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Iodine and human health

Iodine is a chemical element of group VII of the periodic system D.I. Mendeleev - was discovered in 1811 by the French chemist Courtois. This light halogen is so named because of its color in the gaseous state. Iodine is dispersed in all objects of the biosphere, litho- and atmosphere, natural waters and living organisms. It does not form independent deposits and is an exclusively mobile migrant. Its main reservoir is the World Ocean.

In nature, iodine is found in various compounds - organic and inorganic, a significant part of it is represented by iodides and iodates. The concentration of iodide in seawater is 50–60 µg/l, and in air - about 0.7 µg/m3. The iodine content in the soil has significant fluctuations (from 50 to 9000 μg/l), which is associated with the depth of soil freezing during the last ice age.

The main physicochemical properties of iodine are high chemical activity, volatility in its typical elemental state and the ability to exhibit variable valence.

Due to the expansion of human economic activity in the biosphere, the biogeochemical iodine cycle that has developed over centuries is changing. On the one hand, there is an increase in the industrial release of significant quantities of iodine from its reserves and its involvement in the biogeochemical cycle. On the other hand, due to the reduction of plant resources, there is an increase in the scale of iodine removal to places of accumulation - seas and oceans.

Iodine enters the human body with food products of plant (34%) and animal (60%) origin, and only a small proportion of it comes with water and air (3% each).

The concentration of iodine in local drinking water reflects the content of this trace element in the soil. In iodine-deficient regions, the concentration of iodine in the soil does not exceed 2 μg/l. The iodine content of foods varies significantly depending on the region, season, length of storage and cooking of foods. The highest concentration of iodine is in seafood (from 800 to 1000 mcg/kg), seaweed is especially rich in iodine. Fruits and vegetables contain little iodine.

The need for iodine depends on a person’s age and physiological state. The daily requirement for iodine ranges from 100 to 200 mcg and, calculated per 1 kg of body weight per day, decreases with age. If in premature infants it is 30 mcg/kg, then in full-term newborns it is 15 mcg/kg and decreases almost by half by the end of the first year of life.

For an adult, this requirement is 2 mcg/kg. In absolute quantities, for a newborn, an adequate level of iodine intake is at least 90 mcg per day, for children over 6 months - 110–130 mcg per day, during puberty, during pregnancy and lactation, up to 200 mcg per day is required.

Over a lifetime, a person consumes 3–5 grams of iodine, which is equivalent to the contents of about one teaspoon. The total iodine content in the body is 15–20 mg, with almost half of it contained in the thyroid gland.

Iodine enters the body in both inorganic and organic forms, is absorbed in the small intestine, its bioavailability reaches 100%. Already 2 hours after absorption, iodine is distributed in the intercellular space, accumulated by the thyroid gland, in the kidneys, stomach, mammary and salivary glands, and in breast milk in lactating women.

With normal intake of iodine into the body, its concentration in the blood plasma is 10–15 μg/l, while the total extracellular pool of iodine is about 250 μg. Most of the iodine (2/3) is excreted from the body by the kidneys, and the rest by the mammary, salivary and sweat glands.

The importance of iodine for humans is determined by the fact that this micronutrient is an obligatory structural component of the thyroid hormones - thyroxine (T4), containing 4 iodine atoms, and triiodothyronine (T3), which contains 3 iodine atoms. Consequently, its adequate intake into the body is necessary for the physiological synthesis and secretion of thyroid hormones.

The World Health Organization, the United Nations Children's Fund and the International Council on Iodine Deficiency Disorders recommend the following daily iodine intakes:

50 mcg - for children of the first year of life;
90 mcg - for preschool children;
120 mcg - for children 7–12 years old;
150 mcg - for children over 12 years of age and adults;
200 mcg - for pregnant and lactating women.

The role of thyroid hormones

The role of thyroid hormones in the human body throughout his life is extremely large and varied. The main functions of thyroid hormones are to maintain basal metabolism and regulate tissue respiration: they increase overall metabolism, oxygen consumption and heat generation in tissues.

Working mainly at the level of the cell nucleus, they can directly influence processes in the mitochondria and cell membrane, stimulating the formation of RNA and leading to stimulation of proteosynthesis, manifested by both growth and differentiation reactions.

According to Edelman (American immunologist and neurophysiologist, Nobel Prize laureate), thyroxine stimulates energy output (energy expenditure) during active transmembrane transport of sodium in the liver, kidneys and muscles. Stimulation of thyroid hormone receptors determines changes in gene expression with corresponding metabolic effects.

Without thyroid hormones or with their deficiency, the normal development of any functional system of the body is impossible. Thyroid hormones are involved in water-electrolyte and gas exchange, stimulate the absorption of oxygen and the release of carbon dioxide.

Thyroid hormones affect fat and carbohydrate metabolism: they increase the absorption of carbohydrates (glucose and galactose) in the intestines and their utilization in cells, stimulate the breakdown of glycogen, reducing its content in the liver, and reduce cholesterol levels in the blood.

The effects of thyroid hormones are especially significant in the prenatal and early postnatal periods. During pregnancy, under the control of the mother's thyroid hormones, embryogenesis processes take place, all organs and systems, primarily the nervous, cardiovascular and musculoskeletal systems, differentiate and mature.

The mother's thyroid hormones are the most important regulators of the formation and maturation of the unborn child's brain. They provide a complete anatomical and morphological development of the main components of the central nervous system in the first trimester of pregnancy.

It has been proven that the mother's thyroid-stimulating hormones actively form those parts of the brain that subsequently provide the child with the opportunity to develop normally intellectually. Therefore, a decrease in the functional activity of the thyroid gland in the first trimester (gestational hypothyroxinemia in the first trimester) significantly impairs the child’s intellectual development, and it is not possible to correct defects in brain development that arise at this stage.

With iodine deficiency, not only the child’s brain suffers, but also the formation of hearing, visual memory and speech. At this time, the most significant structures of the fetal brain are formed: cortex, corpus callosum, subcortical nuclei, striatum, subarachnoid tract, cochlea, eyes, facial skeleton, lung tissue.

The maturation of interneuronal connections, myelinogenesis, and myelination of nerve endings in the fetal period occur under the regulation of thyroid hormones of the fetus itself. The thyroid gland in the fetus is the first of the endocrine glands to exhibit functional activity after the 12th week of gestation.

In the second half of pregnancy, when the fetal thyroid gland begins to actively function, thyroid hormones carry out a very important process for brain development - the myelination of nerve fibers, which provides the possibility of associative connections, and in the future - abstract thinking.

After the birth of a child, the importance of thyroid hormones in the development of his brain and the formation of cognitive functions does not decrease. On the contrary, throughout the first year of life, thyroid hormones play an important role in the process of differentiation of neurons, growth of axons and dendrites, in the formation of synapses, gliogenesis, maturation of the hippocampus and cerebellum, and also during the three years of the postnatal stage of development they stimulate myelinogenesis and myelination of neurocyte processes .

One of the most important aspects of the physiological role of thyroid hormones is their effect on the immune system. Thyroid hormones are involved in maintaining the optimal level of nonspecific and specific factors that protect the body from infections.

The regulatory effect of triiodothyronine on the synthesis of nucleic acids in thymocytes and monocytes of peripheral blood has been proven.

Thyroid hormones accelerate the processes of transferrin synthesis and enhance the absorption of iron in the gastrointestinal tract, actively stimulating erythropoiesis. Disruption of metabolic processes in the bone marrow in thyroid pathology causes the development of thyroprival anemia and hypercoagulation processes.

A relationship has been discovered between the level of thyroid hormones and intraocular pressure, and its increase in hypothyroidism has been described.

A decrease in the level of thyroid hormones is to a certain extent associated with a change in total peripheral vascular resistance and the development of diastolic arterial hypertension.

During the reproductive period, thyroid hormones stimulate the differentiation of granulosa cells, inhibit the follicle-stimulating and increase the luteinizing function of the pituitary gland, increase the sensitivity of the ovaries to gonadotropic hormones, the endometrium to estrogens, and contribute to the formation of a two-phase menstrual cycle.

Thyroid-stimulating hormones, together with sex steroids, stimulate the completion of physical, sexual and mental differentiation during puberty. With the onset of pregnancy, thyroid hormones stimulate the development of the corpus luteum in the ovary.

It is more convenient to present the main functions of thyroid hormones in the human body in the table:
The main functions of thyroid hormones are normal

Process, system or organ	Action of thyroid hormones
BX	- increase in basal metabolism - increase in appetite - activation of calorie consumption
Carbohydrate, protein, lipid metabolism	- activation of glucose catabolism (release of energy) - stimulation of protein synthesis - activation of lipolysis - increased excretion of cholesterol with bile
Heart	- ensuring normal contractile function - increasing heart rate
Nervous system	- stimulation of normal neuronal development in the fetus and child - ensuring brain maturation and the formation of intelligence - ensuring adequate neuronal function in adults - enhancing the effects of the sympathetic nervous system
Musculoskeletal system	- regulation of maturation and formation of the skeleton, activity of the growth process - regulation of normal maturation and function of muscles
Reproductive system	- participation in the regulation of reproductive function and lactation

Thus, iodine is a vital chemical element for the human body. Its role is varied and significant throughout a person’s life, and especially during the period of intrauterine development and childhood.

Nagornaya N.V., Matsynina N.I. - Donetsk State Medical University. M. Gorky Head of Pediatric Service Maria Aleksandrovna Matsynina