parathyroid

Generality

Parathyroid hormone (PTH) is a hormone produced by the parathyroid glands, located in the neck, near the thyroid.

Its main role is to keep the concentration of calcium constant in the bloodstream (called calcemia ).

Normally, when the levels of calcium in the blood decrease, the parathyroids release the PTH, which causes increases in the calcemia by acting in three different ways:

• Promotes the release of calcium from the bones in the bloodstream;
• Promotes intestinal calcium absorption (through vitamin D);
• It acts on the kidneys, decreasing the elimination of calcium through the urine.

As calcium levels increase in the blood, PTH decreases.

The doctor may prescribe the test to measure the parathyroid hormone in the blood when it wishes to evaluate the functioning of the parathyroid glands, or if abnormal levels of calcium are found in the blood, both in excess (hypercalcemia), and in defect (hypocalcemia).

The examination also serves to establish whether the cause of an altered calcemia is deriving from a parathyroid disorder or from a renal disease .

What's this

Parathyroid hormone (PTH) is a peptide hormone consisting of 84 amino acids. It is secreted by the parathyroid glands, four endocrine glands located in the neck, on the dorsal aspect of the thyroid.

The parathormone, together with calcitonin, is used for the homeostasis of calcium and phosphorus ions. These two minerals, besides being the main constituents of bones and teeth, allow muscle contraction, transmission of nerve impulses, blood coagulation and the normal development of numerous metabolic reactions. It is therefore essential that their concentration remains relatively constant throughout the day.

To maintain these values ​​in balance the body relies mainly on two hormones:

• The parathormone, which performs a hyper-calming action;
• Calcitonin, which plays the opposite role.

1, 25- (OH) ₂ -colecalciferol or calcitriol, which is the active form of vitamin D, also intervenes in calcium homeostasis.

Actions of the parathormone

At the bone level, the parathyroid hormone stimulates the mobilization of calcium by direct and indirect routes.

In the first case it intervenes in first person, positively modulating the activity of osteoclasts (large cells that have the function of eroding and renewing the bone matrix). Since the bone tissue is rich in calcium ions, its catabolism favors the increase in serum calcium.

At renal level the parathormone increases the excretion of phosphate ions with the urine, decreasing the concentration of the mineral in the blood. To rebalance the situation, the body draws phosphate from the bones, in which it is deposited in the form of hydroxyapatite Ca ₅ (PO ₄ ) ₃ (OH). Looking at the molecular formula of this mineral, it is understood that while its degradation leads to an increase in phosphorus (P), on the other it also increases the release of calcium (Ca2 +).

At the intestinal level, thanks to the help of calcitriol (active form of vitamin D), the parathormone stimulates the absorption of calcium. At the kidney level the same hormone stimulates the activation of the aforementioned vitamin.

In addition to favoring the excretion of phosphate with urine, the parathormone acts positively on the reabsorption of calcium.

The parathormone is a hypercalcemizing hormone that acts at three levels (renal, intestinal and bone):

1. increasing the renal reabsorption of Ca2 +

2. increasing renal phosphorus elimination

3. stimulating the formation of Vitamin D3 from D2 (kidney)

4. increasing bone degradation

5. increasing intestinal calcium absorption

The decrease in calcemia represents a strong stimulus on the secretion of parathormone and, in parallel, inhibits the release of its antagonist (calcitonin). Similarly, as soon as the concentration of calcium in the blood exceeds normal values, parathormone secretion is inhibited.

This fine regulatory mechanism keeps the plasma calcium levels within fairly narrow limits; when this mechanism goes haywire the calcemia undergoes important variations. This is the case, for example, of hyper and hypoparathyroidism, which are accompanied, respectively, by hyper and hypocalcemia.

Why do you measure

The parathormone test measures its concentration in the blood.

This test is indicated in the presence of symptoms that may indicate an altered calcium metabolism, for which a parathyroid malfunction is suspected.

PTH is also required for monitoring patients with chronic kidney disease or those undergoing dialysis.

When is the exam prescribed?

The doctor prescribes the examination of the parathyroid hormone when the values ​​of calcium in the blood are higher (hypercalcaemia) or lower (hypocalcemia) than the norm, to identify the origin of the imbalances and place a differential diagnosis between parathyroid or non-parathyroid causes.

The parathyroid test is indicated to evaluate the functioning of the parathyroid glands or when there is a suspicion of a renal disease.

In addition, PTH is monitored regularly in patients with chronically altering calcium balance and to monitor the effectiveness of a treatment performed for parathyroid disorders.

Normal values

Normally, parathyroid hormone concentrations in the blood vary during the course of the day, reaching a peak around 2:00 am. However, the exam is generally performed around 8:00 am.

The reference interval also varies according to the laboratory performing the test but, in any case, on average we speak of high values ​​when they are higher than 7 pmol / L (or 70 pg / mL), considering the interval as normal between 1-7 pmol / L (or 10-70 pg / mL).

High parathormone - Causes

The causes of a high presence of parathyroid hormones in the blood can be attributed to a decompensation of calcium in the body; for this reason, more PTH is produced to counterbalance the imbalance.

Other possible causes of high parathormone or hypercalcaemia may be:

• Primary hyperparathyroidism (frequently caused by an adenoma or a benign parathyroid tumor);
• Hyperparathyroidism (often caused by renal failure or taking specific drugs, including rifampicin, phosphates, anticonvulsants, steroids, isoniazid, etc.);
• Vitamin D deficiency;
• Pregnancy;
• Menopause;
• Lithium based treatments.

Low parathormone - Causes

Among the causes of low parathormone or hypocalcemia there may be:

• Hypoparathyroidism (which may be transient, progressive or persistent);
• Autoimmune diseases;
• High phosphate levels;
• Parathyroid removal surgery.

How to measure it

To perform the parathormone test, it is sufficient to take a small amount of blood from the vein of an arm. Then in the laboratory the dosage is performed, that is the measurement of the concentration of the protein in the blood sample taken.

Preparation

Before undergoing the examination, it is not necessary to observe the "usual" fast for at least 8-10 hours. However, PTH levels vary during the day and seasonal fluctuations may occur due to the inverse relationship of PTH with vitamin D. For these reasons, the sampling time should be agreed with the treating physician.

Interpretation of Results

To correctly interpret the results of the PTH test, it is necessary to compare them with the calcium values:

• If both calcium and PTH are normal, it means that the calcium regulation system works properly.
• If one or both parameters are altered, instead, a specific assessment of the situation must be made.

Low levels of PTH can be due to the presence of pathologies that cause an excessive presence of calcium in the blood (for this reason the parathormone does not intervene in the stimulation), or to a reduced production of the same hormone (hypoparathyroidism).

If the value is higher than normal, instead, it indicates a hyperparathyroidism, which can be caused by:

• A low concentration of calcium in the blood (this hormone, therefore, intervenes to stimulate the activity of the osteoblasts, which constitute the bone structure);
• A tumor of the parathyroid, which stops working properly.