10 Facts About the Parathyroid Gland and Its Function

The parathyroid gland is a small but essential part of the endocrine system that plays a major role in regulating calcium levels throughout the body. Most people have four parathyroid glands located behind the thyroid gland in the neck. Although they are tiny, these glands help control important functions involving the bones, muscles, nerves, and kidneys.

The parathyroid glands produce parathyroid hormone (PTH), which helps maintain a healthy balance of calcium and phosphorus in the bloodstream. When these glands become overactive or underactive, they can lead to a variety of health problems, including osteoporosis, kidney stones, muscle weakness, and abnormal calcium levels.

Understanding how the parathyroid gland works can help you recognize its importance in overall health. In this article, we’ll explore 10 important facts about the parathyroid gland and its function, including its role in calcium regulation and common disorders that can affect it.

The Function of the Parathyroid Gland

The primary function of the parathyroid gland is to secrete parathyroid hormone (PTH), which acts as the main regulator of calcium and phosphorus concentrations in the extracellular fluid. To understand better, one must ask: what is the purpose of parathyroid glands? Their primary purpose is to function as the body’s central thermostat for calcium, continuously monitoring blood levels and making precise adjustments to maintain a stable internal environment, a process known as homeostasis.

What Are the Parathyroid Glands?

The parathyroid glands are typically four separate, tiny, rice-sized endocrine glands located in the neck, most often found on the posterior surface of the thyroid gland. Their sole clinical purpose is to regulate the body’s systemic calcium levels. Despite their small size—each gland weighing only about 30 to 50 milligrams—their role is indispensable for life, as precise calcium control is vital for numerous physiological processes.

The glands are primarily composed of two main types of cells:

Chief Cells

These are the primary functional cells of the parathyroid gland system. They are responsible for synthesizing, storing, and secreting parathyroid hormone (PTH). Chief cells have a special sensor on their surface called the Calcium-Sensing Receptor (CaSR). This receptor detects minute fluctuations in the concentration of ionized calcium in the blood. When calcium levels fall, the CaSR signals the chief cells to release stored PTH into the bloodstream. Conversely, when calcium levels rise, the receptor is activated, which inhibits PTH secretion.

Oxyphil Cells

The exact function of oxyphil cells is still not fully understood. They appear later in life, around puberty, and increase in number with age. They are larger than chief cells and contain a high number of mitochondria, suggesting they are metabolically active. While they do not secrete PTH, some research suggests they may be derived from chief cells and could have a supportive or yet-to-be-discovered regulatory role within the gland.

Anatomically, there is considerable variability in the number and location of these structures. While approximately 85% of people have four glands, some individuals may have as few as two or as many as six or even more. Their location can also vary; while most are found behind the thyroid, they can sometimes be located within the thyroid tissue itself, in the thymus gland in the chest (ectopic parathyroid), or elsewhere along the path of their embryological development.

What Is Parathyroid Hormone (PTH)?

Parathyroid hormone (PTH), also known as parathormone, is a polypeptide hormone secreted by the parathyroid gland that acts as the principal regulator of the body’s blood calcium levels. It functions to increase calcium concentrations in the blood when they fall below the normal range. The hormone is synthesized within the chief cells of the glands as a larger precursor molecule called pre-pro-PTH, which is then cleaved to form pro-PTH, and finally processed into the biologically active 84-amino acid polypeptide.

When the Calcium-Sensing Receptors (CaSR) on the surface of the parathyroid chief cells detect a drop in serum ionized calcium, two things happen almost immediately:

• Rapid Release: Pre-formed PTH stored in secretory granules within the chief cells is quickly released into the bloodstream. This provides an immediate response to correct the low calcium.
• Increased Synthesis: The cells also ramp up the transcription and translation of the PTH gene, leading to the synthesis of new hormone molecules to replenish stores and sustain the response if the low calcium state persists.

Once in circulation, PTH has a very short half-life, typically only three to five minutes. This rapid clearance allows for extremely fine and responsive control over calcium levels. Blood tests can measure “intact PTH,” which refers to the full, biologically active 84-amino acid molecule, providing the most accurate assessment of parathyroid function.

Is the Parathyroid Gland’s Main Job to Control Calcium?

Yes, the exclusive and vital job of these glands is to act as the body’s “calcium thermostat,” meticulously controlling the amount of calcium in the blood and bones. They do not influence any other metabolic processes; their focus is singular and precise. This tight regulation is critical because calcium is not just a structural component of bone; it plays an essential role in numerous moment-to-moment physiological functions, and deviations from its narrow normal range (typically 8.5 to 10.5 mg/dL) can have severe consequences.

The parathyroid gland network maintains calcium homeostasis through a sophisticated negative feedback loop, which operates much like a home heating system:

• The Thermostat: The parathyroid glands, with their calcium-sensing receptors, act as the thermostat, constantly monitoring the calcium levels of the blood.
• The Set Point: The body has a genetically determined set point for what the ideal calcium level should be.
• Turning the Furnace On: If the blood calcium level drops below this set point, the glands switch “on,” releasing PTH.
• Raising the Temperature: PTH then acts on the bones, kidneys, and intestines to increase the blood calcium level, bringing the value back up.
• Turning the Furnace Off: Once the calcium level rises back into the normal range, the calcium-sensing receptors on the parathyroid glands detect this change and signal the glands to dramatically reduce or stop PTH secretion.

Proper calcium balance is essential for nerve impulse transmission, muscle contraction (including the beating of the heart), blood clotting, and the release of hormones. Without the constant vigilance of the parathyroid gland system, these fundamental processes would fail, leading to conditions like tetany (severe muscle cramps) with low calcium, or cardiac arrhythmias and kidney stones with high calcium.

Which Body Systems Does Parathyroid Hormone Affect?

To fully answer what is the purpose of parathyroid glands, one must look at the multiple body systems they influence. Parathyroid hormone primarily targets three main body systems to regulate blood calcium levels: the skeletal system (bones), the urinary system (kidneys), and the gastrointestinal system (specifically, the small intestine).

The Bones (The Calcium Bank)

The skeleton serves as the body’s largest reservoir of calcium, storing over 99% of the total amount. When PTH is released, it stimulates cells called osteoclasts, which are responsible for bone resorption. These cells effectively break down small amounts of bone tissue, releasing the stored calcium and phosphate into the bloodstream. This is the fastest way for the body to increase blood calcium levels. While this process is normal and necessary for minute-to-minute regulation, chronic overstimulation due to hyperparathyroidism can lead to excessive bone loss and conditions like osteoporosis.

The Kidneys (The Conservation and Activation Center)

PTH has a dual effect on the kidneys. First, it acts on the renal tubules to increase the reabsorption of calcium, preventing it from being lost in the urine and returning it to the blood. This is a critical conservation mechanism. Second, PTH stimulates the final and most crucial step in the activation of Vitamin D. The kidneys convert inactive Vitamin D into its active form, calcitriol, a process that is entirely dependent on PTH signaling.

The Small Intestine (The Absorption Site)

The effect of the parathyroid gland hormone on the intestines is indirect but powerful. It is the active form of Vitamin D (calcitriol), produced by the kidneys under PTH stimulation, that acts on the cells of the small intestine. Calcitriol significantly increases the efficiency with which the intestine can absorb calcium from dietary sources. Without sufficient PTH to activate Vitamin D, dietary calcium passes through the digestive system largely unabsorbed, making it impossible to maintain normal calcium levels through diet alone.

How the Parathyroid Gland Regulates Calcium

The parathyroid gland regulates calcium through a sophisticated negative feedback system. It releases Parathyroid Hormone (PTH) in response to low blood calcium, which then acts on the bones, kidneys, and intestines to restore normal levels.

This process involves signaling bones to release stored calcium, instructing the kidneys to conserve calcium and activate Vitamin D, and enabling the intestines to absorb more calcium from food, all while carefully managing phosphorus levels.

How Does PTH Get Calcium from the Bones?

Parathyroid hormone signals bone tissue to release its stored calcium into the bloodstream by stimulating the activity of bone-resorbing cells known as osteoclasts. The skeleton functions as a vast mineral reservoir, and PTH can rapidly tap into this supply to correct low blood calcium levels. This process, called bone resorption, is a dynamic part of bone remodeling, but its rate is directly controlled by the parathyroid gland.

The mechanism involves a complex interaction between different types of bone cells:

• Osteoblasts: These are the bone-forming cells. Interestingly, PTH does not act directly on osteoclasts. Instead, its receptors are located on osteoblasts. When PTH binds to these receptors, it causes the osteoblasts to release a signaling molecule called RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand).

• Osteoclasts: These are large, multinucleated cells responsible for breaking down bone matrix. Precursor cells to osteoclasts have a receptor on their surface called RANK. When RANKL, released by the osteoblasts, binds to the RANK receptor on osteoclast precursors, it triggers their differentiation and activation into mature, bone-resorbing osteoclasts.

• Bone Resorption: The activated osteoclasts attach to the bone surface and secrete acids and proteolytic enzymes that dissolve the mineral (calcium phosphate) and digest the organic matrix (collagen). This process liberates calcium and phosphate from the bone into the circulation.

This bone-releasing action is the body’s first line of defense against hypocalcemia (low blood calcium). However, the effect of PTH on bone is biphasic. While continuous high levels of PTH lead to significant bone loss, intermittent, low-dose administration of PTH can actually be anabolic, stimulating bone formation by osteoblasts—a property utilized in osteoporosis medications.

How Does PTH Work with the Kidneys?

Parathyroid hormone acts directly on the kidneys to conserve calcium by increasing its reabsorption from the fluid that would otherwise become urine, and it also promotes the excretion of phosphorus. The kidneys filter a large amount of calcium from the blood each day, and PTH ensures that most of it is returned to the body. The process occurs in distinct parts of the nephron, the functional unit of the kidney:

Calcium Reabsorption

The primary site of PTH-regulated calcium reabsorption is in the distal convoluted tubules of the nephrons. PTH binds to receptors on the cells lining these tubules, which opens calcium channels on the luminal side facing the filtrate. This allows calcium to enter the cell. On the basolateral side facing the blood, PTH stimulates the activity of pumps that actively transport the calcium out of the cell and back into the bloodstream, salvaging calcium that would have been lost in urine.

Phosphorus Excretion

Simultaneously, PTH inhibits the reabsorption of phosphate in the proximal tubules of the nephrons, increasing the amount of phosphate excreted in the urine. This action is essential because when calcium is released from bone, phosphorus is released with it. If phosphorus levels were allowed to rise along with calcium, much of the newly mobilized calcium would bind to the excess phosphate, forming calcium phosphate and preventing the desired increase in free, ionized calcium in the blood.

Vitamin D Activation

As a third critical function, PTH stimulates the enzyme 1-alpha-hydroxylase in the proximal tubules. This enzyme is responsible for the final conversion step of inactive Vitamin D into its potent, active form, calcitriol.

What Is the Relationship Between PTH and Vitamin D?

The relationship between the parathyroid gland hormone and Vitamin D is synergistic: PTH is essential for activating Vitamin D, and active Vitamin D is, in turn, necessary for PTH to effectively raise blood calcium through intestinal absorption. PTH acts as the metabolic trigger, while Vitamin D acts as the key that unlocks the absorption of dietary calcium from the gut.

The activation and function of Vitamin D is a tightly regulated multi-step process:

• Sources of Vitamin D: Vitamin D can be obtained from dietary sources or synthesized in the skin upon exposure to ultraviolet B (UVB) sunlight. In both cases, it enters the body in an inactive form.

• First Activation Step (Liver): This inactive Vitamin D travels to the liver, where an enzyme converts it to 25-hydroxyvitamin D (calcidiol). This is the main storage form of Vitamin D measured in routine blood tests.

• Second Activation Step (Kidneys): When PTH levels are elevated due to low blood calcium, PTH activates the enzyme 1-alpha-hydroxylase in the kidney tubules. This enzyme converts 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D (calcitriol), the fully active hormone.

• Action on the Intestine: Calcitriol travels to the small intestine, where it promotes the synthesis of calcium-binding proteins (like calbindin). These proteins actively transport calcium from digested food across the intestinal wall and into the bloodstream. Without active Vitamin D, only about 10-15% of dietary calcium is absorbed.

How Are PTH and Phosphorus Levels Related?

Parathyroid hormone and phosphorus levels have an inverse relationship: as PTH acts to raise blood calcium, it simultaneously acts on the kidneys to lower blood phosphorus levels by increasing its excretion in the urine. This phosphate-excreting (phosphaturic) effect is just as important as its calcium-raising effects for maintaining overall mineral balance.

This inverse regulation is necessary to maintain the amount of biologically active, ionized calcium. Calcium and phosphate have a high chemical affinity for each other. If both levels were to rise together in the blood, they would precipitate out of solution, forming solid calcium phosphate crystals. This would lower the level of free, ionized calcium (defeating the purpose of PTH secretion) and lead to dangerous calcification of soft tissues like blood vessels and kidneys.

When the parathyroid gland stimulates bone resorption, both calcium and phosphate are released into the bloodstream from the bone matrix. To counteract the rise in phosphate, PTH downregulates the activity of sodium-phosphate cotransporters on the surface of the kidney’s proximal tubule cells. These transporters are responsible for reabsorbing phosphate from the filtrate back into the blood. By inhibiting them, PTH allows more phosphate to remain in the filtrate and be cleared from the body.

What Is a Negative Feedback Loop in Calcium Regulation?

A negative feedback loop is the self-regulating system that controls parathyroid gland secretion, where high levels of blood calcium act directly on the parathyroid glands to inhibit further PTH production and release. This ensures that PTH is only secreted when needed and that calcium levels do not overshoot into a dangerously high range (hypercalcemia).

The loop centers on a specialized protein on the surface of the parathyroid chief cells called the Calcium-Sensing Receptor (CaSR):

Sensing Low Calcium

When the concentration of ionized calcium in the blood is low, the CaSR remains in an inactive state. This inactivity is the biological signal for the chief cells to ramp up the synthesis and secretion of PTH, releasing it into the blood to correct the deficit.

Sensing High Calcium

As PTH elevates blood calcium levels, calcium ions bind to the CaSR on the chief cells. This binding activates the receptor, initiating a cascade of intracellular signals that blocks the release of pre-formed hormone vesicles and suppresses the transcription of the PTH gene.

System Shutdown

As a result, PTH secretion plummets. Because PTH has a short half-life of only three to five minutes, its levels in the blood fall rapidly. Without PTH stimulation, bones stop releasing calcium, the kidneys excrete more calcium, and Vitamin D activation slows down, bringing blood calcium levels back down toward the normal set point.

Are the Parathyroid and Thyroid Glands Functionally Related?

No, despite their close physical proximity in the neck and similar-sounding names, the parathyroid gland and thyroid gland are functionally completely separate and unrelated endocrine organs. The thyroid gland regulates the body’s overall metabolic rate, while the parathyroid glands have the single, specific job of regulating calcium and phosphorus homeostasis.

Their hormones and regulatory pathways do not overlap in a clinically significant way:

• Thyroid Gland Function: The thyroid produces thyroxine ($T_4$) and triiodothyronine ($T_3$), which act on nearly every cell in the body to control the basal metabolic rate. The thyroid is controlled by Thyroid-Stimulating Hormone (TSH) from the pituitary gland.

• Parathyroid Gland Function: The parathyroid glands produce only parathyroid hormone (PTH). Its secretion is not controlled by the pituitary gland or the brain, but is regulated directly by serum calcium concentrations hitting the CaSR.

The thyroid gland does produce a minor hormone called calcitonin from its parafollicular cells, which acts to lower blood calcium levels. However, in humans, the role of calcitonin in minute-to-minute calcium regulation is very minor compared to the dominant influence of PTH. Patients who have had their thyroid gland completely removed are able to maintain perfect calcium balance as long as their parathyroid glands are left intact.

What Happens When Parathyroid Function Goes Wrong?

When parathyroid gland function goes wrong, it causes significant imbalances in the body’s calcium and phosphorus levels. This leads to conditions like hyperparathyroidism or hypoparathyroidism that directly affect the skeletal, renal, nervous, and digestive systems. These dysfunctions disrupt the delicate hormonal feedback loop responsible for mineral homeostasis, resulting in a cascade of symptoms that can range from mild and non-specific to severe and life-threatening.

To understand these pathological states, it helps to revisit the core physiology: what is the purpose of parathyroid glands? Their fundamental purpose is to maintain a tight, narrow window of serum calcium. When they produce either too much or too little parathyroid hormone (PTH), it alters how much calcium is drawn from bones, absorbed by the intestines, and excreted by the kidneys.

Common Symptoms of High Parathyroid Function (Hyperparathyroidism)

The symptoms of hyperparathyroidism, a condition marked by excess PTH and subsequent high blood calcium (hypercalcemia), are classically remembered by the medical mnemonic: “bones, stones, abdominal groans, and psychic moans.” This phrase effectively groups the wide-ranging systemic effects that elevated calcium has on the body.

• Bones: This refers to skeletal complications. As the overactive parathyroid gland continuously signals for calcium to be released from the skeletal reservoir, it can lead to bone thinning (osteoporosis or osteopenia), an increased risk of fractures, and generalized bone and joint pain.
• Stones: This highlights the impact on the renal system. High levels of calcium filtered by the kidneys can crystallize to form painful kidney stones. This state can also cause increased thirst (polydipsia) and frequent urination (polyuria) as the body attempts to flush out the excess minerals.
• Abdominal Groans: This encapsulates the gastrointestinal disturbances associated with hypercalcemia. Symptoms can include nausea, vomiting, severe constipation, and a loss of appetite, as high calcium levels slow down smooth muscle contraction within the digestive tract.
• Psychic Moans: This describes the neurological and psychological effects. Patients often report profound fatigue, lethargy, depression, anxiety, memory problems, and difficulty concentrating. In severe cases, acute hypercalcemic crisis can lead to confusion, cardiac arrhythmias, or even a coma.

Common Symptoms of Low Parathyroid Function (Hypoparathyroidism)

In contrast to an overactive gland, hypoparathyroidism is characterized by insufficient production of PTH from the parathyroid gland, leading to abnormally low levels of blood calcium (hypocalcemia). The symptoms of this condition primarily revolve around increased neuromuscular irritability caused by the lack of extracellular calcium, which is essential for stabilizing nerve and muscle membranes.

One of the most common and earliest signs is paresthesia, a sensation of tingling or numbness typically felt in the fingertips, toes, and around the mouth. As calcium levels drop further, this progresses to more pronounced muscle issues, such as generalized muscle aches, cramps, and spasms in the hands and legs.

A classic sign of severe hypocalcemia is tetany, a condition involving involuntary muscle contractions. This may manifest as carpopedal spasms, where the hands and feet cramp into contorted, rigid positions. In severe, acute cases, these spasms can affect the larynx (laryngospasm), causing life-threatening airway obstruction.

Chronic hypoparathyroidism also leads to distinct structural changes over time, including dry, coarse skin, brittle nails, patchy hair loss, and profound cognitive “brain fog.”

How Are Parathyroid Disorders Diagnosed?

The diagnosis of a parathyroid gland disorder is primarily established through specific blood tests that measure the relationship between calcium and PTH levels simultaneously from the same blood sample. In a healthy individual, these two values exist in a strict, predictable feedback loop: when calcium is low, PTH should be high, and when calcium is high, PTH should be low. A disruption in this relationship confirms a disorder.

Diagnosing Hyperparathyroidism

The classic diagnostic finding is an elevated serum calcium level occurring at the same time as an elevated, or inappropriately “normal,” PTH level. A normal PTH level is considered pathologically inappropriate when calcium is high, because a healthy gland would have completely shut down production.

Diagnosing Hypoparathyroidism

The diagnosis is confirmed by finding a low serum calcium level along with a low or completely undetectable PTH level, showing that the gland is failing to respond to hypocalcemia.

Supplementary Diagnostic Tests

24-Hour Urine Calcium Test: This measures how much calcium is excreted in the urine over a full day, which helps differentiate types of hyperparathyroidism and assesses the absolute risk of kidney stones.

• Bone Mineral Density Test (DEXA Scan): This imaging test determines if chronic PTH overproduction has caused significant bone loss or osteoporosis.
• Sestamibi Scan and Ultrasound: If surgery is required for hyperparathyroidism, these imaging modalities help locate the specific overactive gland (often a benign tumor called an adenoma) in the neck.

Primary vs. Secondary Hyperparathyroidism

The distinction between primary and secondary hyperparathyroidism lies entirely in the underlying cause of the parathyroid gland overactivity, which results in completely different biochemical profiles.

Primary Hyperparathyroidism

This condition originates from an intrinsic problem within the parathyroid gland tissue itself. The gland becomes autonomous and secretes excessive PTH while completely ignoring the body’s normal negative feedback signals. The most common cause, accounting for about 85% of cases, is a single, benign tumor known as a parathyroid adenoma. Because the problem is internal, the abnormally high PTH level continuously pulls calcium into the blood, leading to definitive hypercalcemia.

Secondary Hyperparathyroidism

This is not a primary disease of the parathyroid glands, but rather a correct and appropriate physiological response to another medical condition that is causing chronically low blood calcium. The parathyroid glands themselves are healthy, but they are working overtime to compensate for a systemic deficiency.

The most common causes are chronic kidney disease and severe Vitamin D deficiency. In chronic kidney disease, the failing kidneys cannot activate Vitamin D or excrete phosphorus efficiently, forcing blood calcium levels downward. In response to this persistent threat, the parathyroid glands ramp up PTH production to try and normalize calcium.

Consequently, treating secondary hyperparathyroidism requires addressing the external root cause (such as active Vitamin D supplementation or managing kidney disease), whereas primary hyperparathyroidism typically requires surgical removal (parathyroidectomy) of the overactive adenoma.

FAQs

What is the parathyroid gland?

The parathyroid gland refers to four small glands located behind the thyroid gland that produce parathyroid hormone (PTH), which regulates calcium levels in the body.

What does the parathyroid gland do?

Its primary function is to maintain healthy calcium and phosphorus levels in the blood and bones through the release of PTH.

How many parathyroid glands does a person have?

Most people have four parathyroid glands, although some individuals may have more or fewer.

Is the parathyroid gland the same as the thyroid gland?

No. Despite their similar names and close location, the parathyroid glands and thyroid gland have different functions and produce different hormones.

What happens if the parathyroid gland becomes overactive?

An overactive parathyroid gland can cause hyperparathyroidism, leading to elevated calcium levels, kidney stones, bone loss, and fatigue.

What happens if the parathyroid gland is underactive?

An underactive parathyroid gland can cause hypoparathyroidism, resulting in low calcium levels, muscle cramps, tingling, and seizures in severe cases.

How are parathyroid disorders diagnosed?

Doctors typically use blood tests to measure calcium, phosphorus, and parathyroid hormone levels, along with imaging studies when needed.

Can parathyroid gland disorders be treated?

Yes. Treatment depends on the condition and may include medications, calcium supplements, vitamin D therapy, or surgery.

What are common symptoms of parathyroid disease?

Symptoms may include fatigue, muscle weakness, bone pain, kidney stones, depression, memory problems, or abnormal calcium levels.

When should I see a doctor?

You should seek medical evaluation if you experience symptoms of abnormal calcium levels or have blood test results suggesting a parathyroid disorder.

Conclusion

The parathyroid gland may be small, but its influence on overall health is significant. By regulating calcium and phosphorus levels, these glands help support healthy bones, proper muscle function, nerve signaling, and many other essential bodily processes.

Because parathyroid disorders can affect multiple systems throughout the body, recognizing symptoms early and seeking appropriate medical evaluation is important. Many conditions involving the parathyroid glands can be effectively managed with modern diagnostic tools and treatment options.

Whether you’re learning about calcium balance, bone health, or endocrine disorders, understanding the function of the parathyroid gland provides valuable insight into one of the body’s most important regulatory systems.

Read more: 7 Warning Signs of Nystagmus That Can Be Easy to Miss

Sources

• National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) – Hyperparathyroidism
• Cleveland Clinic – Parathyroid Glands and Parathyroid Disorders
• Mayo Clinic – Hyperparathyroidism Overview
• MedlinePlus – Parathyroid Disorders
• American Association of Endocrine Surgeons (AAES)
• Hormone Health Network – Parathyroid Gland Information