts on the renal proximal and distal tubules to:

• Increase calcium reabsorption, reducing urinary calcium excretion.
• Decrease phosphate reabsorption by downregulating sodium-phosphate cotransporters in the proximal tubule, leading to phosphaturia.
• Stimulate 1α-hydroxylase enzyme activity in proximal tubular cells, enhancing conversion of 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (calcitriol).

These renal effects contribute to increased serum calcium and decreased serum phosphate levels.

Indirect Effects on the Intestine

Although PTH does not act directly on the intestine, by stimulating renal production of 1,25-dihydroxyvitamin D, it promotes increased intestinal absorption of calcium and phosphate, further contributing to calcium homeostasis.

Physiological Role and Clinical Relevance

Maintenance of Calcium and Phosphate Homeostasis

PTH is the principal hormone responsible for maintaining extracellular calcium concentration within a narrow physiological range. By mobilizing calcium from bone, reducing renal calcium excretion, and enhancing intestinal calcium absorption indirectly, PTH ensures adequate serum calcium for neuromuscular function, blood coagulation, and cellular signaling.

Role in Bone Remodeling

Through its regulation of osteoblast and osteoclast activity, PTH participates in continuous bone remodeling, balancing bone resorption and formation to maintain skeletal integrity and calcium reserves.

Disorders Associated with PTH Dysregulation

• Hyperparathyroidism (primary or secondary) results in excessive PTH secretion, leading to hypercalcemia, bone resorption, nephrolithiasis, and soft tissue calcifications.
• Hypoparathyroidism causes hypocalcemia due to insufficient PTH, leading to neuromuscular irritability and tetany.
• PTH measurement is crucial in diagnosing and managing these disorders.

Molecular Structure and Circulating Forms

PTH is a single-chain polypeptide of 84 amino acids. The N-terminal region (first 34 amino acids) is essential for receptor binding and activation, and synthetic analogs based on this region are used therapeutically (e.g., teriparatide).

Circulating PTH includes:

• Intact PTH (1-84): biologically active hormone.
• N-terminal fragments: rapidly cleared, biologically inactive.
• C-terminal fragments: accumulate especially in renal failure and are inactive.

Accurate assays measure intact PTH to reflect true physiological activity.

Summary of Key Effects

Parathyroid hormone is fundamental to mineral metabolism, integrating signals from serum calcium and phosphate to maintain systemic mineral balance and skeletal health. Its precise regulation and multifaceted actions are essential for normal physiology and are a major focus in clinical endocrinology.