T4 (free): the norm. Hormone T4 (free): a blood test for thyroid hormones

Great importance in the normal functioning of the thyroid gland is hypothalamic-pituitary thyroid system. It provides control over the synthesis, production and activity of thyroid hormones.

General information

The hypothalamus secretes TRP (thyrotropin-releasing factor). It, in turn, stimulates the secretion and synthesis of thyroid-stimulating hormone (thyrotropin - TSH). TTG takes part in the processes associated with other steroids. In particular, it stimulates the secretion, accumulation, metabolism and synthesis of triiodothyronine (T3) and thyroxine (T4). More than 99% of these two steroids circulate in the blood in a form associated with transport proteins. Less than one percent remains in free form. The level of unbound steroid in most people is interrelated with the functional state of the thyroid gland.

Properties of thyroxine

The hormone T4 (free) promotes regulation of normal development and growth, ensuring the maintenance of body temperature and thus maintaining heat formation. The compound affects all stages of the metabolism of carbohydrates, in part - on the vitamin and lipid metabolism. The hormone T4 (free) is an important component of development in the prenatal and neonatal periods. The concentration of the compound indicates a clinical condition of the thyroid status, since a change in the level of the total thyroxin can be triggered by disorders in the thyroid activity or a change in the number of transport proteins. During the day, the maximum content of the steroid is determined from 8 to 12 hours, and the minimum from 23 to 3. During the year, the highest level of T4 (free) reaches from September to February, the minimum - in the summer. In prenatal period (during pregnancy), the concentration of thyroxin rises, reaching a gradually maximum level to the third trimester. Throughout life, the content of thyroxin in people, regardless of sex, remains relatively constant. The decrease in hormone levels is observed after forty years.

What can the level of thyroxin say?

If T4 (free) is clearly elevated, then this is considered a confirmation of hyperthyroidism. Reduced concentration indicates hypothyroidism. Independence of steroid content from thyroxin-binding globulin makes it possible to use it as a reliable diagnostic test. This is especially important in conditions that are accompanied by a change in the level of thyroxin-binding globulin. These include taking contraceptives (oral), pregnancy, receiving androgens or estrogens. Changes are also characteristic for individuals with a hereditary predisposition to an increase or decrease in the concentration of globulin. To give blood to free T4 is recommended for the diagnosis of secondary type hypothyroidism due to pathologies on the hypothalamic-pituitary level. In this case, the TSH content does not change or increases. As a rule, an increase in thyroxin levels may be due to a high serum level of bilirubin, obesity, and the application of a tourniquet, when a blood test is given. T4 (free) does not change in severe diseases, not related to thyroid activity. The level of total thyroxine may decrease.

Preparation for laboratory research

One month before blood donation, hormones are excluded (unless there are special instructions from the endocrinologist). For two or three days before the analysis, the use of iodine-containing drugs is discontinued. Blood should be taken before carrying out X-ray studies using contrast agents. On the eve of donating blood, you need to refrain from physical exertion, to exclude stressful situations. Before the test, for half an hour, you should calm down, bring your breathing back to normal. The analysis is carried out on an empty stomach. With the last meal should take at least eight hours (but preferably 12 hours). Do not drink coffee, juice or tea. You can only water.

Lowered thyroxine level

T4 (free) (the norm in women and men - 9-19 pmol / liter) may decrease in the postoperative period, with secondary hypothyroidism (inflammation of the pituitary gland, thyrotropinoma, Shihan syndrome). Reduction of concentration is also noted on the basis of the intake of anabolic, thyreostatics, anticonvulsant drugs, lithium preparations. The level of thyroxine decreases with the use of oral contraceptives, octreotide, methadone, clofibrate. Possible reasons include a diet with a limited amount of protein, a lack of iodine, the use of heroin, contact with lead. T4 (free) can decrease with tertiary (inflammation of the hypothalamus, TBI), acquired, congenital hypothyroidism (against a background of extensive resection and thyroid tumor, autoimmune thyroiditis, endemic goiter).

Increase in concentration

The level of a hormone such as T4 (free) (the norm in women and men is indicated above) can be increased with toxic goiter, TTG-independent thyrotoxicosis, obesity. The increase in concentration is noted against a background of nephrotic syndrome, heparin therapy. The reasons also include choriocarcinoma, the administration of thyroxin on the grounds of hypothyroidism, the postnatal change in the activity of the thyroid gland, and chronic liver damage. T4 (free) may be increased with a syndrome of resistance to thyroid steroids, genetic dysalbunemic hyperthyroxinemia, conditions that provoke a decrease in the concentration of thyroxin globulin.

Thyroxine and pregnancy

Hormones of the thyroid gland take part in almost all processes in the body. As mentioned above, the compounds regulate metabolic processes, affect the activity of other steroids. Pathologies of thyroid gland can be accompanied by both an increase and a decrease in its functions. Particular importance of violations in the activities of the body have in the period of bearing the child. Change in the functional state of the thyroid gland affects the course of pregnancy, its nature, outcome and the state of the newborn. Rarely the bearing takes place with pronounced endocrine pathologies. Diseases of this kind, as a rule, lead to violations of reproductive function, infertility. Most often during pregnancy, a goiter is diagnosed (an increase in the thyroid gland of a diffuse nature) with the preservation of euthyroidism, as well as an autoimmune thyroiditis that provokes changes in the hormonal background. For the prenatal period, a change in the functional state of the gland is characteristic.

Recommendations

In order to correctly interpret the results of laboratory tests during pregnancy, some points should be taken into account. The definition of general T3 and T4 is not informative. This is due to the fact that during pregnancy their concentration is increased by one and a half times. When studying the level of unbound thyroxin, its concentration should be determined in combination with the content of TSH. Thyroxine is slightly increased in about 2% of pregnant women. In the first half of the antenatal period there is a normal decrease in TSH (approximately in 20-30% of patients with singleton and in all with multiple pregnancies). In the treatment of thyrotoxicosis, only T4 (free) is examined. The level of unrelated thyroxin in later terms can be borderline lowered. In this case, the concentration of TSH will remain within normal limits.