Hormone T4 (thyroxine) - the norm, what it is responsible for, what functions

What is the hormone T4 (thyroxine)?

T4 – thyroid hormone

produced by cells of thyroid follicles. Thyrocytes synthesize thyroglobulin from amino acids and iodine, which is a precursor to thyroxine. Thyroglobulin accumulates in the follicles, and, if necessary, thyroxine is formed from it by dividing into fragments.

The main action of the hormone T4

is to accelerate catabolism - the process of obtaining energy from energetically significant metabolites (glycogen, fat). Excessive concentration of thyroxine in the blood leads to rapid heartbeat, irritability, and weight loss. But this does not mean that the hormone is harmful, these are only symptoms of its overdose. Normally, tetraiodothyronine maintains the tone of the nervous system, heart rate and adequate metabolism.

T4 hormone is not the most active thyroid hormone,

for comparison, its activity is almost ten times lower than triiodothyronine. The latter is also called the T3 hormone, since its formula contains 3 iodine atoms. T3 can be formed in the cells of the gland itself, as well as in the cells of the body from its predecessor, thyroxine. Essentially, it is a more active metabolite of T4.

Hormones T3, T4 are also called thyroid hormones

, since they are secreted by the thyroid gland, called “thyroid” in Latin. TSH is also sometimes referred to as thyroid hormone, but this is erroneous, because it is produced by the pituitary gland, located in the brain, and controls the hormone-forming function of the gland.

Often, together with a blood test for the hormones TSH, T3, T4, antibodies to TPO and thyroglobulin are simultaneously determined. Typically, endocrinologists use these indicators to diagnose thyroid pathologies. Sometimes thyroid hormones are studied during the treatment of the disease to determine its dynamics and the effectiveness of the prescribed therapy. This article will provide information about what the T4 hormone is, what functions it performs in the body, and how the analysis of its content is interpreted.

The T4 hormone belongs to the group of iodine-containing thyroid hormones. Its chemical formula contains two residues of the amino acid tyrosine and four halogen atoms of iodine. Synonyms for the T4 hormone are tetraiodothyronine and thyroxine. The substance received its name due to the number of iodine atoms contained in the molecule. Due to its simple structure, the concentration of tetraiodothyronine can be easily determined in the laboratory. For the same reason, the hormone can be synthesized artificially, which is used in hormonal therapy.

Preparing for analysis

• It is better to take blood from a vein in the morning (before 12.00) and only on an empty stomach (the last meal was 8 or more hours ago). You can drink regular water without gas.
• Smoking and/or using nicotine substitutes is prohibited 2-3 hours before the procedure.
• The day before the test, you must adhere to a diet: exclude fatty, fried, smoked and fast food from your diet. It is strictly forbidden to consume energy and tonic drinks and alcohol.
• Intense physical activity, sports and anxiety can also affect the concentration of thyroxine (increasingly). Therefore, during the day (at least) before the test, you need to protect yourself from any stress and physical strain.
• 48 hours before the test, it is advisable to discontinue steroid and thyroid hormones (in consultation with the endocrinologist). You may need to stop taking other medications.

Sources:

• Rui-ting Ma. Influence Factors on Serum Thyroid Hormone Level in Healthy People, - data from the US National Library of Medicine PubMed, 2015.
• Data from the independent laboratory Invitro, 2021.
• Data from Helix Laboratory, 2021.
• Fran Lowry. High Thyroid-Hormone Levels Linked to Increased CVD Mortality, - English-language database of current medical data Medscape, 2021.
• Muhammad Bader Hammami, MD. Thyroxine, - English-language database of current medical data Medscape, 2021.

Hormone T4 in the blood

In the bloodstream, the T4 hormone is mainly in a protein-bound state. When thyroxine is formed in the thyroid follicles, it is captured by a special protein - thyroxine-binding globulin (TBG). This substance performs a transport function, delivering the hormone to the cells of the body. That small amount of thyroxine not bound by protein is called free T4. It is this fraction that is responsible for providing the biological effect. The part of the hormone that is bound to TSH is called T4 bound. If you separately determine free T4 and bound T4 in the blood, and then add these values, you get total T4.

In the body, the main effect is exerted by free thyroid hormones (free T4, free T3), therefore their content is of greatest importance for determining thyroid pathologies. In laboratories, blood tests are most often performed for thyrotropin and free T4. The TSH indicator is used to assess the regulation of the thyroid gland by the pituitary gland, and free T4, as the main hormone of the gland, directly reflects its function. The concentration of free T4 is increased in hyperthyroid conditions or as a result of an overdose of hormonal drugs used to treat hypothyroidism.

Increase in free T4

• diffuse goiter (increase in the size of the thyroid gland);
• thyroid dysfunction after childbirth;
• therapy with synthetic hormone T4, incl. uncontrolled (self-medication);
• thyrotoxic adenoma or myeloma (thyroid tumors);
• hyperthyroidism (hyperfunction of the gland);
• TSH-independent thyrotoxicosis;
• nephrotic syndrome (kidney damage, generalized edema throughout the body);
• obesity;
• liver disease (chronic form);
• treatment with heparin, amiodarone, furosemide, danazol, valproic acid, tamoxifen.

According to a study that involved about 9,500 people over 65 years of age, high levels of free thyroxine at this age are directly associated with atherosclerotic cardiovascular disease.

Conditions associated with the balance of TSH, free T3 and T4:

T4 hormone test

In the clinical practice of endocrinologists, analysis of the hormones TSH and T4 is the most commonly used diagnostic method. Hormonal tests are prescribed in various combinations, taking into account the symptoms and economic capabilities of the patient.

At the initial visit to the endocrinologist, if the patient does not have severe symptoms, it is often enough to take a test for the hormones TSH, T4, T3. As for the last two hormones, it is better to study their active, i.e., free fractions. In cases where the patient receives thyreostatics for the treatment of the early stage of Graves' disease (diffuse toxic goiter), it is better to determine only the free hormones T3 and T4. Under the influence of thyreostatic drugs, a rapid decrease in these indicators occurs, while the TSH level seems to be delayed and does not have time to decrease.

If a patient is being treated for a long time for insufficient gland function, then for periodic monitoring of the quality of therapy it is enough to determine only the concentration of thyroid-stimulating hormone (TSH). Free T4 is tested only if there are specific indications. You should know that if you are taking thyroxine, you can take a blood test for the T4 hormone only before taking it

. If this rule is not followed, the result of the analysis will be uninformative, since the amount of thyroxine that came with the medication taken will be added to the T4 hormone secreted by the thyroid gland.

During pregnancy, the value of free T4 hormone increases especially

, because the level of thyrotropin may be reduced as a result of the action of hCG - human chorionic gonadotropin produced by the placenta. Therefore, when examining pregnant women, determining only TSH is insufficient for correct diagnosis. It is necessary to be tested simultaneously for TSH and T4.

In the direction or form with the research result you can find various abbreviations:

• FT4, FT3 – T4 and T3 free (English free, which means “free”);
• Hormone St. T4, hormone St. T3 is also a free form of hormone.

Indications

According to the standards of the Russian healthcare system, a thyroxine test is a diagnostic test and is recommended for testing once a year during a routine medical examination. The analysis helps to identify early disorders of the thyroid gland. The study is also prescribed in the following cases:

• thyrotropin (TSH) level is higher or lower than normal;
• suspicion of hypo- or hyperthyroidism;
• control of drug treatment of hypo- or hyperthyroidism;
• thyroid diseases;
• female infertility;
• pregnancy in women with various thyroid disorders;
• control of hormonal levels in newborns whose mothers had endocrine disorders.

What is the normal level of the T4 hormone?

Free hormone T4.

To correctly evaluate the results of an analysis for free T4 hormone, it is not enough to know any specific standards. Normal thyroxine levels largely depend on the laboratory performing the analysis. These indicators are different for different analyzers; even the set of reagents used in each case matters. As a rule, the permissible concentration of T4 in the blood is indicated on the form after the test result. When using high-quality laboratory equipment of the 3rd generation in healthy people, the concentration of thyroxine ranges from about 9 to 20 pmol/l.

General hormone T4.

An indicator such as the total hormone T4 depends on the physiological state of the body. For example, it increases in pregnant women. Therefore, the normal limits for total thyroxine are more variable than for its free fraction.

Total T4 hormone (tetraiodothyronine)

T4 hormone (tetraiodothyronine) free

! Important:

How can thyroid disease be recognized by hormone analysis?

Examination rules

For analysis, venous blood is taken from the patient using a venipuncture procedure. To prevent bleeding and hematoma formation, according to the standard, a cotton ball is placed at the puncture site. After the procedure, it is advisable not to load your hand. The blood is placed in a tube with serum gel and sent for testing.

During the examination, the patient must adhere to the following rules:

• 3 hours before the test, avoid smoking;
• do not drink alcohol during the day, dinner the day before should be light without fatty and spicy foods, and spices;
• 24 hours beforehand, you need to get ready for the examination, cancel training, and eliminate stressful situations, since the hormone reacts to a person’s emotional state.

T4 hormone is increased

The following symptoms are characteristic of excess levels of the T4 hormone:

• Increased sweating
• Fatigue,
• Irritability,
• Increased heart rate and palpitations,
• Arrhythmia (extrasystoles, atrial fibrillation),
• Noticeable weight loss
• Trembling in the arms and legs (tremor).

Increased T4 hormone accelerates the breakdown of fats in the body, therefore body weight decreases. Excessive amounts of released energy have a negative impact on organ function. This manifests itself in acceleration and strengthening of the heart, and increased sweating. Excessive stimulation of the nervous system causes irritability and frequent mood swings, and acceleration of neuromuscular transmission leads to tremors of the limbs. Losing weight in this condition is not physiological, because it occurs against the background of dysfunction of internal organs. If an elevated concentration of thyroxine is maintained for a long time, calcium is washed out of the bones, which is fraught with an increased risk of fractures and osteoporosis.

Reasons for increased T4 hormone (total and free):

• Multiple myeloma with high levels of immunoglobulin G;
• Excess body weight (obesity);
• Glomerulonephritis with nephrotic syndrome;
• HIV infection;
• Postpartum thyroid dysfunction;
• Acute and subacute thyroiditis;
• Choriocarcinoma;
• Diffuse toxic goiter;
• Chronic liver diseases;
• Taking synthetic analogues of thyroid hormones, cordarone, methadone, oral contraceptives, radiopaque iodine-containing substances, prostaglandins, tamoxifen, insulin, levodopa;
• Porphyria

Symptoms of impaired T4 secretion

If the thyroid gland does not produce enough thyroxine or the thyroid-stimulating hormone that regulates it, a person develops hypothyroidism. This disease is accompanied by clinical signs:

• dry skin and mucous membranes;
• excess body weight gain;
• weakness, lethargy, decreased performance;
• increased sensitivity to cold;
• menstrual irregularities in women;
• problems with bowel movements (constipation);
• hair loss;
• ischemic disease.

If the thyroid gland produces too much T4, the metabolism accelerates and the cells absorb energy faster. However, these changes cannot be considered positive - hyperthyroidism is also dangerous and is characterized by symptoms:

• increased heart rate (tachycardia);
• anxiety, nervousness, irritability;
• sleep disorders;
• tremor (trembling of limbs);
• redness and dryness of the eye mucosa;
• decreased visual acuity;
• sudden loss of body weight;
• puffiness of the face, swelling of other parts of the body;
• indigestion.

In most cases, the balance of hormones is disturbed due to autoimmune pathologies: Graves' disease (hyperthyroidism with high free T4), Hashimoto's thyroiditis (hypothyroidism with low T4).

T4 hormone is low

Low T4 hormone is usually characteristic of insufficient hormonal function of the thyroid gland. This pathology is called hypothyroidism.

The causes of low T4 hormone levels include the following conditions:

• Treatment of thyrotoxicosis with thyreostatics or radioactive iodine;
• Autoimmune thyroiditis (the body produces antibodies against the cells of the thyroid follicles, which leads to insufficient production of thyroxine);
• Removal of the gland or part of it through surgery.

There are cases when apparently healthy people have low free T4 hormone.

Most often this is due to errors when performing research in the laboratory.
When rechecked in better laboratory centers, it turns out that the thyroxine concentration is normal. To avoid errors in diagnosis, it is necessary to take into account the clinical picture and the value of the TSH level. In all cases when the T4 hormone is low in the analysis, and the TSH hormone is within the normal range,
the result should be interpreted with caution and, if possible, a repeat test should be performed.

If a recheck in a good laboratory again shows a reduced level of thyroxine, you need to contact an endocrinologist. Insufficiency of thyroid function is usually lifelong. Therefore, continuous therapy with the administration of the synthetic hormone T4 is necessary. Artificially obtained thyroxine is identical in structure to the natural hormone and does not cause any side effects if the dosage is correctly selected.

Reasons for decreased T4 hormone (total and free):

• Sheehan syndrome;
• Congenital and acquired endemic goiter;
• Autoimmune thyroiditis;
• Traumatic brain injuries;
• Inflammatory processes in the pituitary gland and hypothalamus;
• Hypothyroidism
• Treatment with tamoxifen, antithyroid drugs (mercazolyl, propylthiouracil), steroids and anabolics, beta-blockers (metoprolol, propranolol), NSAIDs (diclofenac, ibuprofen), statins (atorvastatin, simvastatin), antituberculosis and anticonvulsants, diuretics, lithium salts, taking X-ray contrast substances.

Summing up

A blood test for the T4 hormone allows you to identify various pathological processes occurring in the thyroid gland and endocrine system. Its excess, as well as its deficiency, entails negative and unnecessary consequences, therefore, if there is the slightest suspicion of thyroid problems, it is extremely necessary to undergo hormonal testing. Based on the results of the analysis, a qualified endocrinologist will make the correct diagnosis and prescribe appropriate therapy, which will allow you to quickly and easily get rid of the existing pathology.

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T4 hormone during pregnancy

Thyroxine levels play a very important role in fetal development. The free hormone T4 in the first 3 months of pregnancy ensures the development and growth of the embryo's nervous system, so its deficiency can lead to various congenital pathologies. The thyroxine content in women and men is approximately the same, but during pregnancy it is not recommended to determine the level of total T4. In a pregnant woman, there is a physiological increase in the synthesis of thyroxine-binding globulin, and it binds most of the T4 in the bloodstream. Determination of total T4 during this period is not very informative, since its value will invariably be elevated, despite the fact that the free fraction of the hormone is normal.

During pregnancy, it is necessary to test free T4 hormone

, this indicator will help to objectively assess the functioning of the gland. It happens that healthy pregnant women experience a slight increase in free thyroxine that does not require treatment. But if this indicator significantly exceeds the upper limit, it is necessary to reduce the indicator through drug therapy. Treatment is carried out very carefully, under constant monitoring of thyroxine, to ensure normal development of the fetus.

Author of the article:

Kuzmina Vera Valerievna |
Endocrinologist, nutritionist Education: Diploma of the Russian State Medical University named after. N.I. Pirogov, specialty “General Medicine” (2004). Residency at the Moscow State Medical and Dental University, diploma in Endocrinology (2006). Our authors

Symptoms

A decrease in T4 does not leave its mark on the body. Most often, symptoms are not observed immediately; they can develop over months and even years, which is why contacting a doctor may be delayed.

Common symptoms when the hormone deviates from the norm:

• fast fatiguability,
• bad memory
• chills,
• problems with skin, hair, nails,
• swelling of the limbs,
• unreasonable weight gain,
• disruptions in the menstrual cycle.

These conditions should not be overlooked, as a chronic decrease in free T4 leads to coma if not diagnosed and treated.

Treatment

The body is not able to independently replace or additionally synthesize this substance if its levels are below normal. Therefore, the basis of treatment is replacement therapy, in which the patient is prescribed the missing hormones.

Most often, therapy is needed for the rest of your life, but there is nothing scary about it. The drugs are not dangerous to any other organs, supporting only the endocrine system.

Iodine deficiency also causes total and free T4 concentrations to fall below normal. Therefore, iodine-containing drugs are offered as therapy. If the course of treatment does not bring results, and the threshold of the hormone in the blood is still low, they resort to the first option.

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Normal values

Clear normal values ​​are not available for determining T4. Reference ranges may vary between laboratories, as they depend on many factors: the age and gender of the patient, analytical methods and instruments used. It should also be remembered that the results must be assessed as a whole by a physician who knows the patient's medical history.

Possible reasons for deviation of T4 from normal values

Elevated thyroxine values ​​indicate:

• hyperthyroidism;
• toxic goiter;
• inflammation of the thyroid gland (thyroiditis);
• Hashimoto's thyroiditis in the early stages.

In general, a high T4 level is a sign of an overactive thyroid gland, which produces excess amounts of hormones, forming the clinical picture of hyperthyroidism:

• tachycardia;
• anxiety;
• weight loss;
• feeling of warmth and intolerance to high temperatures;
• insomnia;
• diffuse tremors;
• feeling of weakness;
• hypersensitivity to light;
• nervousness;
• fragile nails and hair.

Low thyroxine values ​​are mainly due to:

• hypothyroidism;
• endemic goiter;
• late stage of Hashimoto's thyroiditis.

A low level of T4 in the blood indicates that the gland is not producing enough hormones, and the classic symptoms of hypothyroidism appear in many ways the opposite of the previous ones:

• weight gain;
• dry skin;
• constipation;
• intolerance to low temperatures;
• fatigue;
• menstrual irregularity;
• hair loss;
• edema.

Effect of drugs on T4 values

Many drugs can lower or increase total T4 levels. The former include anabolic steroids, androgens, antithyroid drugs, alpha interferon, interleukin-2, lithium, propanolol and phenytoin. Drugs that increase total T4 levels: birth control pills, chlorofibrate, estrogens and methadone. For this reason, before performing a T4 blood test, it is important to inform your doctor about your current drug therapies. Typically, free T4 values ​​are not affected by these drugs.