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Parathyroid Hormone (PTH)
Parathyroid hormone is a peptide hormone secreted by chief cells of the parathyroid glands. It is a major regulator of extracellular calcium and phosphate homeostasis, acting on bone and kidneys and indirectly on the intestine through activation of vitamin D.
Region-
SystemEndocrine System
Parathyroid hormone (PTH) is a peptide hormone secreted primarily by chief cells of the parathyroid glands. It is one of the principal regulators of extracellular calcium concentration and also has major effects on phosphate metabolism.
PTH secretion is controlled primarily by the concentration of ionized calcium in extracellular fluid. When ionized calcium falls, the parathyroid glands rapidly increase PTH secretion. PTH then acts on the skeleton and kidneys and indirectly on the gastrointestinal tract to restore calcium toward its normal physiological range.
The hormone increases renal calcium conservation, promotes phosphate excretion, influences bone remodeling, and stimulates renal formation of active vitamin D. These coordinated actions make PTH central to calcium-phosphate homeostasis.
Hormone Classification
PTH is a peptide hormone. Human PTH is synthesized as a larger precursor and processed to produce the biologically active mature hormone.
Mature PTH contains 84 amino acids and is commonly referred to as PTH(1-84).
Site of Production
PTH is produced by the parathyroid glands, small endocrine glands usually located on the posterior aspect of the thyroid gland.
Most individuals have four parathyroid glands, although variation in number and location is common.
Parathyroid Glands
The parathyroid glands are usually arranged as paired superior and inferior parathyroid glands associated with the posterior surface of the thyroid lobes.
Despite their close anatomical relationship to the thyroid, the parathyroid glands are functionally distinct endocrine organs.
Superior Parathyroid Glands
The superior parathyroid glands tend to have a relatively consistent position near the posterior aspect of the upper to middle thyroid lobes.
They develop from the fourth pharyngeal pouch.
Inferior Parathyroid Glands
The inferior parathyroid glands show greater anatomical variability because of their embryological migration.
They develop from the third pharyngeal pouch and descend in association with the thymus, which also develops from the third pouch.
Parathyroid Histology
The adult parathyroid gland contains predominantly chief cells and variable numbers of oxyphil cells, supported by a rich capillary network and connective tissue stroma.
Adipose tissue commonly becomes increasingly prominent within the glands with age.
Chief Cells
Chief cells, also called principal cells, are the primary PTH-producing cells of the parathyroid gland.
They synthesize, store, and secrete PTH and express calcium-sensing receptors that allow them to continuously monitor extracellular calcium concentration.
Oxyphil Cells
Oxyphil cells are larger than chief cells and contain abundant mitochondria, giving their cytoplasm a strongly eosinophilic appearance.
They become more numerous with age. Their physiological role is less prominent than that of chief cells, although they can possess parathyroid endocrine activity.
Parathyroid Cell Types
| Cell Type | Major Feature | Functional Importance |
|---|---|---|
| Chief cells | Small principal endocrine cells | Primary synthesis and secretion of PTH |
| Oxyphil cells | Large cells rich in mitochondria | Less clearly defined endocrine role |
PTH Synthesis
PTH synthesis begins with production of preproparathyroid hormone, a larger precursor peptide.
Sequential processing within the endoplasmic reticulum and Golgi apparatus generates proparathyroid hormone and ultimately mature PTH.
PTH Processing
| Stage | Product |
|---|---|
| 1 | Preproparathyroid hormone |
| 2 | Proparathyroid hormone |
| 3 | Mature PTH |
| 4 | Storage in secretory vesicles and regulated secretion |
Regulation of PTH Secretion
The concentration of extracellular ionized calcium is the dominant physiological regulator of PTH secretion.
The parathyroid glands detect extracellular calcium through the calcium-sensing receptor (CaSR) expressed on chief cells.
Calcium-Sensing Receptor
The calcium-sensing receptor is a G protein-coupled receptor that responds to changes in extracellular calcium concentration.
It allows the parathyroid gland to regulate PTH secretion rapidly and continuously according to the body's calcium status.
Low Calcium and PTH
A decrease in extracellular ionized calcium reduces activation of the calcium-sensing receptor.
This promotes increased PTH secretion and, over longer periods, can also influence PTH synthesis and parathyroid cell activity.
High Calcium and PTH
When extracellular calcium increases, greater activation of the calcium-sensing receptor suppresses PTH secretion.
This forms a classic negative-feedback system that stabilizes extracellular calcium concentration.
Calcium-PTH Feedback
| Change | Parathyroid Response | Physiological Result |
|---|---|---|
| Ionized calcium decreases | PTH secretion increases | Mechanisms that raise extracellular calcium are activated |
| Ionized calcium increases | PTH secretion decreases | Calcium-raising actions of PTH diminish |
Magnesium and PTH
Magnesium can also influence PTH secretion. Mild decreases in magnesium may stimulate PTH, whereas severe magnesium deficiency can impair PTH secretion and reduce responsiveness to the hormone.
This relationship can contribute to hypocalcemia associated with severe hypomagnesemia.
Vitamin D and PTH
Active vitamin D participates in negative regulation of the parathyroid glands.
Calcitriol can suppress PTH synthesis and influences parathyroid cell growth, forming part of the feedback network controlling mineral metabolism.
Major Target Organs
The major direct target organs of PTH are the kidneys and skeleton.
PTH also increases intestinal calcium and phosphate absorption indirectly by stimulating renal production of active vitamin D.
Major Actions of PTH
| Target | Major Action |
|---|---|
| Kidney | Increases calcium reabsorption and decreases phosphate reabsorption |
| Kidney | Stimulates formation of calcitriol |
| Bone | Regulates remodeling and can increase calcium and phosphate mobilization |
| Intestine | Indirectly increases calcium and phosphate absorption through calcitriol |
PTH and the Kidney
The kidney is a major direct target of PTH and plays a central role in the hormone's ability to regulate calcium and phosphate balance.
PTH alters tubular handling of calcium and phosphate and stimulates production of active vitamin D.
Renal Calcium Reabsorption
PTH increases renal conservation of calcium, particularly by increasing calcium reabsorption in the distal nephron.
This reduces urinary calcium loss and contributes to restoration of extracellular calcium.
Renal Phosphate Handling
PTH decreases phosphate reabsorption primarily in the proximal tubule.
This causes increased urinary phosphate excretion, an effect known as phosphaturia.
Phosphaturia
Phosphaturia is important because PTH-mediated bone mineral mobilization can release both calcium and phosphate.
Increasing renal phosphate excretion helps prevent excessive accumulation of phosphate in extracellular fluid while PTH raises calcium availability.
Renal Effects of PTH
| Renal Process | Effect of PTH |
|---|---|
| Calcium reabsorption | Increases |
| Phosphate reabsorption | Decreases |
| Urinary phosphate excretion | Increases |
| 1-alpha-hydroxylase activity | Increases |
| Calcitriol formation | Increases |
PTH and Vitamin D Activation
PTH stimulates renal 1-alpha-hydroxylase, particularly in the proximal tubule.
This enzyme converts 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D, also called calcitriol, the hormonally active form of vitamin D.
Calcitriol
Calcitriol increases intestinal absorption of calcium and phosphate and participates in regulation of bone and mineral metabolism.
By stimulating calcitriol formation, PTH indirectly increases the amount of dietary calcium available for absorption from the gastrointestinal tract.
PTH and the Intestine
PTH does not exert its major intestinal calcium effect through direct action on intestinal epithelial cells.
Instead, it stimulates renal calcitriol production, and calcitriol subsequently increases intestinal calcium absorption.
PTH-Vitamin D-Intestine Pathway
| Step | Event |
|---|---|
| 1 | Extracellular calcium decreases |
| 2 | PTH secretion increases |
| 3 | Renal 1-alpha-hydroxylase activity increases |
| 4 | Calcitriol production increases |
| 5 | Intestinal calcium absorption increases |
| 6 | Calcium availability increases |
PTH and Bone
Bone contains the majority of the body's calcium and therefore represents an important component of calcium homeostasis.
PTH influences bone remodeling, with its effects depending on the duration and pattern of hormone exposure.
Bone Remodeling
Bone remodeling is a continuous process involving coordinated activity of bone-forming osteoblasts and bone-resorbing osteoclasts.
PTH regulates this cellular system largely through actions on osteoblast-lineage cells rather than by acting directly on mature osteoclasts.
Osteoblasts and PTH
Osteoblast-lineage cells express receptors for PTH and respond by altering production of signals that regulate osteoclast formation and activity.
An important component of this pathway involves the interaction between RANKL and RANK.
RANKL
RANKL, or receptor activator of nuclear factor kappa-B ligand, is expressed by osteoblast-lineage and related stromal cells.
PTH can increase signals favoring RANKL-mediated differentiation and activation of osteoclasts, thereby increasing bone resorption under sustained PTH exposure.
Osteoclasts
Osteoclasts are multinucleated cells specialized for bone resorption.
They dissolve mineral and degrade organic bone matrix, releasing calcium and phosphate from skeletal stores.
Indirect Activation of Osteoclasts
| Step | Event |
|---|---|
| 1 | PTH acts on osteoblast-lineage cells |
| 2 | RANKL-mediated signaling increases |
| 3 | Osteoclast differentiation and activity increase |
| 4 | Bone resorption increases |
| 5 | Calcium and phosphate are released from bone mineral |
Continuous PTH Exposure
Persistently elevated PTH tends to favor increased bone turnover and, when sufficiently sustained, increased bone resorption.
This pattern is important in disorders such as hyperparathyroidism.
Intermittent PTH Exposure
Intermittent exposure to PTH or PTH-receptor agonism can produce a different skeletal response and may favor bone formation.
This difference between continuous and intermittent exposure forms the physiological basis for the use of certain PTH-related therapies in osteoporosis.
Net Effect on Extracellular Calcium
The overall physiological effect of PTH is to increase extracellular ionized calcium.
It accomplishes this through coordinated renal calcium conservation, regulation of skeletal mineral turnover, and increased calcitriol-mediated intestinal calcium absorption.
Net Effect on Phosphate
Although bone resorption and increased intestinal absorption can add phosphate to extracellular fluid, PTH strongly increases renal phosphate excretion.
Consequently, sustained PTH excess generally tends to lower serum phosphate.
Calcium and Phosphate Effects
| Variable | Major PTH Effect |
|---|---|
| Blood calcium | Increases |
| Renal calcium loss | Decreases |
| Renal phosphate reabsorption | Decreases |
| Urinary phosphate | Increases |
| Calcitriol production | Increases |
| Intestinal calcium absorption | Indirectly increases |
PTH Receptors
The principal receptor mediating the classical actions of PTH is the PTH1 receptor (PTH1R).
PTH1R is a G protein-coupled receptor expressed prominently in bone and kidney.
PTH1 Receptor
The PTH1 receptor responds to PTH and parathyroid hormone-related peptide in appropriate tissues.
It activates multiple intracellular signaling pathways, including pathways involving cyclic AMP and protein kinase A.
PTH Signaling
Binding of PTH to PTH1R can activate Gs-mediated stimulation of adenylyl cyclase, increasing intracellular cAMP.
Additional signaling pathways, including phospholipase C-related mechanisms, can also contribute to PTH actions depending on the target cell.
Calcium Homeostasis
Calcium is essential for neuromuscular excitability, intracellular signaling, membrane function, blood coagulation, enzyme activity, and skeletal mineralization.
Because relatively small changes in extracellular ionized calcium can have important physiological consequences, its concentration is tightly regulated.
Ionized Calcium
Ionized calcium is the biologically active fraction of calcium in extracellular fluid and is the form directly sensed by the calcium-sensing receptor.
Total serum calcium also includes calcium bound to proteins, particularly albumin, and calcium complexed with small anions.
Forms of Circulating Calcium
| Form | Characteristic |
|---|---|
| Ionized calcium | Biologically active and directly sensed by CaSR |
| Protein-bound calcium | Predominantly associated with albumin |
| Complexed calcium | Bound to small circulating anions |
PTH and Calcitonin
PTH and calcitonin both participate in mineral physiology, but PTH has a much more prominent role in moment-to-moment regulation of extracellular calcium in humans.
Calcitonin is secreted by thyroid parafollicular cells in response to increased calcium and can inhibit osteoclastic bone resorption.
PTH Versus Calcitonin
| Feature | PTH | Calcitonin |
|---|---|---|
| Source | Parathyroid chief cells | Thyroid parafollicular cells |
| Major calcium stimulus | Low ionized calcium | Elevated calcium |
| Effect on blood calcium | Raises calcium | Can lower calcium |
| Importance in adult human calcium homeostasis | Major | More limited |
PTH and Calcitriol
PTH and calcitriol work together to maintain calcium availability.
PTH stimulates calcitriol production in the kidney, while calcitriol increases intestinal calcium absorption and feeds back on the parathyroid glands.
Parathyroid Negative Feedback
The PTH regulatory system contains several interacting feedback mechanisms.
Rising extracellular calcium suppresses PTH through CaSR activation, while calcitriol also suppresses PTH synthesis. These mechanisms prevent excessive calcium-raising activity when calcium availability is adequate.
Primary Hyperparathyroidism
Primary hyperparathyroidism results from autonomous or inappropriate secretion of PTH by one or more parathyroid glands.
A parathyroid adenoma is a common cause. The biochemical pattern commonly includes elevated calcium with elevated or inappropriately nonsuppressed PTH and often reduced serum phosphate.
Parathyroid Adenoma
A parathyroid adenoma is a benign neoplasm of parathyroid tissue capable of secreting excessive PTH.
It commonly involves a single gland and is an important cause of primary hyperparathyroidism.
Parathyroid Hyperplasia
Hyperplasia involving multiple parathyroid glands can also produce excessive PTH secretion.
Multiglandular disease may occur sporadically or as part of inherited endocrine syndromes.
Secondary Hyperparathyroidism
Secondary hyperparathyroidism is an adaptive increase in PTH secretion caused by a persistent physiological stimulus rather than autonomous primary parathyroid disease.
Important causes include chronic kidney disease and prolonged vitamin D deficiency.
Chronic Kidney Disease and PTH
Chronic kidney disease can disturb phosphate excretion, vitamin D activation, and calcium balance.
These abnormalities can chronically stimulate the parathyroid glands, producing secondary hyperparathyroidism and eventually parathyroid hyperplasia.
Tertiary Hyperparathyroidism
Tertiary hyperparathyroidism can develop after longstanding secondary hyperparathyroidism when parathyroid tissue becomes increasingly autonomous.
PTH secretion may then remain excessive even after the original stimulus has been reduced or corrected.
Hypoparathyroidism
Hypoparathyroidism is characterized by deficient PTH secretion.
Loss of PTH reduces renal calcium conservation and calcitriol production while increasing renal phosphate retention, tending to produce hypocalcemia and hyperphosphatemia.
Postsurgical Hypoparathyroidism
Parathyroid glands may be damaged, devascularized, or inadvertently removed during surgery involving the thyroid or central neck.
For this reason, preservation of the glands and their vascular supply is an important consideration during thyroid surgery.
Hypocalcemia
Hypocalcemia increases neuromuscular excitability and can produce paresthesias, muscle cramps, tetany, seizures, and cardiac electrical abnormalities when sufficiently severe.
Normally, falling ionized calcium produces a rapid compensatory increase in PTH secretion.
Hypercalcemia
Hypercalcemia normally suppresses PTH secretion through activation of the calcium-sensing receptor.
An elevated or inappropriately normal PTH concentration in the presence of hypercalcemia therefore suggests PTH-dependent dysregulation.
Pseudohypoparathyroidism
Pseudohypoparathyroidism refers to disorders in which target tissues demonstrate resistance to PTH signaling.
PTH concentrations can therefore be elevated despite biochemical findings reflecting inadequate PTH action.
Calcium-Sensing Receptor Disorders
Abnormalities of the calcium-sensing receptor can alter the relationship between extracellular calcium and PTH secretion.
Changes in CaSR function can therefore produce inherited disorders characterized by abnormal calcium concentrations and altered parathyroid regulation.
Measurement of PTH
Clinical evaluation commonly uses an intact PTH assay together with serum calcium and other measures of mineral metabolism.
PTH values should be interpreted in relation to calcium rather than in isolation because the appropriateness of a PTH concentration depends strongly on the simultaneous calcium concentration.
PTH and Calcium Interpretation
| Calcium | PTH Pattern | Possible Physiological Interpretation |
|---|---|---|
| Low | High | Appropriate compensatory response or secondary hyperparathyroid state |
| Low | Low or inappropriately normal | Possible deficient parathyroid response |
| High | Suppressed | Appropriate physiological suppression |
| High | Elevated or inappropriately normal | Suggests PTH-dependent hypercalcemia |
Relationship to Parathyroid Anatomy
PTH physiology is directly connected to the small size and specialized organization of the parathyroid glands. Chief cells are positioned within a richly vascularized endocrine tissue, allowing rapid detection of extracellular calcium and rapid delivery of PTH into the circulation.
The glands' variable locations, particularly those of the inferior parathyroids, are clinically important during neck surgery and when abnormal parathyroid tissue must be localized.
Key Regulatory Pathway
| Step | Event |
|---|---|
| 1 | Extracellular ionized calcium decreases |
| 2 | CaSR activation on parathyroid chief cells decreases |
| 3 | PTH secretion increases |
| 4 | Renal calcium conservation increases |
| 5 | Renal phosphate excretion increases |
| 6 | Renal calcitriol production increases |
| 7 | Intestinal calcium absorption increases through calcitriol |
| 8 | Bone remodeling contributes to calcium availability |
| 9 | Extracellular calcium rises toward its regulated range |
| 10 | Increased calcium suppresses further PTH secretion |
Key Features of Parathyroid Hormone
| Feature | Key Point |
|---|---|
| Abbreviation | PTH |
| Hormone class | Peptide hormone |
| Mature length | 84 amino acids |
| Primary source | Parathyroid chief cells |
| Major regulator | Extracellular ionized calcium |
| Calcium sensor | Calcium-sensing receptor |
| Major receptor | PTH1 receptor |
| Major direct targets | Bone and kidney |
| Blood calcium effect | Increases |
| Renal calcium reabsorption | Increases |
| Renal phosphate reabsorption | Decreases |
| Urinary phosphate | Increases |
| Calcitriol synthesis | Increases |
| Intestinal calcium absorption | Indirectly increases |
| Primary physiological role | Calcium and phosphate homeostasis |
Anatomical and Physiological Importance
Parathyroid hormone connects the microscopic organization of the parathyroid glands with the regulation of calcium throughout the body. Chief cells continuously monitor extracellular ionized calcium through calcium-sensing receptors and can rapidly adjust PTH secretion when calcium concentration changes.
When ionized calcium falls, increased PTH acts on the kidneys to conserve calcium while simultaneously increasing phosphate excretion. It also stimulates renal formation of calcitriol, which increases intestinal calcium absorption and helps restore calcium availability from dietary sources.
The skeleton provides the body's largest calcium reservoir. PTH influences bone remodeling through actions on osteoblast-lineage cells and their regulation of osteoclasts. Sustained elevations in PTH can favor bone resorption, whereas intermittent PTH-receptor stimulation can favor bone formation.
The relationship between calcium and PTH forms a tightly controlled negative-feedback loop. Falling calcium stimulates PTH secretion, PTH activates mechanisms that increase calcium availability, and restoration of calcium increases CaSR activation and suppresses further PTH release.
This regulatory system also demonstrates the close functional relationship among the parathyroid glands, kidneys, skeleton, intestine, and vitamin D endocrine system. Dysfunction at any of these levels can alter calcium-phosphate balance and secondarily affect PTH secretion.
Through calcium sensing by parathyroid chief cells, rapid hormone secretion, renal calcium conservation, phosphaturia, stimulation of calcitriol production, and regulation of skeletal remodeling, PTH serves as one of the central endocrine regulators of mineral homeostasis.
Published on September 29, 2026
Last updated on September 29, 2026
Last updated on September 29, 2026