The parathyroid glands are very small, rounded masses located in the neck, just behind the thyroid gland. More precisely, these are four small glands partially embedded in the posterior aspect of the thyroid lobes. Usually, we therefore have four parathyroid glands, two higher and two lower. Hence the distinction into upper parathyroid and lower parathyroid glands. Normally, the parathyroid glands are about the size of a grain of rice and, in adults, each weighs roughly between 20 and 50 mg. We are therefore talking about a very small average weight, between one fiftieth and one twentieth of a gram. however, there is some individual variability regarding the number, location and size of the parathyroid glands. Before moving on to the next slide, I would like to point out that the thyroid and parathyroid glands are not connected. of the endocrine system and have similar names, the thyroid and parathyroid are independent structures with different functions.
Regarding the functions of the parathyroid glands, the main task of these glands is to regulate the level of calcium in the blood. In fact, the parathyroid glands produce a protein hormone - called parathyroid hormone or parathyroid hormone (PTH) - which participates in the metabolism of calcium. Together with the calcitonin secreted by the thyroid, and vitamin D, the parathyroid hormone then regulates the plasma concentration of calcium, known as calcemia. In practice, the parathyroid hormone has the task of keeping the calcium levels in the blood constant; in fact, even before entering the bone structure, calcium is essential for nerve conduction, for muscle contraction and for many enzymatic activities. Due to their importance in the metabolism of calcium, parathyroid hormone, calcitonin and vitamin D constitute the so-called calciumotropic hormones. They keep the concentration of calcium in the blood constant, acting mainly in the intestine, bone and kidney. The secretion of these calcium hormones is extremely sensitive to even the smallest changes in calcium.
Before going further, I want to say a word about calcium and explain why the regulation of its plasma levels is so critical for many physiological processes. First of all, in addition to being the major component of bones, calcium is very important for the proper functioning of the nervous system and the muscular system. Calcium also allows the normal transmission of electrical impulses along the nerves. Because of these important functions of calcium, the most common symptoms in parathyroid disorders are related to the malfunction of the nervous system and include depression and fatigue. In addition to nerves, muscles also use the changes in calcium levels within muscle fibers to contract. This helps explain why when calcium levels drop below normal people experience a feeling of weakness or muscle cramps. he last important function of calcium is related to the skeletal system. Bones, in a certain sense, also act as a reservoir of calcium, and this allows us to always have a good supply of calcium to keep calcium stable. they continuously carry out withdrawals and deposits of money, the level of calcium in the blood is the result of a balance between the withdrawal and deposit of calcium in the bone. If the calcium level drops, the mobilization of calcium from the bone tissue is stimulated and increases the amount absorbed by the digestive tract. Conversely, when plasma calcium levels are high, the mineral is deposited in the bones. mo of calcium regulation, a primary role is played by our parathyroid glands.
Despite their very small size, the parathyroid glands are particularly rich in blood vessels. This abundant vascularization makes them suitable for monitoring calcium at any time of the day. In fact, when the blood circulates inside them, the parathyroid glands capture the blood concentration of calcium and react by producing more or less parathyroid hormone depending on the needs. Let's now see in detail how this hormonal control takes place. Parathyroid hormone is secreted in response to the decrease in plasma calcium concentration, which in medical terms is referred to as hypocalcemia. Not surprisingly, the parathyroid hormone works by stimulating the release of calcium from the bones and decreasing the excretion of calcium through the kidneys; this effect is exactly opposite to that of calcitonin, which is the hormone produced by the parafollicular cells of the thyroid. In addition to reducing the urinary excretion of calcium and increasing its release from the bones, in the kidney the parathyroid hormone converts vitamin D into its active form, called calcitriol. Once activated, vitamin D is able to stimulate the absorption of dietary calcium in the intestine. Ultimately, all these biological actions carried out by the parathyroid hormone determine an increase in the plasma concentration of calcium.
As we have seen, the parathyroid hormone acts on the kidneys, bones and intestines, with the common aim of increasing, when necessary, the calcium level. The calcium level must in fact be included in a very narrow range, between 8.5 and 10.5 mg per 100 ml of serum. We have also seen that parathyroid activity is directly regulated by changes in blood calcium: an increase in blood calcium levels depresses parathyroid secretion, while a decrease increases it.
As we will explore in the next video, dysfunction of the parathyroid glands can lead to both excessive and insufficient secretion of parathyroid hormone, with serious consequences for the patient's health.
