Thyroid Hormone T4 - Increased / Decreased, What Is The Norm?

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Video: Thyroid Hormone T4 - Increased / Decreased, What Is The Norm?

Video: Thyroid Hormone T4 - Increased / Decreased, What Is The Norm?
Video: Thyroid Gland, Hormones and Thyroid Problems, Animation 2023, March
Thyroid Hormone T4 - Increased / Decreased, What Is The Norm?
Thyroid Hormone T4 - Increased / Decreased, What Is The Norm?

Thyroid hormone T4

What is T4 hormone (thyroxine)?

Thyroid hormone T4
Thyroid hormone T4

T4 - Thyroid hormone is produced by the cells of the thyroid follicles. Thyrocytes are synthesized from the amino acid and iodine thyroglobulin, which is a precursor of thyroxine. Thyroglobulin accumulates in follicles, and thyroxine is formed from it, if necessary, by division into fragments.

The main effect of the T4 hormone is to accelerate catabolism - the process of obtaining energy from energetically significant metabolites (glycogen, fat). Excessive concentration of thyroxine in the blood leads to a rapid heartbeat, irritability, and a decrease in body weight. But this does not mean that the hormone is harmful, these are only symptoms of an overdose. Normally, tetraiodothyronine maintains the tone of the nervous system, pulse rate and adequate metabolism.

The T4 hormone is not the most active thyroid hormone, for comparison, its activity is almost ten times lower than that of 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 precursor, thyroxine. In fact, it is the more active metabolite of T4.

Hormones T3, T4 are also called thyroid hormones, as they are secreted by the thyroid gland, called in Latin "thyroid". TSH is also sometimes referred to as thyroid, but this is erroneous, because it is formed by the pituitary gland 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. Usually, these indicators are used by endocrinologists to diagnose thyroid pathologies. Sometimes thyroid hormones are examined during the treatment of a disease in order to determine its dynamics and the effectiveness of the prescribed therapy. This article will provide information on what the T4 hormone is, what functions it performs in the body and how the analysis for its content is interpreted.

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

T4 hormone 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 called thyroxine-binding globulin (TSH). This substance performs a transport function, delivering the hormone to the cells of the body. That small amount of non-protein bound thyroxine is called free T4. It is this fraction that is responsible for providing the biological effect. The part of the hormone that is associated with TSH is called T4 associated. If we separately determine free T4 and T4 bound in the blood, and then add these values, we get T4 total.

In the body, the main effect is exerted by free thyroid hormones (free T4, free T3), therefore their content is of the greatest importance for determining thyroid pathologies. In laboratories, blood tests for thyrotropin and free T4 are most often carried out. The TSH index 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 agents used to treat hypothyroidism.

T4 hormone test

T4 hormone test
T4 hormone test

In the clinical practice of endocrinologists, the analysis for the hormones TSH, T4 is the most commonly used diagnostic method. Hormone tests are prescribed in various combinations based on the patient's symptoms and economic capabilities.

At the initial visit to an endocrinologist, if the patient does not have severe symptoms, it is enough to pass an analysis for the hormones TSH, T4, T3. As for the last two hormones, it is better to investigate their active, i.e., free fractions. In cases where the patient receives thyrostatics for the treatment of early stages of Graves' disease (diffuse toxic goiter), it is better to determine only free hormones T3 and T4. Under the influence of thyrostatic agents, these indicators rapidly decrease, while the TSH level seems to be delayed and does not have time to decrease.

If a patient is treated for a long time for insufficient gland function, then for periodic monitoring of the quality of therapy, it is sufficient to determine only the concentration of thyroid-stimulating hormone (TSH). Free T4 is examined only in the presence of special indications. You should know that in the case of taking thyroxine, a blood test for T4 hormone can be taken 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 taken medicine will be added to the T4 hormone secreted by the thyroid gland.

During pregnancy, the value of free T4 hormone is especially increased, because the level of thyrotropin can be reduced as a result of the action of hCG - the human chorionic gonadotropin produced by the placenta. Therefore, when examining pregnant women, the determination of TSH alone is insufficient for correct diagnosis. It is necessary to be tested simultaneously for TSH and T4.

In the direction or form with the result of the study, you can find various abbreviations:

  • FT4, FT3 - T4 and T3 are free (English free, which means "free");
  • Hormone St. T4, St. T3 - also free forms of hormones.

What is the rate of T4 hormone?

Free hormone T4. To correctly evaluate the results of the analysis for free hormone T4, it is not enough to know any specific norms. The normal content of thyroxine largely depends on the laboratory conducting the analysis. These indicators are different for different analyzers; even the set of reagents used in each case matters. As a rule, the acceptable 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 thyroxine concentration ranges from about 9 - 20 pmol / l.

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

T4 hormone (tetraiodothyronine) total

Patient age

nmol / l

μg / dl


59 - 135

4.6 - 10.5


71 - 142

5.5 - 11


75 - 230

5.8 - 17.9

Children: 1-5 years old

90 - 194

7 - 15

Children: 5-10 years old

83 - 172

6.5 - 13.4

T4 hormone (tetraiodothyronine) free

Patient age

pmol / l

ng / dl


9.0 - 22.0

0.93 - 1.71


7.6 - 18.6

0.6 - 1.45

Children: 5-10 years old

10.7 - 22.2

0.83 - 1.73

Children: 10-15 years old

12.1 - 26.9

0.94 - 2.09

! Important: How can thyroid disease be recognized by the analysis of hormones?

T4 hormone increased

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

  • Increased sweating
  • Fatigue,
  • Irritability,
  • Rapid pulse and feeling of increased heartbeat
  • Arrhythmia (extrasystoles, atrial fibrillation),
  • Noticeable weight loss,
  • Tremors in the arms and legs (tremors).

The increased T4 hormone accelerates the breakdown of fats in the body, therefore, body weight decreases. The excess amount of energy released has a negative effect on organ function. This is manifested in the acceleration and intensification of the work of the heart, increased sweating. Over-stimulation of the nervous system causes irritability and frequent mood swings, and the acceleration of neuromuscular transmission leads to limb tremors. Losing weight in this condition is not physiological, because it occurs against the background of dysfunction of internal organs. With prolonged preservation of an increased concentration of thyroxine, calcium is washed out of the bones, which is fraught with an increased risk of fractures and osteoporosis.

The reasons for the increase in the hormone T4 (total and free):

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

T4 hormone lowered

T4 hormone lowered
T4 hormone lowered

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

The reasons for low levels of T4 hormone 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 a gland or part of it by surgery.

There are times when apparently healthy people have low T4 free hormone. This is most often due to inaccuracies in performing research in the laboratory. When rechecking in better quality laboratory centers, it turns out that the concentration of thyroxine is normal. To avoid mistakes in diagnosis, it is necessary to take into account the clinical picture and the significance of the TSH level. In all cases, when the T4 hormone is lowered in the analysis, and the TSH hormone is within the normal range, the result should be interpreted with caution and, if possible, re-examined.

If a recheck in a good laboratory again showed a reduced level of thyroxine, it is necessary to consult an endocrinologist. Lack of thyroid function is usually lifelong. Therefore, it is necessary to carry out constant therapy with the appointment of synthetic T4 hormone. The artificially obtained thyroxine is identical in structure to a natural hormone and, with a properly selected dosage, does not cause any side effects.

Reasons for lowering the T4 hormone (total and free):

  • Sheehan's syndrome;
  • Congenital and acquired endemic goiter;
  • Autoimmune thyroiditis;
  • Traumatic brain injury;
  • Inflammatory processes in the pituitary gland and hypothalamus;
  • Hypothyroidism
  • Treatment with tamoxifen, antithyroid drugs (mercazolil, propylthiouracil), steroids and anabolic steroids, beta-blockers (metoprolol, propranolol), NSAIDs (diclofenac, ibuprofen), statins (atorvastatin, simvastatin), anticonvulsants, anti-vascular drugs, and anti-tuberculosis drugs substances.

T4 hormone during pregnancy

Thyroxine levels play a very important role in fetal development. Free T4 hormone in the first 3 months of pregnancy ensures the development and growth of the nervous system of the embryo, 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 total T4 level. 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. The determination of total T4 during this period is not very informative, since its value will invariably be increased, despite the fact that the free fraction of the hormone is normal.

During pregnancy, it is necessary to take free T4 hormone, this indicator will help to objectively assess the work of the gland. It happens that in healthy pregnant women there is a slight increase in free thyroxine, which does not require treatment. But if this indicator significantly exceeds the upper limit, it is necessary to reduce the indicator by means of drug therapy. Treatment is carried out very carefully, under constant control of thyroxine, to ensure the normal development of the fetus.


The author of the article: Kuzmina Vera Valerievna | Endocrinologist, nutritionist

Education: Diploma of the Russian State Medical University named after NI Pirogov with a degree in General Medicine (2004). Residency at Moscow State University of Medicine and Dentistry, diploma in Endocrinology (2006).

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