Parathyroid hormone (PTH)

Sample for Parathyroid hormone (PTH)

1. The serum of the patient is required.
   1. A fasting sample is preferred.
   2. Place on ice.
2. The serum should be separated immediately because the PTH is unstable at room temperature and even on refrigeration.
   1. Refrigerate during centrifugation.
   2. Immediately freeze the sample.
   3. It can store at -20 °C to -70 ° C.
3. A lower level is seen if the sample was left at room temperature for 4 hours.
4. Also, a lower level has seen if the sample is kept at 4 °C for one day.
5. The EDTA plasma is the best choice if the chemical method allows it.
6. Simultaneous estimation of total calcium, ionized calcium, and phosphorus is recommended.

Purpose of the test (Indications) for Parathyroid hormone (PTH)

1. This is done to evaluate hypercalcemia or hypocalcemia.
2. This is also done to establish the diagnosis of hyperparathyroidism.
3. PTH differentiates hyperparathyroidism from Nonparathyroid causes leading to hypercalcemia.

Precaution for Parathyroid hormone (PTH)

1. lipemic serum and hemolysis interfere with the method.
2. Some drugs may affect the result and increase PTH value like:
   1. Anticonvulsants.
   2. Isoniazid.
   3. Lithium.
   4. Rifampicin.
   5. Steroids.
3. Drugs that decrease PTH are:
   1. Cimetidine.
   2. Propranolol.

Pathophysiology of Parathyroid hormone (PTH)

1. This is a polypeptide hormone produced in the parathyroid gland.
   1. These are 4 in number and present close to or on the posterior surface of the thyroid gland.
   2. Additional parathyroid glands may be found in the mediastinum (in the thymus gland) or neck.

Formation of Parathyroid hormone (PTH):

1. Preparathyroid (115 AA) is synthesized in the ribosomes of Cheif cells.
2. Proteolysis converted Parathyroid hormone (PTH) into Pro-PTH (90 AA).
3. By the second, proteolysis converted into Intact-PTH (1- 84 AA).
4. PTH-1-84 is metabolized in the peripheral tissue, mainly the liver and kidney.
5. This is stored in the gland, and some go into blood circulation.
6. The biological activity of the PTH resides in the first 30 AA of the N-terminal.
7. The concentration of the biologically active form of PTH is very low in blood circulation (10 pmol/L or 0.1 ng/mL).
8. PTH secretion is directly controlled by plasma calcium.

9. This is the main hormone regulating calcium concentration in the extracellular fluid.
   1. A decrease in the ionized calcium is the stimulus for the PTH secretion.
   2. PTH + 1,25- hydroxy D3 rais the calcium level by the following mechanisms:
      1. It promotes the dissolving of the bone.
      2. Increases renal tubular reabsorption of calcium in the kidney.
      3. Increase intestinal absorption of calcium.
   3. A rise in Calcium inhibits PTH secretion.

10. Magnesium also influences the PTH level.
    1. Hypermagnesemia suppresses the PTH secretion, although not like Calcium.

Primary hyperparathyroidism:

1. It is caused by parathyroid adenoma or rarely by cancer.
   1. The patient will have high PTH.
   2. And high Calcium levels.

Secondary hyperparathyroidism:

1. It is due to chronic renal failure, where the patient has low calcium and high phosphate.
   1. Now parathyroid glands persistently produce PTH to maintain Calcium levels.
   2. These patients have high PTH and low calcium.

Tertiary hyperparathyroidism:

1. In this case, patients bypass the compensatory mechanism.
   1. Develops a high PTH level.
   2. This PTH leads to hypercalcemia.
   3. These patients have high PTH and High calcium levels.
2. Intact PTH molecules metabolized into three different molecules:
   1. N-terminal.
   2. Mid-Region.
   3. C- terminal.
3. Variation of the blood PTH level:
   1. PTH level has diurnal variation. It is the highest at around 2 AM and lowest at around 2 PM.

The normal level of Parathyroid hormone (PTH)

Source 1

• C- terminal and midmolecule
  • Serm (by RIA)
  • 1 to 16 years = 51 to 217 pg/mL
  • Adult = 50 to 300 pg/mL
• N- Terminal
  • Serum (by RIA)
  • 2 to 13 years = 14 to 21 pg/mL
  • Adult = 8 to 24 pg/mL
• Intact molecule
  • Serum (by ICMA))
  • Cord blood = ≤ 3 pg/mL
  • 2 to 20 years = 9 to 52 pg/mL
  • Adult = 10 to 65 pg/mL

Source 2

• Intact PTH molecule = 10 to 65 pg/mL (10 to 65 ng/mL)
• N-terminal intact PTH = 8 to 24 pg/L (8 to 24 ng/L)
• C-terminal  intact PTH = 50 to 330 pg/L (50 to 330 ng/L)
  • Mostly the intact PTH molecule is recommended.

Increased Parathyroid hormone (PTH) level is seen in:

1. Primary hyperparathyroidism.
2. Pseudohypoparathyroidism ( Secondary hyperparathyroidism ).
3. Vit. D deficiency ( hereditary ) and rickets.
4. Zollinger Ellison syndrome.
5. Non-PTH-producing tumors give rise to paraneoplastic syndrome; They produce PTH-like protein that acts like PTH.
6. Chronic renal failure.
7. Hypocalcemia.
8. Malabsorption.

Decreased PTH level is seen in:

1. Grave’s disease ( Hypoparathyroidism ).
2. Non-Parathyroid hypercalcemia.
3. Surgical, secondary hypoparathyroidism.
4. Sarcoidosis.
5. Metastatic bone tumors.
6. Vit.D intoxication.
7. Milk-alkali syndrome.
8. DiGeorge syndrome.

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