Parathyroid Hormone: The 84-Amino-Acid Calcium Controller

Calcium-Regulating Peptides

84 amino acids

Parathyroid hormone is an 84-amino-acid peptide that maintains blood calcium within a range of roughly 8.5 to 10.5 mg/dL through coordinated actions on bone, kidneys, and the intestine.

Leung, Advances in Clinical Chemistry, 2021

Leung, Advances in Clinical Chemistry, 2021

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Your blood calcium concentration sits within a remarkably narrow band, roughly 8.5 to 10.5 mg/dL, and parathyroid hormone (PTH) is the primary peptide responsible for keeping it there. Secreted by four tiny glands embedded in the thyroid, PTH is an 84-amino-acid peptide hormone that raises blood calcium by acting on bone, kidneys, and (indirectly) the intestine.^[1] When calcium drops, PTH rises within seconds. When calcium climbs, PTH secretion shuts down. This feedback loop is so critical that its failure in either direction produces life-threatening disease: hypocalcemia with tetany and seizures when PTH is absent, or hypercalcemia with kidney stones and bone loss when PTH runs unchecked.^[2] This article covers the full biology of PTH, from the calcium-sensing receptor that triggers its release to the paradox that lets a bone-dissolving hormone build bone when given as a drug. For focused coverage of related peptides in this family, see our articles on calcitonin, CGRP and migraine, and PTH-related peptide in cancer.

A Century of Parathyroid Hormone Research

The parathyroid glands were first described by Ivar Sandstrom in 1880, but their function remained a mystery for decades. Surgeons noticed that removing the thyroid sometimes caused fatal tetanic convulsions, while other times it did not. The variable outcomes depended on whether the tiny parathyroid glands, which sit behind the thyroid, were inadvertently removed with it.^[3]

The breakthrough came in 1925 when Canadian biochemist James Collip prepared acid extracts of parathyroid glands and demonstrated that injecting them into parathyroidectomized dogs reversed tetany and raised blood calcium.^[2] This was the first evidence that the parathyroids are endocrine glands secreting a calcium-regulating hormone. Collip also showed that excess PTH administration caused hypercalcemia, establishing the hormone's dose-dependent relationship with blood calcium.^[3]

Progress stalled for decades because purifying PTH proved extraordinarily difficult. The glands are tiny (30-50 mg each in humans), and the hormone degrades rapidly. It took until the 1970s for researchers to determine PTH's complete 84-amino-acid sequence and identify that only the first 34 residues, PTH(1-34), are required for full biological activity at the receptor.^[2] This discovery would later enable the development of teriparatide, the first anabolic drug for osteoporosis. For more on PTH's history alongside the broader calcium-regulating peptide family, see our article on calcitonin, which was discovered in the same era.

Structure and Synthesis

PTH is synthesized as a 115-amino-acid precursor called preproPTH. A 25-amino-acid signal peptide is cleaved during translation, yielding 90-amino-acid proPTH. A further 6-amino-acid pro-sequence is removed in the Golgi apparatus, producing the mature 84-amino-acid hormone that is packaged into secretory granules.^[1]

The biologically active region occupies the amino-terminal 34 residues. Fragments lacking the first two amino acids lose receptor binding capacity almost entirely. The region between residues 15-34 contributes to receptor affinity, while residues 1-14 are critical for activating the downstream cAMP signaling cascade.^[5] The C-terminal portion (residues 35-84) does not bind the classical PTH1R receptor but may have independent biological effects that are still being characterized.

Once secreted, intact PTH(1-84) circulates with a half-life of approximately 4 minutes before being cleared primarily by the liver and kidneys. This rapid clearance is functionally important: it means that PTH levels can change almost as fast as calcium levels do, enabling minute-to-minute regulation of blood calcium.^[1]

The Calcium-Sensing Receptor: How PTH Knows When to Act

Parathyroid cells detect blood calcium through the calcium-sensing receptor (CaSR), a class C G protein-coupled receptor expressed on the parathyroid cell surface. CaSR can detect changes in extracellular calcium concentration as small as 0.1 mM.^[1]

When blood calcium is normal or high, calcium ions bind CaSR and activate intracellular phospholipase C signaling, which suppresses PTH secretion. When calcium drops, CaSR activation decreases, releasing the brake on PTH secretion. The response is rapid: PTH levels begin rising within seconds of a calcium decline.^[2]

CaSR also regulates PTH gene transcription and parathyroid cell proliferation over longer timescales. Chronic hypocalcemia upregulates both PTH mRNA production and parathyroid gland size. Chronic hypercalcemia does the opposite. This is why patients with chronic kidney disease, who have persistent hypocalcemia and low vitamin D, often develop parathyroid hyperplasia with massively elevated PTH levels (secondary hyperparathyroidism).^[1]

The discovery of CaSR in 1993 by Edward Brown and colleagues was a landmark in endocrinology. It was the first receptor identified where an inorganic ion served as the primary ligand. This discovery led directly to the development of calcimimetic drugs like cinacalcet, which activate CaSR to suppress PTH secretion in hyperparathyroidism.^[2]

Three Target Organs: How PTH Raises Calcium

PTH raises blood calcium through coordinated actions on three tissues. Each mechanism operates on a different timescale.

Kidney (minutes)

The fastest PTH effect occurs in the kidneys. PTH binds to PTH1R receptors on distal tubular cells and increases calcium reabsorption from the urine filtrate back into the blood. Simultaneously, PTH decreases phosphate reabsorption in the proximal tubule, causing phosphate to be excreted in urine. This phosphate-lowering effect prevents the calcium x phosphate product from rising too high, which would cause soft tissue calcification.^[1]

PTH also stimulates the enzyme 1-alpha-hydroxylase in the proximal tubule, which converts 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (calcitriol). This sets up the slower intestinal calcium absorption pathway.^[2]

Bone (hours to days)

PTH's relationship with bone is the most complex and pharmacologically exploitable of its three target-organ effects. PTH binds to PTH1R receptors on osteoblasts (bone-building cells) and osteocytes (mature bone cells embedded in the mineralized matrix). It does not bind osteoclasts directly.^[5]

When PTH activates osteoblasts, two competing processes begin. First, osteoblasts upregulate RANKL (receptor activator of nuclear factor kappa-B ligand) and simultaneously downregulate osteoprotegerin (OPG), a decoy receptor that normally blocks RANKL. The net effect is increased RANKL signaling to osteoclast precursors, promoting osteoclast formation and bone resorption.^[5] This releases calcium from the mineralized bone matrix into the blood.

Second, PTH activates Wnt signaling and cAMP/PKA pathways in osteoblasts, stimulating new bone formation. PTH also inhibits sclerostin, a Wnt pathway inhibitor produced by osteocytes, further promoting bone formation.^[5]

Whether the net outcome is bone building or bone destruction depends on the pattern of PTH exposure. This is the central paradox that made PTH-based osteoporosis drugs possible. For a detailed exploration of abaloparatide, a PTHrP-based analog that exploits this paradox, see our article on abaloparatide for osteoporosis.

Intestine (days)

PTH does not act directly on intestinal cells. Instead, it increases intestinal calcium absorption indirectly by stimulating calcitriol production in the kidney. Calcitriol then acts on enterocytes in the duodenum and jejunum to upregulate calcium transport proteins including TRPV6, calbindin-D9k, and the basolateral calcium pump PMCA1b.^[1] This pathway takes 1-2 days to fully activate, making it the slowest of PTH's calcium-raising mechanisms.

The Anabolic Paradox: Building Bone with a Bone-Dissolving Hormone

The most pharmacologically significant property of PTH is that intermittent exposure builds bone while continuous exposure destroys it. This paradox, first observed experimentally in the 1930s, was not fully exploited therapeutically until 2002 when teriparatide (recombinant PTH 1-34) was approved for osteoporosis.^[6]

Martin and colleagues (2021) provided a comprehensive framework for understanding this paradox through the lens of PTH1R signaling.^[6] Intermittent PTH pulses (as produced by a daily injection) cause brief activation of the cAMP/PKA pathway in osteoblasts. This transiently increases osteoblast number, activity, and survival while activating Wnt signaling. Because the PTH pulse clears within hours, the bone-forming response has time to outpace the resorptive response before the next dose arrives.

Continuous PTH exposure, by contrast, sustains RANKL upregulation and sclerostin suppression around the clock, tipping the balance toward persistent osteoclast activity and net bone loss. This is exactly what happens in hyperparathyroidism, where chronically elevated PTH leads to cortical bone thinning and fractures.^[6]

Chen et al. (2021) detailed the molecular pathways involved: intermittent PTH primarily activates the cAMP/PKA and Wnt/beta-catenin pathways (pro-formation), while continuous PTH sustains cAMP/PKC and RANKL/RANK/OPG signaling (pro-resorption).^[5]

This biology creates a narrow therapeutic window. Teriparatide must be injected once daily to produce the intermittent exposure pattern. Continuous infusion of the same drug at the same total daily dose would have the opposite effect.

PTH1R: The Shared Receptor

PTH acts through the parathyroid hormone 1 receptor (PTH1R), a class B G protein-coupled receptor. The same receptor also mediates the effects of PTH-related peptide (PTHrP), a distinct hormone that shares significant N-terminal sequence homology with PTH.^[6] For more on PTHrP's role in development and disease, see our article on PTH-related peptide.

PTH1R exists in two high-affinity conformations. The R0 conformation binds ligands independently of G proteins and produces sustained intracellular signaling from endosomes. The RG conformation requires G protein coupling and produces transient signaling from the cell surface.^[7]

PTH(1-34) (teriparatide) binds both R0 and RG conformations with high affinity. This dual binding means teriparatide produces both sustained and transient signaling. Abaloparatide, a synthetic PTHrP(1-34) analog, binds preferentially to the RG conformation with an R0:RG selectivity ratio of 1,580:1, compared to teriparatide's 12:1.^[7] This RG selectivity produces a more transient signaling pattern that favors bone formation over resorption, with less hypercalcemia as a side effect. For detailed evidence on this mechanism, see our article on abaloparatide for osteoporosis.

Zhai et al. (2022) used cryo-EM structural analysis to explain why PTH and abaloparatide produce different signaling durations despite activating the same receptor. PTH-bound PTH1R-Gs complexes show reduced molecular motion and tolerate more receptor mutations, indicating a more stable interaction that supports prolonged endosomal signaling. Abaloparatide-bound complexes are less stable, dissociate faster, and allow more rapid receptor recycling.^[8]

Sato et al. (2021) confirmed that teriparatide, abaloparatide, and long-acting PTH produce comparable initial activation of cAMP, intracellular calcium, beta-arrestin recruitment, and SIK2 signaling at PTH1R. The differences between these ligands are not in which pathways they activate but in how long those signals persist.^[9]

A second receptor, PTH2R, is expressed primarily in the brain and pancreas and responds to PTH but not PTHrP. Its physiological role remains poorly defined, though it may be involved in nociception and pancreatic function.^[6]

Teriparatide: PTH as a Drug

Teriparatide (Forteo), recombinant human PTH(1-34), was approved by the FDA in 2002 for osteoporosis in postmenopausal women and men at high fracture risk. It was the first anabolic (bone-building) drug for osteoporosis, in contrast to the existing antiresorptive therapies like bisphosphonates that only slow bone loss.

The Neer Trial

The pivotal trial was published by Neer et al. in the New England Journal of Medicine in 2001.^[10] The researchers randomized 1,637 postmenopausal women with prior vertebral fractures to receive daily subcutaneous injections of PTH(1-34) at 20 mcg, 40 mcg, or placebo. After a median of 21 months:

• New vertebral fractures occurred in 5% of the 20 mcg group versus 14% of the placebo group, a 65% risk reduction (relative risk 0.35, 95% CI 0.22-0.55)
• Nonvertebral fractures occurred in 6% of the 20 mcg group versus 10% of the placebo group, a 35% risk reduction (relative risk 0.54, 95% CI 0.37-0.79, adjusted for weight)
• Lumbar spine BMD increased by 9% in the 20 mcg group versus a 1% gain in placebo
• The 40 mcg dose produced greater BMD gains but also more side effects including hypercalcemia

The trial was stopped early because of a concurrent finding in rats: lifetime teriparatide exposure at high doses caused osteosarcoma. This finding led to a boxed warning and a recommendation limiting treatment duration to 24 months. The osteosarcoma signal has not been replicated in humans across more than two decades of clinical use and post-marketing surveillance.^[10]

Beyond postmenopausal osteoporosis

Teriparatide has since been studied in glucocorticoid-induced osteoporosis, where it proved superior to alendronate for increasing spine BMD, and in male osteoporosis. Kato et al. (2020) demonstrated in an ovariectomized mouse model that teriparatide prevents bone loss and also reduces pain-related behavior, reducing CGRP and TRPV1 expression in dorsal root ganglia.^[11] This pain-reducing effect is separate from the bone-building mechanism and may involve PTH1R signaling in sensory neurons. The relationship between PTH-family peptides and pain pathways connects to CGRP biology, since CGRP is itself a calcitonin gene product.

Abaloparatide: The Next-Generation PTH1R Agonist

Abaloparatide (Tymlos), a synthetic analog of PTHrP(1-34), was approved in 2017 for postmenopausal osteoporosis. While it acts through the same PTH1R receptor as teriparatide, its preferential binding to the RG conformation produces a distinct pharmacological profile.^[4]

In the ACTIVE trial, Miller et al. (2016) randomized 2,463 postmenopausal women to abaloparatide 80 mcg, placebo, or open-label teriparatide 20 mcg for 18 months. Abaloparatide reduced new vertebral fractures by 86% versus placebo (0.58% vs 4.22%) and nonvertebral fractures by 43%.^[12]

The ACTIVExtend study (Bone et al., 2018) followed ACTIVE completers with 24 months of alendronate. Women who received abaloparatide first had a vertebral fracture rate of only 0.9% at 43 months versus 5.6% in the placebo-then-alendronate group.^[13]

In the ATOM trial, Czerwinski et al. (2022) tested abaloparatide in 228 men with osteoporosis. After 12 months, lumbar spine BMD increased by 8.48% with abaloparatide versus 1.17% with placebo.^[14]

Matsumoto et al. (2023) demonstrated a dose-dependent BMD response to abaloparatide in postmenopausal Japanese women, confirming the anabolic effect across different populations.^[15]

For comprehensive coverage of abaloparatide's clinical evidence and mechanism, see our dedicated pillar article on abaloparatide for osteoporosis.

When PTH Goes Wrong: Hyperparathyroidism and Hypoparathyroidism

Primary hyperparathyroidism

In primary hyperparathyroidism (PHPT), one or more parathyroid glands produce excess PTH autonomously, typically due to a benign adenoma. The continuous PTH elevation raises blood calcium and accelerates bone resorption. About 80% of cases are caused by a single adenoma; the remainder involve multi-gland hyperplasia or, rarely, parathyroid carcinoma.^[2]

PHPT affects roughly 1 in 500 women over age 50 and is often discovered incidentally through routine blood work showing elevated calcium. Symptoms can include kidney stones, bone pain, fatigue, and cognitive difficulty, though many patients are asymptomatic at diagnosis. Surgery (parathyroidectomy) is curative in over 95% of cases.^[2]

Secondary hyperparathyroidism

In secondary hyperparathyroidism, the parathyroid glands produce excess PTH in response to a genuine physiological stimulus, most commonly chronic kidney disease. As kidney function declines, phosphate retention and reduced calcitriol production drive a sustained drop in blood calcium. The parathyroid glands compensate by increasing PTH secretion, but the chronically elevated PTH causes progressive bone disease (renal osteodystrophy) and vascular calcification.^[1] Treatment targets the underlying calcium-phosphate imbalance with phosphate binders, calcitriol, and calcimimetics (cinacalcet or etelcalcetide) that activate CaSR to suppress PTH secretion without raising calcium further.

Hypoparathyroidism

Hypoparathyroidism, insufficient PTH production, most commonly results from inadvertent damage to the parathyroid glands during thyroid surgery. The loss of PTH causes blood calcium to fall, producing symptoms ranging from perioral tingling and muscle cramps to seizures and cardiac arrhythmias.^[1]

Standard treatment is oral calcium and vitamin D supplementation, but this approach bypasses the normal PTH-mediated fine-tuning of calcium balance. PTH replacement therapy with recombinant PTH(1-84) (Natpara) was approved in 2015 as an adjunct in patients inadequately controlled on calcium and vitamin D alone, though it carries the same osteosarcoma boxed warning as teriparatide. TransCon PTH, a long-acting prodrug of PTH(1-34), is in late-stage clinical development and aims to provide more physiological PTH replacement with once-daily dosing.^[1]

The Broader PTH Peptide Family

PTH does not operate alone. It belongs to a family of structurally and functionally related peptides that share the PTH1R receptor and play roles far beyond calcium homeostasis.

PTH-related peptide (PTHrP) was discovered in the 1980s when researchers identified the factor responsible for humoral hypercalcemia of malignancy, a condition in which cancers produce a PTH-like substance that raises blood calcium without involving the parathyroid glands. PTHrP shares 8 of its first 13 amino acids with PTH and activates PTH1R with similar potency. Under normal conditions, PTHrP functions as a paracrine/autocrine factor that regulates chondrocyte differentiation, mammary gland development, and smooth muscle relaxation.^[6] For full coverage, see our article on PTH-related peptide and cancer.

Calcitonin is often described as PTH's physiological antagonist. Produced by thyroid C cells, calcitonin lowers blood calcium by inhibiting osteoclast activity. Its biological role appears more limited than PTH's, as patients with thyroidectomy (and therefore no calcitonin) do not develop significant calcium abnormalities. See our article on calcitonin.

Calcitonin gene-related peptide (CGRP) is produced by alternative splicing of the calcitonin gene and is one of the most potent vasodilators known. CGRP has become a major therapeutic target in migraine through monoclonal antibodies (erenumab, fremanezumab, galcanezumab) that block CGRP or its receptor. See our article on CGRP and migraine.

Evidence Limitations

Several important gaps remain in our understanding of PTH biology and pharmacology.

The osteosarcoma risk from PTH-based drugs is extrapolated entirely from rat models where animals received high doses for most of their lifespan. No human osteosarcoma signal has emerged from over 20 years of teriparatide use, but the 24-month treatment limit persists out of caution. Whether this restriction unnecessarily limits access to effective treatment remains debated.^[10]

The exact mechanism by which intermittent PTH shifts the balance from resorption to formation is still incompletely understood at the molecular level. Multiple pathways (Wnt, cAMP, RANKL, sclerostin) are involved, but the precise timing, cell-type specificity, and dose-response relationships are still being mapped.^[5]

Most large teriparatide and abaloparatide trials enrolled postmenopausal women. Evidence in men, premenopausal women, and children is substantially thinner.^[14]

The C-terminal fragment of PTH(1-84), residues 35-84, may have biological activities independent of PTH1R. Some evidence suggests it has anti-resorptive or even pro-apoptotic effects on osteoclasts, but this remains preliminary and contested.^[2]

Cross-connections between PTH signaling and other peptide systems (collagen metabolism, endocannabinoids, incretins) are increasingly recognized but poorly characterized. For coverage of peptides in bone-building beyond PTH, see our article on collagen peptides for bone density.

The Bottom Line

Parathyroid hormone is a fast-acting 84-amino-acid peptide that maintains blood calcium within a narrow range through coordinated effects on bone, kidneys, and the intestine. Its signaling through PTH1R produces opposite skeletal effects depending on exposure pattern: intermittent pulses build bone, while continuous elevation destroys it. This biology has been translated into two approved anabolic osteoporosis drugs (teriparatide and abaloparatide) that reduce fracture risk by 65-86%. Evidence gaps remain around long-term treatment safety, mechanism details, and populations beyond postmenopausal women.

Frequently Asked Questions

What does parathyroid hormone do?

Parathyroid hormone (PTH) raises blood calcium levels by acting on three target organs: it increases calcium reabsorption in the kidneys, stimulates calcium release from bone, and boosts intestinal calcium absorption indirectly through vitamin D activation. PTH is an 84-amino-acid peptide secreted by four small glands behind the thyroid, and blood levels adjust within seconds when calcium concentrations change (Leung, Adv Clin Chem, 2021).

What is a normal parathyroid hormone level?

Normal intact PTH levels typically range from 10 to 65 pg/mL, though reference ranges vary by laboratory and assay. PTH levels must always be interpreted alongside blood calcium: a PTH level that is "normal" in absolute terms may be inappropriately high if calcium is also elevated, suggesting primary hyperparathyroidism (Leung, Adv Clin Chem, 2021).

How does teriparatide build bone if PTH breaks bone down?

The effect depends on exposure pattern. Daily injections produce brief PTH pulses lasting 2-4 hours that activate osteoblast bone-formation pathways (Wnt, cAMP/PKA) before resorption pathways can fully engage. Continuous PTH elevation, as in hyperparathyroidism, sustains RANKL-driven osteoclast activity and tips the balance toward net bone loss. In the Neer 2001 trial, once-daily PTH(1-34) injections reduced vertebral fractures by 65% over 21 months (Neer et al., NEJM, 2001).

What is the difference between teriparatide and abaloparatide?

Teriparatide is a fragment of human PTH (amino acids 1-34), while abaloparatide is a synthetic analog of PTH-related peptide (PTHrP). Both act through the PTH1R receptor, but abaloparatide binds preferentially to the RG receptor conformation, producing more transient signaling. In clinical trials, abaloparatide showed comparable or greater BMD gains at the hip with approximately 47% less hypercalcemia than teriparatide (Hattersley et al., Endocrinology, 2016).

What happens when parathyroid hormone is too high?

Chronically elevated PTH (hyperparathyroidism) causes persistent calcium mobilization from bone, leading to osteoporosis, kidney stones, elevated blood calcium, fatigue, and cognitive symptoms. Primary hyperparathyroidism affects about 1 in 500 women over age 50 and is most commonly caused by a benign parathyroid adenoma. Surgery cures the condition in over 95% of cases (Potts, J Endocrinol, 2005).

Can you live without parathyroid glands?

Survival without parathyroid glands is possible but requires lifelong calcium and vitamin D supplementation to prevent dangerous hypocalcemia. Without PTH, the body loses its ability to fine-tune blood calcium, and patients are at risk for muscle cramps, seizures, and cardiac arrhythmias. PTH replacement therapy with recombinant PTH(1-84) is available for refractory cases (Leung, Adv Clin Chem, 2021).