physiology

Physiology of the Thyroid

Edited by Ivan Mercolini

The thyroid is an endocrine gland, located at the base of the neck, weighing about 20 g in an adult. Its characteristic shape is called H or papillon. It is generally mentioned by highly overweight subjects as defective, to justify their obesity. In reality, less than 3% of the obese are due to reasons related to thyroid dysfunction ...

The thyroid is responsible for the production of the hormones triiodothyronine (T3) and tetraiodothyronine or L-thyroxine (T4) (the number behind the molecule indicates the number of iodine atoms present), through vesicles called follicles. The thyroid always produces calcitonin via parafollicular cells. Calcitonin enters the mechanism of calcium homeostasis in antagonism to the parathyroid produced by the parathyroid glands. However, calcitonin is not the hormone that interests us, so I will not treat it. What we are interested in are instead the hormones T3 and T4 as we will see that they regulate the metabolism as the main activity: a correct level of these hormones is one of the keys to a constant and satisfying weight loss.

Iodine is a fundamental constituent of T3 and T4. It must be taken with the diet (about 150 mcg / day is its requirement), although a percentage derives from the thyroid itself (which in this sense acts as an iodine reservoir) and from the liver following the inactivation of T3 and T4. Excess iodine is excreted through the kidneys and a small part through the bile, therefore mainly with the urine and less with the faeces. Iodine enters the thyroid via active transport, a mechanism that requires ATP and works against gradient. Once inside the thyroid. Iodine is organized and undergoes some transformations up to the formation of monoiodothyrosine (MIT) and diiodotyrosine (DIT) which are the precursors of T3 and T4. I know it's a beard, but please follow me because these concepts are important not only to understand my lesson, but also for other purposes no less interesting. For example, you will find, if you look further, that diiodotyrosine is a substance used in competitive BB for the cutting phase (it was discovered to have action on the brown fat where the thermogenesis process is based, therefore it would have fat burning effects). Maybe in a future lesson I'll talk about it.

I will try, as far as possible to be summarized (??? And here you can begin to laugh ....).

Continuing ... a DIT + an MIT forms triiodothyronine (T3), while two DITs form T4. T3 is also obtained at the peripheral level (especially in the liver) from T4. About 80% of the circulating T3 is produced by thyroxine through this mechanism, while only 20% is produced by the thyroid, which therefore produces much more T4.

The metabolic effects of these two hormones are practically identical, what changes substantially is the speed of action. T3 acts quickly and more effectively (4 times more), while T4 is slower, but more durable (it uses the mechanism of the second messenger).

So far I have written T3 and T4, but it would be more correct to say FT3 and FT4, which are free, active molecules, not bound to plasma proteins. The body's technique of binding proteins allows the regulation of these hormones on an enzymatic key, as happens for example for testosterone. The normal blood values ​​of FT3 are in the range between 3 and 8 pmoli / L, while of T3 (bound fraction) are between 1.1 and 2.6 nmoles / L. The FT4 standard is between 10 and 25 pmoles per liter of blood, T4 is between 60 and 150 nmoles / L. In order not to forget me during the course of the discussion, I anticipate that the blood value considered normal of the pituitary hormone TSH (thyrotropin - we will soon see what it is) is between 0.15 and 3.5 mU / L.

Normal thyroid values
Thyroxine (T4) total (TT4)60 - 150 nmoles / L
Thyroxine (T4) free (fT4)10 - 25 pmoli / L
Total triiodothyronine (T3) (TT3)1.1 - 2.6 nmol / L
Triiodothyronine (T3) free (fT3)3.0 - 8.0 pmoli / L
Thyroid-stimulating hormone (or thyrotropin) (TSH)0.15 - 3.5 mU / L

NOTE: the normal ranges may vary from laboratory to laboratory; moreover, different units of measure are sometimes used (eg mcg / dl and ng / dl) and in this case the numerical values ​​are completely different from those listed. The reference values ​​may also vary according to age and during pregnancy.

It is interesting to consider that a peculiarity of the thyroid is to store its hormones, to the point that the effects of hypothyroidism will appear only months later.

I said that once released from the thyroid, these hormones are mostly related to plasma proteins such as albumin. Only a small part remains free and active (much less than 1%). These hormones in free form act on almost all body tissues and their main effect is to regulate metabolism and thermogenesis. A higher metabolism equals more calories consumed and more definition. Not only that, but high thyroid levels increase diuresis. Ergo: good levels of T3 and T4 reduce fluid retention, increase lipolysis, and increase metabolism and thermogenesis; we can say that they are the cutting hormones. It is on these effects that we will concentrate, but in the meantime I name other actions for cultural completeness. They are essential for the correct development of the nervous system in the child: their lack or insufficiency leads to a disease known as thyroid cretinism.

Thyroid hormones stimulate intestinal peristalsis, erythropoiesis. They then have a biphasic effect on the deposition of minerals on the bones: moderate levels of T3 / T4 improve calcification, while supernormal levels lead to decalcification. The same applies to protein synthesis: it increases with normal levels of thyroid hormones, while it decreases, with a prevalence of catabolism, in the case of high values ​​of T3 and T4 ..

The glucose metabolism is also influenced by thyroid hormones, which always work with insulin in a biphasic way: at normal levels, T3 and T4 improve the sensitivity of the cells to insulin, favoring glycogen synthesis, acting on the receptors; high levels of T3 and T4 instead lead to glycogenolysis with a consequent rise in blood sugar (this biphasic effect will bring cortisol back to mind ....).

Catecholamines and thyroid hormones, as we will see shortly, support each other: high levels of adrenaline increase TSH by increasing the production of iodized hormones. Similarly, high levels of T3 and T4 make the cells more sensitive to the action of catecholamines. The net result is an increase in heart rate, contraction, glycogenolysis and lipolysis, and metabolism in general. And with these words I answer to those who, in these days, asked themselves in the Forum the question whether the coffee is anorexant or rather if it is hyperglycaemic and leads to insulin resistance.

See, it depends on the conditions. Beta stimulants and anorexants in general are products that should be used during low-calorie diet regimes associated with physical activity. Under these conditions there are the benefits of increasing the catabolism of storage energy reserves. When instead they are used during a hyperalimentation, they accentuate a situation of hyperglycemia with the consequence, over time, of the development of insulin resistance up to type II diabetes.

And this should not surprise you. As high levels of adrenergic hormones and hyperalimentation, they are a paradox, an artifice improperly created by modern man. Evolutionary high adrenaline meant hunting, work, struggle, danger, food procurement. STOMACH EMPTY. Once recovered the prey ate to satiety and rested, with consequent relaxation of the adrenal gland. Stimulating the adrenal gland on a full stomach creates a paradox whose clear consequence is a condition of hyperglycemia that only serves to further stimulate insulin production. Therefore to those who ask you if the coffee is hyperglycaemic, answer that this is true only if you are not following a correct weight loss program, but you are using this alkaloid in a normocaloric or high calorie / glucose diet (as in general do the non-sportsmen, who drink coffee at the end of a meal for pure pleasure on the palate). Sorry if I opened this parenthesis, but the biphasic nature of hormones is an important concept and serves to make you understand how the human body does not work in black and white, but rather in "multi-color". Therefore there is no truth, but as many truths as there are conditions in which the question is posed. And with this I also launch a push to the various promoters of fashionable diets that divide food into GOOD and MALE in a very simplistic way, forgetting concepts such as glycemic load, individuality, condition, versatility and complication of the human body.

It is time we stopped a second to summarize the main points of the action of these iodized hormones.

  • What interests us most is their action in regulating energy metabolism: adequate levels allow to keep calorie consumption and thermogenesis high. Not only that: adequate levels significantly reduce water retention by increasing urine output. This is to remain in the two fundamental points for lovers of beauty and well-being. And again: at low / medium values, thyroid hormones facilitate nutrient entry into the cell, amplifying insulin sensitivity. At medium / high values ​​they activate glycolysis with an increase in blood sugar.
  • Thyroid hormones are important for a child's nervous development.
  • The or Thyroid optimizes the effects of catecholamine by increasing the sensitivity of cellular receptors.
  • The o.tiroidei serve for the function of intestinal peristalsis and therefore for a healthy digestive physiology.
  • T3 and T4 improve bone mineralization, but if their level is too high for too long (eg hyperthyroidism), the opposite effect occurs. The same applies to protein synthesis (muscular anabolism).