Parathyroid Hormone (PTH) is a protein hormone secreted by the four small parathyroid glands, typically located in the neck behind the thyroid gland. These glands function as a thermostat, balancing specific substances within the body. The primary purpose of PTH is to maintain a stable internal environment, ensuring critical processes like nerve signaling and muscle contraction function without disruption. The body constantly monitors and adjusts PTH release through a feedback loop to preserve equilibrium.
How Parathyroid Hormone Regulates the Body
PTH acts as the body’s primary regulator of mineral balance, specifically managing calcium and phosphate levels. The hormone targets three main organs: the bones, the kidneys, and the intestines. When blood calcium levels drop, the parathyroid glands immediately release PTH to restore the concentration to an optimal range.
In the skeletal system, PTH indirectly stimulates osteoclasts to break down bone tissue. This process, known as bone resorption, releases stored calcium and phosphate minerals into the bloodstream, increasing their circulating levels. This action provides a rapid way for the body to access its largest mineral reserve when needed.
The kidneys are a direct target for PTH, where the hormone conserves calcium while promoting phosphate excretion. PTH signals the kidney tubules to reabsorb more calcium back into the blood, reducing calcium loss in the urine. Concurrently, it blocks phosphate reabsorption, ensuring this mineral is flushed out to prevent imbalance.
PTH also influences the production of the active form of Vitamin D within the kidneys by activating the enzyme 1-alpha-hydroxylase. This active Vitamin D then travels to the small intestine, enhancing the absorption of dietary calcium. This indirect effect completes the three-pronged mechanism by which PTH works to elevate and stabilize blood calcium concentrations.
Interpreting the Normal Range
Measuring intact PTH in the blood helps medical professionals assess parathyroid gland function and diagnose calcium metabolism disorders. The typical normal range for intact PTH generally falls between 10 and 65 picograms per milliliter (pg/mL). Reference values can vary slightly depending on the laboratory and testing method, so the range listed on a patient’s specific lab report should be the guiding reference.
Interpreting a PTH result rarely happens in isolation; the level must be considered alongside the patient’s serum calcium concentration. A seemingly “normal” PTH level can be considered abnormal if the calcium level is simultaneously elevated. In a healthy state, increased blood calcium should trigger a decrease in PTH secretion. Therefore, a PTH level within the reference range paired with high calcium suggests a malfunction in the feedback mechanism.
When PTH Levels Are Elevated
An elevated PTH level, known as hyperparathyroidism, indicates the parathyroid glands are producing too much hormone. The most common form is Primary Hyperparathyroidism, usually caused by a benign tumor (adenoma) developing on one of the four glands. This adenoma acts independently of the feedback loop, continuously secreting PTH and causing persistently high blood calcium levels.
Secondary Hyperparathyroidism occurs when the glands overwork in response to an underlying medical issue, not due to a primary gland problem. Chronic kidney disease is a frequent cause, as failing kidneys cannot efficiently activate Vitamin D or excrete phosphate. This leads to low calcium, which triggers excessive PTH release. Severe long-term Vitamin D deficiency can also cause this form, as the body compensates for poor intestinal calcium absorption.
Persistently high PTH and calcium levels can cause symptoms affecting multiple body systems.
Symptoms of Hyperparathyroidism
Patients may experience:
- Kidney stones (“stones”) due to excess calcium filtered through the kidneys.
- Bone pain or fractures (“bones”) as calcium is pulled from the skeleton.
- Generalized fatigue, constipation, and nausea (“groans”).
- Issues like depression, anxiety, or difficulty concentrating (“psychic overtones”).
If secondary hyperparathyroidism is left untreated, it can progress to Tertiary Hyperparathyroidism. In this stage, the glands become permanently overactive and autonomously secrete high PTH, even after the underlying cause is corrected.
When PTH Levels Are Low
Hypoparathyroidism is the state where PTH levels are lower than normal, resulting in an inability to maintain adequate blood calcium levels. The most frequent cause is iatrogenic hypoparathyroidism, where the glands are accidentally damaged or removed during thyroid or other neck surgeries. Because the glands are small and located near the thyroid, they are susceptible to injury during these procedures.
Less common causes include autoimmune destruction, where the immune system mistakenly attacks the parathyroid tissue. The resulting low PTH leads to hypocalcemia (low blood calcium), which manifests with distinct neurological and muscular symptoms. Patients often report tingling sensations, particularly around the mouth, fingers, and toes.
Muscle symptoms include spasms, cramps, and in severe cases, tetany, which involves painful, involuntary muscle contractions. Because calcium is integral to nerve and muscle function, the deficiency can also cause fatigue and weakness. Treatment is necessary to prevent these symptoms and protect organ systems from prolonged low calcium effects.
Testing and Treatment
Diagnosing an abnormal PTH level begins with a simple venipuncture, a routine blood draw from a vein in the arm. A healthcare provider may request that the patient fast before the test, as diet can temporarily influence calcium and phosphate levels. For the most accurate diagnostic picture, the PTH test is rarely ordered alone.
Simultaneous measurement of serum calcium, Vitamin D, and phosphate levels is necessary to properly interpret the PTH result and pinpoint the cause of the imbalance. For instance, high PTH with high calcium points toward primary hyperparathyroidism. Conversely, high PTH with low calcium is consistent with the secondary form. In some cases, a 24-hour urine collection may also assess how much calcium the kidneys are excreting.
The appropriate treatment for an abnormal PTH level depends entirely on the underlying diagnosis. For Primary Hyperparathyroidism, surgical removal of the overactive gland (parathyroidectomy) is often the definitive cure. Management for hypoparathyroidism involves lifelong supplementation with calcium and active Vitamin D to aid mineral absorption. Medications called calcimimetics may manage secondary hyperparathyroidism by making the glands more sensitive to circulating calcium, thus reducing PTH production.