Thyroid
The thyroid gland consists of two lobes connected by an isthmus and located on the neck on both sides of the trachea below the thyroid cartilage. It has a lobed structure. The glandular tissue consists of follicles filled with a colloid, in which there are iodine-containing hormones thyroxine (tetraiodothyronine) and triiodothyronine in a bound state with thyroglobulin protein. In the interfollicular space are located parafollicular cells that produce the hormone thyrocalcitonin. The content of thyroxine in the blood is greater than triiodothyronine. However, the activity of triiodothyronine is higher than thyroxine. These hormones are formed from the amino acid tyrosine by its iodination. Inactivation occurs in the liver through the formation of paired compounds with glucuronic acid.
Iodine-containing hormones perform the following functions in the body: 1) enhancing all types of metabolism (protein, lipid, carbohydrate), increasing basic metabolism and enhancing energy formation in the body; 2) the impact on growth processes, physical and mental development; 3) an increase in heart rate; 4) stimulation of the digestive tract: increased appetite, increased intestinal motility, increased secretion of digestive juices; 5) increase in body temperature due to increased heat production; 6) increased excitability of the sympathetic nervous system.
The secretion of thyroid hormones is regulated by the thyroid-stimulating hormone of the adenohypophysis, the thyroliberin of the hypothalamus, and the iodine content in the blood. With a lack of iodine in the blood, as well as iodine-containing hormones, the production of thyroliberin is enhanced by the positive feedback mechanism, which stimulates the synthesis of thyroid-stimulating hormone, which, in turn, leads to an increase in the production of thyroid hormones. With an excessive amount of iodine in the blood and thyroid hormones, a negative feedback mechanism works. Excitation of the sympathetic division of the autonomic nervous system stimulates the hormone-forming function of the thyroid gland, excitation of the parasympathetic division inhibits it.
Thyroid dysfunction is manifested by its hypofunction and hyperfunction. If insufficiency of the function develops in childhood, then this leads to growth retardation, violation of the proportions of the body, sexual and mental development. This pathological condition is called cretinism. In adults, hypothyroidism leads to the development of a pathological condition – myxedema. In this disease, inhibition of neuropsychic activity is observed, which manifests itself in lethargy, drowsiness, apathy, decreased intelligence, decreased excitability of the sympathetic part of the autonomic nervous system, impaired sexual function, inhibition of all types of metabolism and decreased basal metabolism. Such patients have increased body weight for due to an increase in the amount of tissue fluid and puffiness of the face is noted.
Hence the name of this disease: myxedema – mucous edema
Hypofunction of the thyroid gland can develop in people living in areas where iodine deficiency is noted in water and soil. This is the so-called endemic goiter. The thyroid gland in this disease is enlarged (goiter), the number of follicles increases, however, due to the lack of iodine, hormones o6 are formed little, which leads to corresponding disorders in the body, manifested in the form of hypothyroidism.
With hyperthyroidism, the disease develops thyrotoxicosis (diffuse toxic goiter, Bazedov’s disease, Graves disease). The characteristic signs of this disease are an increase in the thyroid gland (goiter) exophthalmos, tachycardia, increased metabolism, especially the main one, weight loss, increased appetite, impaired heat balance of the body, increased irritability and irritability.
Calcitonin, or thyrocalcitonin, together with parathyroid hormone is involved in the regulation of calcium metabolism. Under its influence, the level of calcium in the blood decreases (hypocalcemia). This occurs as a result of the action of the hormone on bone tissue,
where it activates the function of osteoblasts and enhances the mineralization processes. The function of osteoclasts that destroy bone tissue, in contrast, is inhibited. In the kidneys and intestines, calcitonin inhibits calcium reabsorption and enhances phosphate reabsorption. The production of thyrocalcitonin is regulated by the level of calcium in the blood plasma according to the type of feedback. With a decrease in calcium, the production of thyrocalcitonin is inhibited, and vice versa.
Parathyroid (parathyroid) glands
A person has 2 pairs of parathyroid glands located on the back surface or submerged inside the thyroid gland. The main, or oxyphilic, cells of these glands produce parathyroid hormone, or parathyrin, or parathyroid hormone (PTH). Parathyroid hormone regulates the metabolism of calcium in the body and maintains its level in the blood. In bone tissue, parathyroid hormone enhances the function of osteoclasts, which leads to bone demineralization and an increase in calcium in the blood plasma (hypercalcemia). In the kidneys, parathyroid hormone enhances calcium reabsorption. In the intestine, an increase in calcium reabsorption occurs due to the stimulating effect of parathyroid hormone on the synthesis of calcitriol, an active metabolite of vitamin D3. Vitamin D3 is formed in an inactive state in the skin under the influence of ultraviolet radiation. Under the influence of parathyroid hormone, it is activated in the liver and kidneys. Calcitriol increases the formation of calcium-binding protein in the intestinal wall,
which contributes to the reverse absorption of calcium. Influencing calcium metabolism, parathyroid hormone simultaneously affects phosphorus metabolism in the body: it inhibits the reverse absorption of phosphates and enhances their excretion in the urine (phosphaturia).
The activity of the parathyroid glands is determined by the calcium content in the blood plasma. If the concentration of calcium in the blood increases, then this leads to a decrease in the secretion of parathyroid hormone. A decrease in the level of calcium in the blood causes an increase in the production of parathyroid hormone.
Removal of the parathyroid glands in animals or their hypofunction in humans leads to an increase in neuromuscular excitability, which is manifested by fibrillar twitching of single muscles, which turn into spastic contractions of muscle groups, mainly limbs, face and neck. The animal dies from tetanic convulsions.
Hyperfunction of the parathyroid glands leads to demineralization of bone tissue and the development of osteoporosis. Hypercalcemia increases the tendency to stone formation in the kidneys, contributes to the development of disturbances in the electrical activity of the heart, the appearance of ulcers in the gastrointestinal tract as a result of increased amounts of gastrin and Hcl in the stomach, the formation of which is stimulated by calcium ions.