Parathyroid hormone and its physiological role
Parathyroid hormone is a polypeptide consisting of 84 amino acid residues. Its secretion is strictly controlled by the calcium-sensing receptor located on the membranes of parathyroid gland cells. As soon as blood calcium ion concentration drops, the sensor responds, and hormone secretion increases.
The main task of parathyroid hormone is to increase calcium levels. This is achieved through three pathways:
- Bone tissue: the hormone stimulates osteoclasts, leading to the mobilization of calcium from bone reservoirs directly into the bloodstream.
- Kidneys: calcium reabsorption is enhanced (urinary excretion is reduced), while phosphate excretion, conversely, increases (antagonism with calcium).
- Intestines: calcium absorption is indirectly increased.
Additionally, parathyroid hormone works in tandem with vitamin D, increasing the level of calcitriol, which replicates the effects of retaining calcium in the body.
Parathyroid gland preparations
In medical practice, to compensate for parathyroid hypofunction (e.g., in spasmophilia or tetany), the preparation Parathyreoidinum is used. This is a natural agent obtained from cattle glands. It is administered parenterally — subcutaneously or intramuscularly.
An important pharmacokinetic feature of the preparation is its prolonged latent period. The effect develops only 4 hours after injection and lasts for about a day. For this reason, in acute tetany, treatment tactics require administering calcium preparations first for immediate symptom relief, followed by parathyroidin.
With long-term therapy, tolerance ( tachyphylaxis) may develop. As soon as hypocalcemia is corrected, the drug is immediately discontinued. Further maintenance therapy is based on vitamin D preparations and a special diet rich in calcium, but low in phosphorus.
Calcitonin: mechanism of calcium reduction
Calcitonin is a 32-amino acid polypeptide synthesized by thyroid follicular C-cells. Unlike parathyroid hormone, its secretion is triggered by hypercalcemia (excess plasma calcium).
The mechanism of action of calcitonin aims to lower calcium levels:
- In bones: it suppresses osteoclast activity. This reduces bone resorption (destruction) and halts decalcification.
- In the kidneys: it decreases the reabsorption of both calcium and phosphates in the proximal tubules.
- In the CNS: the hormone exerts a pronounced analgesic (pain-relieving) effect.
Clinical application of calcitonin
Pharmacology utilizes both natural calcitonin preparations and synthetic analogues. A classic preparation is calcitrin (Calcitrinum), which is obtained from porcine thyroid glands and administered intramuscularly or subcutaneously.
Main indications for prescribing:
- Prevention of osteoporosis progression.
- Paget's disease (osteitis deformans).
- Delayed fracture healing.
- Bone pain caused by osteolysis.
Administration may be accompanied by side effects: vascular reactions (increased blood pressure, «hot flashes») and dizziness. The preparations are strictly contraindicated in hypocalcemia and in children under 6 years of age due to a lack of clinical data.
Synthetic salmon calcitonin
Special attention should be given to Miacalcic — a synthetic analogue of salmon calcitonin. Its main advantage over mammalian hormones lies in its higher receptor affinity. This provides a more potent and prolonged therapeutic effect.
The preparation is available in a convenient metered nasal spray form (one bottle typically contains 14 doses of 200 IU). A daily dose of 200–400 IU can be administered at once. The effect after instillation occurs within an hour, and the half-life ranges from 16 to 43 minutes. The safety profile, indications, and contraindications are completely identical to those of regular calcitonin.
Briefly: answers to topical questions
What is the pharmacotherapy strategy during an acute tetany attack?
- Strategy in acute tetany — Due to the long latent period, calcium preparations are administered first, followed by parathyroidin.
To avoid confusing the effects: Parathormone — Raises calcium (and Lowers phosphates), while Calcitonin — Puts calcium into Bones (lowering it in the blood).
Class and exam questions
Why is it impossible to rely solely on parathyroidin administration during acute tetany?
The drug has a long latent period — its action begins only after 4 hours. Therefore, for rapid relief of the attack, calcium preparations are administered first.
What is the advantage of synthetic salmon calcitonin over mammalian gland preparations?
It has a higher affinity for human receptors, providing a stronger and more prolonged therapeutic effect. Furthermore, it is convenient to use (nasal spray).
How does parathyroid hormone affect blood phosphate levels?
It acts as a calcium antagonist regarding phosphates: it enhances their excretion through the kidneys, thereby decreasing their concentration in blood plasma.
What else to study on this topic
- The mechanism of action of the calcium-sensing receptor on parathyroid gland membranes.
- The interaction between parathyroid hormone and vitamin D (calcitriol).
- Dietary features during maintenance therapy for hypocalcemia.
- The analgesic effect of calcitonin on the central nervous system.
- Pharmacokinetics of nasal calcitonin formulations (using Miacalcic as an example).
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