The thyroid gland (Latin name – Glandula thyroidea) is the basic hormone-forming organ that regulates important physiological processes in the human body. Thyroid hormones (and these are thyroid hormones TK and T4, as well as calcitonin) affect the growth and general development of the body, are involved in maintaining the functionality of almost all systems and organs, and regulate tissue metabolism. This is why it is so important to control the level of these hormones in the blood.
- Anatomy and physiology of the thyroid gland
- Function of the thyroid gland and its thyroid hormones
- What hormones does the thyroid gland produce?
- Thyroid hormone production
- What happens to hormones in the circulatory system
- Thyroid hormone tests
- Indications to Assignment Analysis
- Preparation for research
- The rate of thyroid hormones in adults and children by age
- Features of the release of thyroid hormones in pregnant women and the elderly
- Additional diagnostic methods
- What does the increased production of hormones indicate?
- Lowered hormone levels
- The dangers and consequences of a violation of the production of thyroid hormones
Anatomy and physiology of the thyroid gland
The thyroid gland (thyroid gland) by its structure is devoid of excretory ducts and therefore releases secretions directly into the blood or lymph. It is located on the anterior plane of the neck, at the level of the larynx. It resembles the letter H in shape, has two lateral lobes and an isthmus between them, plus one non-constant (pyramidal) lobe found in 30% of people.
Women normally have a larger organ than men. The thyroid gland is in contact with the esophagus and the carotid artery, as well as with the laryngeal nerves. The parathyroid glands (from 2 to are adjacent to it. All this makes thyroid surgery extremely difficult, threatening, in case of damage to vital organs, disability or death.
Function of the thyroid gland and its thyroid hormones
In addition to performing its main secretory function, the thyroid gland also acts as a storage site for iodine in the body. The release of hormones directly into the blood allows them to quickly approach the nucleus of any cell, where they interact with the regions of the chromosomes responsible for redox processes. The influence of the gland and its secrets on all basic systems is reflected in the ability:
- regulate the metabolic rate, create new cells from protein molecules, the synthesis of which they control,
- maintain a healthy and age-appropriate hormonal status of a person,
- control the formation of red blood cells, glucose and its level in the blood,
- control heat exchange, saturate tissues with O2, ensuring cell respiration,
- participate in the normalization of the digestive tract, the breakdown of fats, maintenance of weight,
- adjust the heart rate, vascular function, normalize blood circulation,
- preserve human reproductive function, ensuring the synthesis of sex hormones, the development of sexual characteristics,
- be responsible for vision, preservation of memory and intelligence in accordance with age,
- ensure the normal activity of the central nervous system,
- regulate the normal development and growth of the body as a whole, as well as the differentiation of bone and other tissues.
What hormones does the thyroid gland produce?
The algorithm for the production of iodine-containing hormones allows them to control the work of the autonomic nervous system and energy metabolism, which helps to maintain human health. The list of thyroid hormones is not so long, which cannot be said about the importance of each of them:
- T3 (triiodothyronine) is the main thyroid hormone. It is secreted initially in a biologically active form, therefore, entering the bloodstream, it immediately participates in metabolic processes. In the blood, T3 circulates in free or protein-bound form. The hormone level reaches a peak in autumn and winter and a minimum in summer. A deviation from the norm indicates that a failure has occurred in the functioning of the thyroid gland, a person needs a detailed examination.
- T4 (thyroxine) increases the basal metabolic rate with the release of heat calories. It warms our body, with the exception of the brain, spleen and testes in men. In the tissues of the body, it is transformed into a more active form – T3. The hormone has its own daily cycle: the maximum is at noon, and the minimum is at night. In summer, its level steadily decreases, in winter, on the contrary, it increases. Its production and activation occurs under the action of pituitary thyroid stimulating hormone (TSH).
- Calcitonin (a neuroendocrine hormone) is produced by special parafollicular C-cells of a neuroendocrine nature. These cells are born in the unborn baby in the womb. The hormone is responsible for the regulation of calcium in the blood and bones.
There are a number of other substances synthesized by the thyroid gland. By assessing their level, a complete organ screening is performed:
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- TG (thyroglobulin) is a protein involved in the synthesis of thyroid hormones and a marker of tumor growth.
- Thyroxine-binding globulin (TG) – transports hormones to tissues and organs.
- Thyroid peroxidase (TPO) is an enzyme responsible for the formation of the active form of iodine.
Thyroid hormone production
The thyroid gland is capable of producing iodinated thyronines by combining the amino acid tyrosine and iodine in follicular A cells filled with a special colloid – thyroglobulin. Regulates the secretion of the pituitary hormone TSH. As soon as the villi of the follicular cells turn towards the colloid and come into contact with it, and the command “start work” comes from the pituitary gland, the production of T3 and T4 starts. In this case, the synthesis undergoes 6 consecutive biochemical transformations:
- Organization (oxidation). Iodine molecules enter the human digestive system and are absorbed into the blood in the form of iodides, then stored in the thyroid cells. There they come into contact with a special protein pump (Na – I), which pumps them into the colloid. Here iodine is oxidized, in this form it binds and is retained in the gland.
- Iodization. It is carried out with the participation of TPO, which is responsible for the formation of a compound of iodine and the amino acid tyrosine in the thyroglubulin molecule. Iron helps in this.
- Condensation is the next action in hormone synthesis. Thyroid peroxidase in thyroglobulin changes the structure of tyrosine, forming mono- and polytyronines. Only 30% of them are thyroid hormones.
- Moving thyroglobulin into the colloid of the follicle. A huge iodinated molecule is stored in the colloid, but seeks to get rid of its contents, throwing it into the blood. Therefore, the next stage of biochemical transformations begins.
- Proteolysis (breakdown) of thyroglobulin. Intracellularly, the mechanism of movement of a huge thyroglobulin molecule to the basement membrane is triggered, where it is met by lysosomes and broken into its component parts. This is how molecules of pure T3 and T are obtained
- Diffusion. Thyroid hormones enter the bloodstream through the pores of the cell membrane. In this case, the remaining iodinated material (mono-, dithyronines and iodine) follows them and again begins to participate in the synthesis of T3 and T4. This process is ongoing.
Biosynthesis of calcitonin is carried out in thyroid C-cells, which have a large number of mitochondria – energy stations that synthesize protein. They are located next to A-cells, separated by circulating lymph. The same lymph washes the tonsils. That is why all drugs that treat tonsil inflammation negatively affect the thyroid gland.
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What happens to hormones in the circulatory system
Once in the bloodstream, iodinated hormones are transported to all tissues, vital organs. T4 is converted to T3 by the enzyme deiodinase. In the blood, more than 90% of hormones are inactivated by liver proteins (in reserve). In addition, sex hormones, glucocorticoids, can affect the synthesis of binding proteins. Thus, only about 0,5% of thyroid hormones control the body. All this is taken into account when taking tests if at least one symptom of thyroid dysfunction appears.
T4 has a less pronounced biochemical index, so about 80% of it is simply metabolized in the body, that is, it is transferred to the active T3 state. The result can be either a free T3, or a reverse, inactive one. It all depends on how the enzyme (selenium-dependent monodeiodinase), catalyzing the process, behaves. The norm is 50 to 50.
Thyroid hormone tests
As a rule, clinically revealed disorders of the thyroid gland require a comprehensive examination of the patient for an accurate diagnosis, control of the treatment of pathological changes in the gland, and postoperative observation after surgery.
T3 and T4 are almost never determined without TSH, TG, AT-TPO, Calcitonin, AT-TG and other substances closely related to the work of the gland. Therefore, a laboratory study of thyroid pathology is always a combination of tests, depending on the cause that caused the disease. Along with blood tests, an ultrasound of the thyroid gland is almost always required.
Indications to Assignment Analysis
The reason for checking the thyroid gland is the patient’s complaints or the doctor’s suspicions of organ dysfunction. Disruptions in the work of the main endocrine gland occupy the second line in the list of endocrine pathologies after diabetes mellitus. Therefore, the analysis for thyroid hormones is so important for doctors of various specialties: pediatricians, gastroenterologists, hematologists, endocrinologists, vascular surgeons, immunologists, psychiatrists. All diseases in which you have to donate blood for various thyroid hormones are associated with the following factors:
- disorders of hormone synthesis,
- incorrect function of thyroid hormones,
- pathological changes in the brain.
Problems of this nature arise if a deficiency or excess of iodine supplied with food, gastrointestinal tract pathology, genetically determined diseases, hereditary predisposition to functional malfunctions of the thyroid gland, thyroid hyperplasia are ascertained. Indications for examination are the following symptoms:
- An increase in the size of the thyroid gland, follicular tumor, heterogeneity of the tissues of the gland, a single node. In this case, the diagnosis involves the use of a research method such as histology.
- Lagging of a child or adolescent in physical and mental development from peers.
- Infertility, impaired reproductive function, irregular periods, early menopause, planning pregnancy, frequent miscarriages.
- Weight problems: overweight, unreasonably low.
- Violation of the heart rhythm, cardiopathy, inflammation of the endocrine genesis.
- Incorrect functioning of the nervous system, hand tremors, memory loss, poor test results for the likelihood of developing Alzheimer’s disease.
- Dermatological disorders: hair loss, baldness, poor skin tone, layering of the nail plates, acne, dysfunction of the glands of the dermis.
- Sexual disorders.
- Removal of the hormone-producing organ, replacement therapy.
Preparation for research
A blood test for thyroid hormones is taken in the morning on an empty stomach. At least a month before, you need to properly prepare for the study:
- Any drug, even antipyretics and sweeteners, can play a role in getting adequate results. If it is impossible to cancel therapy, then a preliminary consultation with a doctor is required.
- No later than a week before the analysis, you should exclude any alcohol from the diet, stop smoking.
- On the eve of testing, preparation should include a reduction in active physical activity.
- Food is prohibited 8 hours before sample collection, only water can be drunk.
The rate of thyroid hormones in adults and children by age
When decoding any results, it should be borne in mind that in different laboratories the indicators may differ from each other (this is influenced by the choice of reagents and equipment). Therefore, it is necessary to evaluate research data taking into account the reference values of a particular laboratory. The table of average norms will help you navigate the indicators of the content of thyroid hormones in the blood:
|Free triiodothyronine (FT3)||Newborns: |
|16–20 years old: |
|16–20 years old: |
|1-14 months: |
|20–50 years old: |
|20–50 years old: |
|1,2 years – 18 years: |
|Over 50: |
|Over 50: |
|Total triiodothyronine – a combination of free and bound (TT3)||1,19-3,10 pmol/l||1,19-3,10 pmol/l|
|Free thyroxine (FT4)||Newborns: |
|9,0-22,10 pmol/l||9,0-22,10 pmol/l|
|1-14 months: |
|Older than 60 years: |
|1,2 years – 5 years: |
|5–14 years old: |
|> 14 years old: |
|Total thyroxine (T4)||1–6 years old: |
5,5 -14,63 pmol/l
|71,2-142,3 pmol/l||60,7-137 pmol/l|
|5–10 years old: |
|Older than 60 years: |
|Under 18 years old: |
|Thyroid stimulating hormone (TSH)||Newborns: |
|0,39-4,01 mU/l||17–18 years old: |
|1-3 months: |
|Adult over 18 years old: |
|4 months-17 years: |
|Thyroglobulin (TG)||Less than 65 ng/ml||No more than 60 ng/ml|
|Thyroxine binding globulin (TCG)||220–497 nmol/l||200–500 nmol/l|
|Thyroid hormone absorption test||24-35%||22-34%|
|Antibodies to thyroglobulin (AT-TG)||Up to 18 units/ml||Up to 20 units/ml|
|Antibodies to thyroperoxidase |
|Up to 5,7 units/ml||Up to 5,7 units/ml|
|5,4–25,01 nmol/l||8,0 -28,0 nmol/l|
Features of the release of thyroid hormones in pregnant women and the elderly
During the period of gestation, the normal growth of the fetus, its physical development, and successful childbirth depend on the production of thyroid hormones. Toxicosis can also be associated with a change in the level of thyroid hormones. Specifically, thyroidin is also responsible for conception and comfortable delivery.
Even at the stage of family planning, the indicator of thyroid hormonal background is very important. Lack of hormones, as well as their overabundance, is a reason for examination or treatment.
The content of T3 and T4 should be checked first of all for women with a history of thyroid problems and a hereditary predisposition to hypothyroidism or hyperthyroidism.
In the later stages, a lower level of free T3 and an increased concentration of T4 are normally observed, and the level of thyroxine increases gradually from 1 to 3 trimester.
Pregnant women are also characterized by the effect of a slight increase in the gland itself in size, therefore it is important to differentiate pathology from the norm in time. Assessment of the condition of a pregnant girl and the appointment of treatment is carried out only after a comprehensive examination. Deficiency of T3 and T4 negatively affects the bearing of a baby, insufficient secretion threatens an early miscarriage (normally, in pregnant women, the thyroid gland produces 50% more hormones). With high risks of endocrine disorders, you should prepare for pregnancy and pass the necessary tests in advance.
Elderly people are also at risk, especially women during menopause. Pathology of the thyroid gland is associated with age-related changes in the tissues of the gland, a violation of its blood supply, medication, unfavorable ecology, and bad habits. After 65 years, a slight decrease in T3 is considered normal. Other changes in hormonal synthesis lead to negative symptoms and various diseases. Hormone therapy helps to increase or decrease the concentration of hormones to normal in old age.
Additional diagnostic methods
Usually, examinations of the thyroid gland begin with examination, palpation of the organ, and the appointment of standard blood tests. Complex cases of thyroid pathology require additional, non-standard examination:
- X-ray – reveals tissue calcification.
- Ultrasound – allows you to assess the size, volume, mass and degree of blood supply to the gland.
- A biopsy is used if cancer is suspected.
- The safety of the thyroid function is confirmed by the absorption of radioactive iodine by the organ, calculated by a special sensor.
- Radionuclide scanning allows you to clarify the contours of the gland, the size and location of the nodes.
- Laryngoscopy visually records the condition of the vocal cords and laryngeal nerves.
What does the increased production of hormones indicate?
High levels of thyroid hormones – hyperthyroidism. Normally, signs of thyroid hyperfunction are observed only during pregnancy (the fetus forms its own thyroid gland). Pathological hyperthyroidism can be primary, developing against the background of infections, radiation, postpartum dysfunction of the gland, autoimmune processes, and secondary – due to dysfunction of the pituitary gland (for example, due to a tumor). High calcitonin is a marker of medullary thyroid cancer.
Lowered hormone levels
Hypothyroidism as a result of low levels of thyroxine (T4) in the blood is diagnosed much more often. It can be congenital or develop as a result of:
- iodine deficiency
- hyperfunction of the pituitary gland,
- Down syndrome
- irradiation or removal of the gland,
- taking medications,
- autoimmune diseases,
- Turner syndrome (congenital infantilism).
The dangers and consequences of a violation of the production of thyroid hormones
A high concentration of thyroid hormones in the blood leads to intoxication and the development of nausea, vomiting, dyspepsia, and yellowness of the skin. In addition, weight fluctuations, irritability, sweating, deterioration of the condition of the skin, hair, nails, acyclic menstruation, arrhythmia, and hand tremors are observed.
As a result of high hormonal levels, the following diseases develop:
- an increase in the thyroid gland of non-inflammatory or neoplastic genesis,
- tumors or metastases capable of producing hormones,
- Graves’ disease (diffuse toxic goiter),
- thyroiditis of autoimmune genesis.
These pathologies can further increase the degree of thyrotoxicosis.
As a result of low levels of thyroidins, chronic fatigue syndrome develops, muscle weakness, shortness of breath, memory impairment, decreased libido, nervousness, hyperhidrosis of the palms, deterioration of the skin condition. Hypothyroidism leads to constant edema of all body tissues: the face becomes puffy, the voice changes due to laryngeal edema, and limbs swell. In addition, reproductive function is impaired.
Complications of low hormone levels can manifest themselves in the form of oligophrenia, cretinism, ischemic heart disease, arterial hypertension with the development of stroke or heart attack, hypothyroid coma, infertility.