The Thyroid Hormones

What are

Thyroid hormones (T3 and T4) are secreted by the thyroid in response to the pituitary hormone TSH, whose production is in turn regulated by the hypothalamic hormone TRH.

TSH, or thyrotropin, is a glycoprotein produced by the thyrotropic cells of the pituitary. It has a pulsatile secretion, with periods of two hours, which increases in the period before sleep.

The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced by the follicular cells of the thyroid in response to TSH.

The secretion of thyroid hormones follows a circadian rhythm; the highest levels of T3 and T4 are reached during the night and early morning, while the lowest levels are found between 12 and 21.

Functions

Thyroid hormones:

they regulate the brain development of the fetus and infant
they are necessary for the development of the fetal skeleton
they are essential for the normal body growth of the child and for the maturation of the various systems, especially the skeletal one
they regulate the metabolic activity of the adult by influencing the function of each organ and tissue, in particular:
they have thermogenetic action
regulate glucose metabolism
they are involved in lipolysis and lipogenesis
regulate protein synthesis
have effects on the cardiovascular system

THERMOGENETIC ACTION: thyroid hormones increase the consumption of oxygen at rest, raising the basal metabolism, body temperature and daily caloric needs.

EFFECTS ON GLUCIDIC METABOLISM: they favor glycogenolysis and gluconeogenesis, and also increase the activity of the enzymes involved in the oxidation of glucose.

LIPOLYSIS AND LIPOGENESIS: thyroid hormones stimulate both lipolysis (use of fat for energy purposes) and lipogenesis (synthesis of adipose tissue), with a prevailing effect on lipolysis

PROTEIN SYNTHESIS: thyroid hormones increase protein synthesis and therefore have a trophic effect on the muscle; however if synthesized in excess they cause the opposite effect (muscle catabolism)

EFFECTS ON THE CARDIOVASCULAR SYSTEM: thyroid hormones increase myocardial contractility (positive inotropic effect), increase heart rate (positive chronotropic effect) and increase venous return to the heart; they are therefore essential for cardiac function

OTHER METABOLIC EFFECTS: thyroid hormones increase intestinal motility, favor the absorption of cyanocobballamine (vitamin B12) and iron; increase the synthesis of erythropoietin, renal flow and glomerular filtration; stimulate the endogenous production of other hormones (GH ); they have a permissive role on reproductive functions and regulate the trophism of skin and appendages.

Thyroid Hormones - Video: T3, T4 and TSH

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Thyroid Hormones and Exercise

Thyroid function is not greatly influenced by physical activity, but is greatly affected by the nutritional status. In particular, during fasting, the sensitivity of the tissues to thyroid hormones decreases, a sort of defense mechanism to lower metabolism and avoid waste of precious energies for the very survival of the organism.

Synthesis

Role of iodine

For the synthesis of thyroid hormones we need:

tyrosine, made available by thyroglobulin
the iodine which is transported against the gradient inside the thyrocyte thanks to the action of the NIS
a catalyst enzyme (thyroperoxidase)

The active form of thyroid hormones is represented by T3, while T4 is about 15 times less active; at the peripheral level there are some enzymes, called deiodinase, capable of removing an iodine atom from thyroxine (T4) transforming it into triiodothyronine (T3).

It is very important to introduce the right quantities of iodine with the diet to ensure optimal functioning of the thyroid gland; the lack of this mineral can in fact lead to the development of pathologies such as endemic goiter.

The intake of iodine in the diet is generally low; it is mainly contained in fish, while its presence in vegetables depends on the growing medium. In any case, to provide the body with an adequate supply of this microelement, it is sufficient to use iodized salt instead of traditional cooking salt.

To promote proper thyroid function, iodized salt must be used raw; it is useless to add it to pasta during cooking as high temperatures destroy this very important mineral.

Doping and Thyroid Hormones

As we have seen, one of the main functions of thyroid hormones is to regulate basal metabolism and body temperature; the higher the level of these hormones, the more calories are burned.

The use of thyroid hormones in sport has the purpose of increasing the body's metabolism, consequently reducing excess fat reserves. Many slimming drugs are also based on thyroid hormone precursors.

Obviously, such an approach is not without side effects: introducing thyroid hormones or their precursors from the outside inhibits the stimulus for endogenous production; when the drug is stopped, there will also be quite serious problems, such as hypothyroidism, thyroid dysfunctions. o chronic hypothyroidism (in case of prolonged administration).

Undesirable effects such as tachycardia, heavy sweating, excessive weight loss, nervousness, diarrhea, bone demineralization and heart problems are common during the intake cycle.