Thyroid hormones are produced in the thyroid gland (thyroid gland). Thyroxine and triiodothyronine are the two main hormones of this organ, in which there is an iodine atom. These hormones are produced by the cells of the follicular epithelium.
The role of thyroid hormones
Thyroxine and triiodothyronine are synthesized on the basis of tyrosine (alpha amino acid) and are of great importance for the normal development of the whole organism.
- Support the work of the respiratory center.
- With their participation, the formation of heat by the body is controlled.
- Affect oxygen metabolism and increase tissue oxygen demand.
- Affect the strength of heart contractions (ionotropic effect).
- Regulate heart rate (chronotropic effect).
- Due to thyroid hormones, the number of b-adrenergic receptors in skeletal and cardiac muscles, as well as those located in adipose tissue and lymphocytes, increases.
- Activate the motility of the entire gastrointestinal tract.
- Under their influence, the synthesis of various structural proteins and tissue differentiation takes place.
- Stimulate the development and growth of the whole organism.
- Stimulate the central nervous system and accelerate mental associations.
- They regulate glucose in the blood and increase its uptake and utilization by cells, thereby activating the glycolysis process.
- Affect the breakdown of adipose tissue (lipolysis) and delay its formation and deposition.
Violation of the secretion of thyroid hormones in the human body can lead to a delay in mental and physical development.
Synthesis of thyroid hormones
The main protein of thyroid is thyroglobulin. It serves as a matrix for the formation of hormones produced by the gland. Thyroxine and triiodothyronine are synthesized based on thyroglobulin protein. This protein contains in its composition more than 5 thousand amino acid residues and only 18 of them are iodized. Thyroxine is synthesized with the participation of only 2 to 4 amino acid residues. In addition to thyroglobulin, cholesterol also takes part in the production of thyroid hormones. Thus, thyroxine and triiodothyronine are synthesized on the basis of cholesterol, as are steroid hormones. The thyroid hormone T4 produces 10 times more than T3.
Triiodothyronine hormone (T3) is formed by combining the molecules of di- and monoiodotyrosine, which are part of the thyroglobulin protein.
How do thyroid hormones get to tissues?
SCH hormones are bound by blood plasma proteins and in this form are delivered to tissues and organs. There are three main blood proteins that can bind the hormones T3 and T4:
- TSH is a thyroxin-binding globulin.
- TSAA is a thyroxin-binding prealbumin.
- Albumen.
The level of thyroxine and triiodothyronine is influenced by the secretory activity of the thyroid gland and the binding ability of blood serum.
The synthesis of T3 and T4 is controlled by the pituitary hormone TSH (thyroid-stimulating hormone). In turn, the synthesis of TSH is carried out under the influence of the hormone of the hypothalamus TRH (thyrotropin-releasing hormone).
Triiodothyronine Free (fT3)
The percentage of fT3 is only 0.25% of the total T3 in the blood. As we have already found out, T3 in the blood is much less than T4, but, despite this, its free form is only half the free form of T4.
It is the free form of thyroid hormones that determines their biological activity. Triiodothyronine (hormone) is much more active than thyroxine. That is why its free level is characterized by the general metabolic effect of thyroid hormones.
Thyroxine: functions
T4 is biologically inactive. But if necessary, it can be converted into more active T3 under the influence of the enzyme selenium-dependent monodeiodinase.
The hormone thyroxine functions in the body perform as important as triiodothyronine. Namely, T4 is responsible for the production of vitamin A in the liver, stimulates protein metabolism, affects lipid (fat) metabolism, regulates the level of triglycerides and bad cholesterol in the blood, affects the proper formation of bone tissue, which is especially important in childhood.
How to pass a test for thyroid hormones ?
In order to study the level of thyroid hormones, it is enough to take blood from a vein for analysis. This can be done in the treatment room.
To ensure that the study is most reliable and external factors do not affect its results, one month before blood donation, all hormonal drugs must be excluded from the analysis. Of course, this should be agreed with your doctor. 3-5 days before delivery, it is recommended that medicines containing iodine be excluded.
Immediately before blood sampling, the patient should not undergo any x-ray studies. On the eve of blood donation, high physical exertion and stressful situations (sports competitions, exams, etc.) are not recommended. Before entering the office and passing the analysis, the patient is recommended to sit and relax for 15-30 minutes.
Normal values of thyroid hormones
We found out what role thyroxin and triiodothyronine play in the body, are synthesized on the basis of which components, which substances are responsible for their binding in the blood serum, and how to correctly pass the analysis for these hormones. Now we will consider their normal values and under what diseases they can increase or decrease. The norms of these hormones are shown in the table below:
| Hormone | Minimum and maximum values | Units |
| TTG | 0.4-4.0 | mIU / ml |
| T3 | 1.2-2.2 | nmol / l |
| fT3 | 2.63-5.7 | pmol / l |
| T4 | 60-156 | nmol / l |
| fT4 | 9.0-25.0 | pmol / l |
| AT-TG (antibodies to thyroglobulin) | 0-18,0 | U / ml |
| TG (thyroglobulin) | 1.6-5.9 | ng / ml |
| Thyroid Peroxidase Antibodies | <5.6 | U / ml |
What diseases increase fT3?
Triiodothyronine free may increase with the following diseases:
- Hyperthyroidism is primary or secondary.
- T3 toxicosis isolated.
- Thyroiditis.
- Hypothyroidism T4-resistant.
- Choriocarcinoma.
- Syndrome of resistance to thyroid hormones.
- Decrease in concentration of thyroxin-binding globulin.
- Chronic liver disease.
- With hemodialysis.
- With nephrotic syndrome.
- After therapy with drugs of radioactive iodine.
When does fT3 decrease?
A decrease in fT3 in the blood can occur in the following situations:
- Hypothyroidism is primary, secondary or tertiary.
- Severe non-thyroid pathology, including mental and somatic diseases (stroke, heart attack, etc.).
- Prolonged fasting or a diet low in protein.
- During the recovery period after severe pathologies and operations.
- In primary uncompensated adrenal insufficiency.
- In women with regular heavy physical exertion.
- In the third trimester of pregnancy (the level of fT3 during pregnancy steadily decreases starting from the first trimester and by the end of the third becomes the most pronounced).
- When taking the following drugs: amiodarone, androgens, propranolol, salicylates, radiopaque iodine-containing drugs.
In addition, seasonal variations in fT3 are possible. The maximum falls on the time period from September to February, and the minimum on the summer months.
Symptoms of Hypothyroidism
Hypothyroidism is an insufficient synthesis of thyroid hormones. In this case, the following symptoms may occur:
- Lethargy, fatigue and drowsiness.
- The appearance of excess weight, which can not be regulated due to physical activity and diets.
- Constant depression.
- A decrease in body temperature to 35.6 degrees can be observed.
- Itching of the skin, their dryness and swelling.
- Hair loss and dandruff, which does not go away even after using therapeutic shampoos.
- Decreased heart rate (bradycardia).
- Low blood pressure.
- Decreased memory and response.
- Regular constipation.
- In women, it can provoke a violation of the menstrual cycle and infertility.
Symptoms of Hyperthyroidism
Hyperthyroidism is an excess of the synthesis of thyroid hormones. In this case, the following symptoms are observed:
- Increased appetite and weight loss.
- General weakness, against which a flash of excitement can be observed.
- Laxity and dryness of the skin.
- Heart palpitations and high blood pressure.
- Increase in body temperature to 37.5 degrees.
- Violation of the menstrual cycle in women and infertility.
- In severe cases, a noticeable increase in thyroid gland and buccal eye.
Prevention of thyroid disease
The thyroid gland and its hormones perform the most important functions for the body, therefore it is necessary to monitor the state of this organ. This is done at annual preventive examinations. If necessary, the doctor will give a referral for ultrasound and thyroid hormone tests.
To prevent thyroid diseases and lack of hormones of this body, it is necessary to include products with a high iodine content in your diet. They include almost all seafood (tuna, flounder, salmon, shrimp, seaweed), fruits (persimmons, bananas, oranges), vegetables (onions, garlic, sorrel, eggplant). By following simple recommendations, you can permanently preserve the health of the thyroid gland and the entire body as a whole.