The thyroid is a small, butterfly-shaped organ located at the base of the neck. It is very small, but it is very important, as it controls many key functions of our body.For example, body temperature and weight, appetite and mood, fatigue, laziness and sleep, heart rate, intestinal functioning and even calcium metabolism depend on it.
All these functions of the thyroid are mediated by hormones, real chemical messengers produced and secreted by the thyroid to act at a distance. On the one hand we have the hormones thyroxine and triiodothyronine, more simply called T3 and T4; they regulate the body's metabolism in general. On the other hand, there is a third, lesser known hormone called calcitonin, which keeps calcium levels in the blood in balance by working together with the parathyroid hormone.
In this video lesson we will learn more about the various functions of the thyroid gland. Understanding how the activity of this gland affects most of the body's cells leads to understanding why it is absolutely essential that thyroid hormones are produced in adequate quantities. But let's go in order and let's focus on the actions performed by thyroid hormones: thyroxine (T4) and triiodiothyronine (T3).
First of all, the thyroid regulates metabolism, that is the complex of chemical reactions that take place in every single cell of the organism. In practice, the thyroid hormones T3 and T4 signal to our body how fast it must work and how it must use food and chemicals to produce energy.
Thyroid hormones therefore contribute in a fundamental way to energy expenditure, directly regulating the notorious basal metabolism. This parameter is measured in kilocalories, or kilojoules, and indicates the body's energy expenditure in conditions of rest; the basal metabolic rate therefore reflects the minimum amount of energy needed to ensure basic vital functions, such as breathing, blood circulation and nervous system activities.
If thyroid hormones increase, metabolic activity in most tissues also increases and the basal metabolism rises. There is thus an increase in the consumption of oxygen and the speed of use of energy substances; consequently also increases the production of energy and heat, the so-called thermogenesis. All of this explains some of the classic symptoms of an overactive thyroid, such as increased sweating, heat intolerance and weight loss despite increased appetite. The consumption of extra calories also explains why some unconscious people resort to synthetic thyroid hormones, such as sodium levothyroxine, to lose weight, but paying dearly for this risky choice.
In addition to stimulating the use of energy, thyroid hormones also regulate energy reserves, stimulating their synthesis or degradation based on their levels. In this regard, we speak of a biphasic effect, to underline how the thyroid hormones act in a diametrically opposite way depending on their doses. In general, there is a predominantly anabolic (ie building) effect at low doses, while when we have an excess of thyroid hormones there is a "catabolic action (ie demolition of energy reserves).
As regards the metabolism of sugars, at normal concentrations the thyroid hormones favor the entry of glucose into the cells, enhancing the action of insulin. They therefore have a hypoglycemic and stimulating effect on the so-called glycogenosynthesis, that is, on the synthesis of glycogen which I remember being the complex reserve carbohydrate typical of animals (in plants and vegetable foods we have starch instead). The glycogen reserves, stored above all in the muscles and liver, are instead demolished in a process called glycogenolysis which will then be stimulated by an excess of thyroid hormones, with a consequent increase in blood sugar.
Also in lipid metabolism, thyroid hormones are involved with different effects depending on their dosage. In case of thyroid hyperactivity, an increase in lipolysis may occur, with reduction of lipid deposits and increased availability of fatty acids; conversely, a lack of thyroid hormones causes the opposite effect, namely lipogenesis, or the synthesis of adipose tissue. This is why people with a slow thyroid work tend to gain weight.
Finally, thyroid hormones stimulate protein synthesis; once again, however, if present in excess, they can cause the opposite effect, increasing the catabolism of proteins, which are then broken down into individual amino acids, often to the detriment of muscle mass. It is therefore not surprising that a hyperthyroid subject is a very thin subject, with reduced muscle mass and who gets tired easily.
The functions performed by the thyroid gland on body development are especially important as regards the nervous system. Going into more detail, thyroid hormones are needed in the fetus and in the first weeks of life, because they play a very important role in the differentiation and growth of nerve structures, as well as ensuring normal brain development. It is interesting to note how a deficiency of thyroid hormones in childhood can lead to a form of irreversible brain damage, called cretinism and characterized by incomplete development of the central nervous system and mental retardation.
Normal thyroid function is also important for the reproductive system. Thyroid hormones, in fact, influence the development and maturation of the testicles and ovaries, ensuring correct spermatogenesis and reproductive activity for men, and for the regularity of the menstrual cycle and the maintenance of pregnancy for women. A thyroid dysfunction can therefore cause consequences, such as infertility, sexual problems and menstrual disorders.
Thyroid hormones also have important effects on the cardiovascular system. In general, they favor myocardial contractility, increase heart rate and decrease vascular resistance, dilating peripheral arterioles. All this has the purpose of guaranteeing the tissues the supply of oxygen necessary to support the increased metabolism. To achieve this goal, the thyroid hormones can also determine an increase in pulmonary ventilation, which, to be efficient, requires a increase in cardiac output, ie the heart is induced to pump more, which also results in an increase in renal function.
Thyroid hormones also regulate normal intestinal peristalsis and are therefore important for a healthy digestive physiology. In the presence of hypothyroidism, problems of meteorism and constipation are common; conversely, in the case of hyperthyroidism, there is an increase in the frequency of bowel movements.
From what we have seen so far, we can say that thyroid hormones - rather than intervening in a single site of action - modulate multiple and coordinated activities, allowing the normal physiological functions of the entire organism to be maintained. Other specific biological effects vary from one tissue to another.
It is worth adding that thyroid hormones are essential for the action of growth hormone or GH and produce sensitive effects on the musculoskeletal system, promoting bone remodeling and increasing the capacity for muscle contraction. Finally, many of the stimulating effects on metabolism are amplified by catecholamines, such as adrenaline and noradranalin, which act in synergy with thyroid hormones.
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