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Parathyroid Hormone (PTH)

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Parathyroid hormone (PTH) is produced by the parathyroid glands, which are located in pairs behind the thyroid gland. Intact PTH (the full hormone molecule) consists of 84 amino acids, has a short half-life (~4 minutes), and is the biologically active form of the hormone. PTH fragments (N-terminal and C-terminal) are longer, and their metabolism is still being studied.

PTH and Calcium-Phosphorus Homeostasis

PTH plays a crucial role in calcium-phosphorus metabolism, maintaining stable levels of calcium and phosphorus in extracellular fluids.

  • PTH secretion is tightly linked to calcium, vitamin D, phosphorus, and magnesium levels.
  • Regulation follows a negative feedback mechanism:
    • When calcium levels decrease (hypocalcemia), PTH secretion increases.
    • When calcium levels rise (hypercalcemia), PTH secretion decreases.

These mechanisms help maintain calcium balance by:

  • Activating osteoclasts, increasing bone resorption and calcium release.
  • Enhancing calcium absorption in the intestines.
  • Reducing calcium excretion by the kidneys.
  • Inhibiting phosphorus reabsorption in the kidneys, leading to phosphorus excretion.

PTH’s physiological antagonist is calcitonin, a hormone secreted by thyroid C-cells. When blood calcium levels normalize, PTH secretion decreases.

  • Avoid fatty foods for 24 hours before the test.
  • Fasting for at least 12 hours before the test.
  • Stop taking medications 24 hours before the test (if approved by a doctor).
  • Avoid physical and emotional stress 24 hours before the test.
  • No smoking for at least 3 hours before the test
  • Assessment of parathyroid gland function.
  • Evaluation of causes of hypo- or hypercalcemia and calcium metabolism disorders.
  • Differentiation of primary, secondary, and tertiary hyperparathyroidism.
  • Diagnosis of hypoparathyroidism.
  • Monitoring patients with chronic calcium metabolism disorders.
  • Assessing the effectiveness of parathyroid disease treatments and post-surgical follow-up.
  • Detection of parathyroid tumors and monitoring after surgical removal.
  • Evaluation of osteoporosis and bone structure changes.
  • Chronic kidney disease and low glomerular filtration rate.
  • Calcium level abnormalities (hypercalcemia or hypocalcemia).
  • Symptoms of hypercalcemia (fatigue, nausea, abdominal pain, thirst) or hypocalcemia (abdominal pain, muscle cramps, tingling in fingers).
  • Changes in parathyroid gland size or structure on imaging (e.g., CT scan).

Causes of Increased PTH Levels

Primary Hyperparathyroidism (↑ PTH, ↑ Calcium, ↑ Calcitonin, ↓/Normal Phosphorus)

  • Parathyroid hyperplasia
  • Parathyroid adenoma or carcinoma

Secondary Hyperparathyroidism (↑ PTH, ↓/Normal Calcium, ↓ Calcitonin, ↑ Phosphorus)

  • Chronic kidney disease
  • Vitamin D or calcium deficiency
  • Malabsorption syndromes (e.g., celiac disease, Crohn’s disease)

Tertiary Hyperparathyroidism (Autonomous parathyroid adenoma with prolonged secondary hyperparathyroidism)

Other Causes of Elevated PTH

  • Pseudohypoparathyroidism (Albright’s hereditary osteodystrophy – tissue resistance to PTH action)
  • Multiple endocrine neoplasia (MEN syndrome)
  • Zollinger-Ellison syndrome
  • Chronic kidney disease
  • Renal hypercalciuria
  • Rickets
  • Ectopic PTH production (e.g., kidney or lung cancer)
  • Bone metastases
  • Breastfeeding
  • Pregnancy
     

    Causes of Decreased PTH Levels

    Primary Hypoparathyroidism (↓ PTH, ↑ Calcium, ↓ Phosphorus)

  • Parathyroid gland insufficiency

  • Secondary Hypoparathyroidism (↓ PTH due to post-surgical removal or damage to parathyroid glands)

  • Post-thyroidectomy or parathyroidectomy complications

  • Other Causes of Low PTH

  • High vitamin A and D levels
  • Idiopathic hypercalcemia
  • Autoimmune diseases with autoantibodies to calcium receptors
  • Wilson-Konovalov disease, hemochromatosis
  • Graves' disease, severe thyrotoxicosis
  • Magnesium deficiency
  • Multiple myeloma
  • Sarcoidosis