Parathyroid Hormone: What It Is, Function, Levels, Symptoms & Treatment
Definition Of Parathyroid Hormone (PTH)
Parathyroid hormone is one of the peptide hormones produced by the parathyroid glands, which have a crucial role in maintaining the calcium and phosphate levels within the blood. PTH is involved in the maintenance of calcium homeostasis by its action on bone remodelling, renal reabsorption of calcium, and intestinal calcium absorption.
Commonly Asked Questions
Q: What is parathyroid hormone-related protein (PTHrP)?
A:
PTHrP is a protein that is structurally similar to PTH but is produced by various tissues in the body, not just the parathyroid glands. It plays a role in fetal development and can cause hypercalcemia in certain cancers (paraneoplastic syndrome).
Anatomy Of The Parathyroid Glands
These are small endocrine glands located near the thyroid gland and have an important function in maintaining calcium levels within the body.
Structure And Location Of The Parathyroid Glands:
Four small glands on the posterior surface of the thyroid gland.
Typically, two superior and two inferior glands.
Each gland is about the size of a grain of rice.
Commonly Asked Questions
Q: What is the significance of PTH measurements in parathyroid carcinoma?
A:
In parathyroid carcinoma, a rare malignancy of the parathyroid glands, PTH levels are typically extremely elevated, often 3-10 times the upper limit of normal. The degree of PTH elevation, along with clinical features, can help differentiate carcinoma from benign parathyroid conditions.
Synthesis And Secretion Of Parathyroid Hormone
Synthesis and secretion are tightly regulated processes crucial for calcium homeostasis.
Pth Synthesis Process:
PTH is synthesised as a preprohormone.
Cleaved to form prohormone and then active PTH.
Factors Influencing PTH Secretion:
Serum calcium levels: Low levels stimulate secretion.
Serum phosphate levels: High levels stimulate secretion.
Vitamin D levels: Inhibit secretion.
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Q: What is parathyroid hormone (PTH) and where is it produced?
A:
Parathyroid hormone (PTH) is a peptide hormone produced by the parathyroid glands, which are four small endocrine glands located on the back of the thyroid gland in the neck. PTH plays a crucial role in regulating calcium levels in the blood and bones.
Q: How does PTH affect magnesium levels in the body?
A:
PTH helps regulate magnesium levels by increasing its reabsorption in the kidneys and promoting its release from bone. However, severe magnesium deficiency can impair PTH secretion and action, potentially leading to hypocalcemia.
Q: What is the role of PTH in calcium-sensing receptor disorders?
A:
Mutations in the calcium-sensing receptor can lead to disorders of calcium homeostasis. In some cases, these mutations result in the receptor being less sensitive to calcium, leading to inappropriate PTH secretion and hypercalcemia.
Q: How does PTH influence phosphate homeostasis in the context of fibroblast growth factor 23 (FGF23) resistance?
A:
In conditions of FGF23 resistance, such as in some genetic disorders or advanced kidney disease, the phosphate-lowering effects of FGF23 are impaired. This can lead to elevated phosphate levels, which stimulate increased PTH secretion as a compensatory mechanism.
Q: What is the role of PTH in calcium-alkali syndrome (milk-alkali syndrome)?
A:
In calcium-alkali syndrome, excessive intake of calcium and absorbable alkali leads to hypercalcemia and metabolic alkalosis. This condition suppresses PTH secretion. Understanding PTH levels helps differentiate this syndrome from other causes of hypercalcemia.
Regulation Of Calcium And Phosphate Homeostasis
PTH is integral in the regulation of calcium and phosphate balance in the body.
Role Of PTH In Calcium Regulation:
Increases calcium release from bones.
Increases calcium reabsorption in the kidneys.
Stimulates calcium absorption in the intestines by facilitating the action of vitamin D.
Mechanism Of Action On Bone, Kidney, And Intestine:
Bone: The osteoclasts get stimulated to release calcium.
Kidney: Increased calcium reabsorption and phosphate excretion
Intestine: Calcium absorption is increased indirectly due to active vitamin D.
Commonly Asked Questions
Q: What triggers the release of parathyroid hormone?
A:
The primary trigger for PTH release is a decrease in blood calcium levels (hypocalcemia). The parathyroid glands have calcium-sensing receptors that detect even small changes in blood calcium concentration and respond by secreting PTH.
Q: How does the body maintain a negative feedback loop for PTH secretion?
A:
As PTH increases blood calcium levels, the rising calcium concentration is detected by the calcium-sensing receptors in the parathyroid glands. This leads to a decrease in PTH secretion, forming a negative feedback loop that helps maintain calcium homeostasis.
Q: How does PTH interact with calcitonin?
A:
PTH and calcitonin work antagonistically to regulate calcium levels. While PTH increases blood calcium, calcitonin (produced by the thyroid gland) decreases it. This interplay helps maintain calcium homeostasis within a narrow range.
Q: What is the relationship between PTH and phosphate levels?
A:
PTH decreases phosphate reabsorption in the kidneys, leading to increased phosphate excretion in urine. This action helps maintain the balance between calcium and phosphate in the blood, as high phosphate levels can lead to decreased calcium levels.
Q: How does PTH influence intestinal calcium absorption?
A:
PTH indirectly enhances intestinal calcium absorption by increasing the production of active vitamin D (calcitriol) in the kidneys. Calcitriol then acts on the intestines to increase calcium absorption from the diet.
Physiological Effects Of Parathyroid Hormone
PT H has several physiological effects that are of importance to calcium equilibrium and bone health.
Bone Remodeling And Calcium Mobilisation
Stimulates the activity of osteoclasts to resorb the bone, thereby making calcium available.
Renal Reabsorption Of Calcium And Phosphate Excretion
Increases reabsorption of calcium in renal tubules.
Promotes phosphate excretion to maintain its levels.
Intestinal Absorption Of Calcium
Increases absorption indirectly through stimulation of vitamin D.
Commonly Asked Questions
Q: How does parathyroid hormone differ from thyroid hormones?
A:
While both are produced in the neck area, parathyroid hormone is produced by the parathyroid glands and regulates calcium levels, whereas thyroid hormones (T3 and T4) are produced by the thyroid gland and regulate metabolism. They have distinct functions and target different body systems.
Q: What is the primary function of parathyroid hormone?
A:
The primary function of parathyroid hormone is to maintain calcium homeostasis in the body. It increases blood calcium levels when they are low by acting on bones, kidneys, and the intestines to release, retain, and absorb more calcium, respectively.
Q: How does parathyroid hormone affect bone tissue?
A:
Parathyroid hormone stimulates osteoclasts (bone-resorbing cells) to break down bone tissue and release calcium into the bloodstream. It also indirectly activates osteoblasts (bone-forming cells) to remodel bone, which helps maintain bone strength over time.
Q: What role does PTH play in kidney function?
A:
In the kidneys, PTH increases calcium reabsorption in the renal tubules, reducing calcium loss through urine. It also promotes the production of active vitamin D (calcitriol) in the kidneys, which enhances calcium absorption in the intestines.
Q: How does parathyroid hormone influence vitamin D metabolism?
A:
PTH stimulates the conversion of inactive vitamin D (25-hydroxyvitamin D) to its active form (1,25-dihydroxyvitamin D or calcitriol) in the kidneys. This active form of vitamin D enhances calcium absorption in the intestines.
Parathyroid Hormone Disorders
An abnormal concentration of PTH can result in several clinical disorders.
Hyperparathyroidism (Primary, Secondary, And Tertiary):
Primary: Gland overproduction due to adenoma or hyperplasia.
Secondary: Compensation due to low calcium as seen in chronic renal failure.
Tertiary: Autonomous PTH secretion following long-standing secondary hyperparathyroidism.
Hypoparathyroidism:
Insufficient production of PTH resulting in hypocalcemia.
Symptoms And Complications Of Pth Disorders:
Hyperparathyroidism: Bone pain, kidney stones, fatigue.
Hypoparathyroidism: Muscle cramps, tetany, seizures.
Commonly Asked Questions
Q: What conditions can cause elevated PTH levels?
A:
Elevated PTH levels (hyperparathyroidism) can be caused by parathyroid gland tumors, chronic kidney disease, vitamin D deficiency, or certain medications. This condition leads to excessive calcium release from bones and can result in various health issues.
Q: What are the symptoms of hyperparathyroidism (elevated PTH levels)?
A:
Symptoms of hyperparathyroidism can include bone pain, kidney stones, frequent urination, abdominal pain, weakness, fatigue, depression, and cognitive issues. However, many people with mild hyperparathyroidism may be asymptomatic.
Q: How does low PTH levels (hypoparathyroidism) affect the body?
A:
Low PTH levels lead to decreased blood calcium and increased phosphate levels. This can cause muscle cramps, tingling sensations, seizures, and in severe cases, tetany (prolonged muscle contractions). It can also affect heart rhythm and bone health over time.
Q: What causes hypoparathyroidism?
A:
Hypoparathyroidism can be caused by accidental removal or damage to the parathyroid glands during thyroid surgery, autoimmune disorders, genetic conditions, or radiation therapy to the neck area. In some cases, the cause may be unknown (idiopathic).
Q: What is primary hyperparathyroidism?
A:
Primary hyperparathyroidism is a condition where one or more of the parathyroid glands produce excessive amounts of PTH, usually due to a benign tumor (adenoma) or gland enlargement. This leads to elevated blood calcium levels and associated symptoms.
Parathyroid Hormone Levels Diagnostic Tests
For an accurate diagnosis of disorders related to the parathyroid hormone, the following tests are necessary.
Blood Tests For PTH Levels:
Serum PTH levels
Serum calcium and phosphate levels
Imaging Studies (Ultrasound, Sestamibi Scan):
Ultrasound: Visualization of parathyroid glands
Sestamibi Scan: Nuclear medicine scan to detect overactive glands
Commonly Asked Questions
Q: What are the normal blood levels of parathyroid hormone?
A:
Normal PTH levels typically range from 10 to 65 picograms per milliliter (pg/mL). However, the exact reference range may vary slightly between different laboratories and testing methods.
Q: How is parathyroid hormone measured in the body?
A:
PTH is typically measured through a blood test called a PTH assay. This test measures the amount of intact PTH in the bloodstream. It's often done in conjunction with tests for calcium, phosphate, and vitamin D levels to assess overall calcium metabolism.
Q: What is the difference between intact PTH and PTH fragments?
A:
Intact PTH is the biologically active form of the hormone. PTH fragments are breakdown products of intact PTH. Modern PTH tests typically measure intact PTH, as it provides a more accurate representation of parathyroid function than measuring PTH fragments.
Q: What is the half-life of parathyroid hormone in the bloodstream?
A:
The half-life of PTH in the bloodstream is very short, typically only 2-4 minutes. This rapid turnover allows for quick adjustments in calcium regulation in response to changing physiological needs.
Q: How does chronic kidney disease affect PTH levels?
A:
In chronic kidney disease, the kidneys' ability to excrete phosphate and activate vitamin D is impaired. This leads to low calcium and high phosphate levels, which stimulate continuous PTH secretion, resulting in secondary hyperparathyroidism.
Treatment Options For Parathyroid Hormone Imbalances
Treatment will depend on the type and degree of PTH imbalance.
Vitamin D and calcium supplements in hypoparathyroidism.
Calcimimetics in hyperparathyroidism.
Surgical Options
Parathyroidectomy involves removing the overactive glands in hyperparathyroidism.
Lifestyle And Dietary Changes
Adequate intake of calcium and vitamin D.
Regular check on calcium levels.
Commonly Asked Questions
Q: How do bisphosphonate medications interact with PTH?
A:
Bisphosphonates, used to treat osteoporosis, work by inhibiting bone resorption. This can lead to a temporary increase in PTH levels as the body tries to maintain calcium homeostasis. However, this increase is usually not clinically significant.
Q: What is the role of PTH in osteoporosis?
A:
While PTH is essential for bone health, chronically elevated levels can contribute to osteoporosis by excessively stimulating bone resorption. Paradoxically, intermittent administration of PTH (as in some osteoporosis treatments) can actually increase bone formation.
Q: What is tertiary hyperparathyroidism?
A:
Tertiary hyperparathyroidism occurs when long-standing secondary hyperparathyroidism (often due to chronic kidney disease) leads to autonomous PTH production by the parathyroid glands, persisting even after the original stimulus is corrected.
Q: How does age affect parathyroid hormone levels?
A:
PTH levels tend to increase slightly with age, partly due to decreased calcium absorption in the intestines and reduced kidney function. This age-related increase is usually mild and helps maintain calcium balance in older adults.
Q: What is the role of PTH in maintaining bone density?
A:
While chronic elevation of PTH can lead to bone loss, normal PTH levels are crucial for maintaining bone density. PTH stimulates bone remodeling, which is necessary for repairing microdamage and maintaining overall bone strength.
Frequently Asked Questions (FAQs)
Q: How does PTH contribute to the pathophysiology of osteitis fibrosa cystica?
A:
Osteitis fibrosa cystica is a skeletal disorder caused by hyperparathyroidism. Chronically elevated PTH leads to excessive osteoclast activity, resulting in bone resorption, fibrosis of the marrow spaces, and the formation of cystic bone lesions. This condition illustrates the dramatic effects of long-term PTH excess on bone structure.
Q: How does PTH interact with sex hormones in bone metabolism?
A:
PTH interacts with sex hormones, particularly estrogen, in regulating bone metabolism. Estrogen tends to inhibit bone resorption, while PTH can stimulate it. The decline in estrogen during menopause can lead to increased bone sensitivity to PTH, contributing to postmenopausal osteoporosis.
Q: How does PTH influence bone marrow function and hematopoiesis?
A:
PTH has been found to have effects on the bone marrow microenvironment. It can stimulate the production of hematopoietic stem cells and influence their mobilization from the bone marrow. This has potential implications for bone marrow transplantation and hematological disorders.
Q: What is the relationship between PTH and parathyroid hormone-related protein (PTHrP) in cancer?
A:
Some cancers can produce PTHrP, leading to a condition called humoral hypercalcemia of malignancy. PTHrP mimics the effects of PTH, causing increased calcium levels. In these cases, actual PTH levels are typically suppressed due to the negative feedback from high calcium.
Q: What is the role of PTH in brown fat activation?
A:
Recent research suggests that PTH may play a role in activating brown adipose tissue, which is involved in thermogenesis and energy expenditure. This connection may have implications for metabolism and body weight regulation.
Q: How does PTH affect cardiac function?
A:
Chronic elevation of PTH, as seen in hyperparathyroidism, can have detrimental effects on the cardiovascular system. It may contribute to left ventricular hypertrophy, vascular calcification, and increased risk of cardiovascular events.
Q: What is the significance of PTH measurements in renal osteodystrophy?
A:
In renal osteodystrophy, a bone disorder associated with chronic kidney disease, PTH levels are used to assess the type and severity of bone disease. Different PTH levels can indicate high-turnover or low-turnover bone disease, guiding treatment decisions.
Q: What is the role of PTH in familial hypocalciuric hypercalcemia?
A:
Familial hypocalciuric hypercalcemia is a genetic disorder characterized by mutations in the calcium-sensing receptor gene. This leads to altered calcium and PTH regulation, resulting in mildly elevated calcium levels but inappropriately normal or slightly elevated PTH levels.
Q: How does PTH interact with fibroblast growth factor 23 (FGF23)?
A:
FGF23 is a bone-derived hormone that decreases phosphate reabsorption in the kidneys and suppresses vitamin D activation. PTH stimulates FGF23 production, while FGF23 can suppress PTH secretion, forming a complex feedback loop in mineral metabolism.
Q: How does PTH influence the risk of kidney stones?
A:
Elevated PTH levels, as seen in hyperparathyroidism, increase the risk of kidney stone formation. This is due to increased calcium excretion in the urine (hypercalciuria) and alterations in other stone-forming factors like citrate and oxalate.
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