Of great importance in the normal functioning of the thyroid gland is the hypothalamic-pituitary thyroid system. It provides control over the synthesis, production, and activity of thyroid hormones.
General information
The hypothalamus secretes TRF (thyrotro-pin-releasing factor). It, in turn, stimulates the release and synthesis of thyroid-stimulating hormone (thyrotropin - TSH). TSH is involved in 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 the form associated with transport proteins. Less than one percent remains free. The level of unbound steroid in most people is interconnected with the functional state of the thyroid gland.
Thyroxine Properties
The hormone T4 (free) helps to regulate normal development and growth, ensuring the maintenance of body temperature and thus maintaining heat generation. The compound has an effect on all stages of carbohydrate metabolism, partly on vitamin and lipid metabolism. Hormone T4 (free) is an important component of development in the prenatal and neonatal periods. The concentration of the compound indicates the clinical state of thyroid status, since a change in the level of total thyroxine can be triggered by disorders in the activity of the thyroid gland or a change in the number of transport proteins. Throughout the day, the maximum steroid content 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 the prenatal period (during pregnancy), the concentration of thyroxine rises, gradually reaching a maximum level by the third trimester. Throughout life, the content of thyroxine in people, regardless of gender, remains relatively constant. A decrease in hormone levels is noted after forty years.
What can the thyroxine level say?
If T4 (free) is clearly elevated, then this is considered confirmation of hyperthyroidism. Reduced concentration indicates hypothyroidism. The independence of the 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, androgens or estrogens. Changes are also characteristic of individuals with a hereditary predisposition to increase or decrease the concentration of globulin. Blood donation for free T4 is recommended for the diagnosis of hypothyroidism of the secondary type, due to pathologies at the hypothalamic-pituitary level. In this case, the content of TSH does not change or increases. Typically, an increase in thyroxine may be due to high serum bilirubin, obesity, and the application of a tourniquet when a blood test is taken. T4 (free) does not change in severe diseases not related to the activity of the thyroid gland. In this case, the level of total thyroxine may decrease.
Preparation for laboratory research
A month before blood donation, hormones are excluded (unless there is a specific prescription from an endocrinologist). Two to three days before the analysis, the use of iodine-containing drugs is stopped. Blood should be donated before radiological examinations using contrast agents. On the eve of blood donation, you need to refrain from physical exertion, eliminate stressful situations. Before the study, in half an hour, you should calm down, bring your breathing back to normal. The analysis is performed on an empty stomach. At least eight hours should pass from the last meal (but preferably 12 hours). It is not allowed to drink coffee, juice or tea. You can only water.
Low thyroxine
T4 (free) (the norm in women and men is 9-19 pmol / liter) can decrease in the postoperative period, with secondary hypothyroidism (inflammation of the pituitary gland, thyrotropinoma, Sheehan syndrome). A decrease in concentration is also noted on the basis of taking anabolics, thyreostatics, anticonvulsants, and 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, iodine deficiency, heroin use, and contact with lead. T4 (free) can decrease with tertiary (inflammation of the hypothalamus, TBI), acquired, congenital hypothyroidism (against the background of extensive resection and tumor of the thyroid gland, autoimmune thyroiditis, endemic goiter).
Increase concentration
The level of such a hormone as T4 (free) (the norm for women and men is indicated above) can be increased with toxic goiter, TSH-independent thyrotoxicosis, and obesity. An increase in concentration is noted against the background of nephrotic syndrome, heparin therapy. The causes also include choriocarcinoma, thyroxine intake due to hypothyroidism, a postpartum change in the activity of the thyroid gland, chronic liver damage. T4 (free) can be elevated in case of thyroid steroid resistance syndrome, genetic dysalbunemic hyperthyroxinemia, conditions that provoke a decrease in the concentration of thyroxine-binding globulin.
Thyroxine and pregnancy
Thyroid hormones are involved in almost all processes in the body. As mentioned above, compounds regulate metabolic processes, affect the activity of other steroids. Pathologies of the thyroid gland can be accompanied by both an increase and a decrease in its functions. Of particular importance are violations in the activities of the body during the gestation period. A change in the functional state of the thyroid gland affects the course of pregnancy, its nature, outcome and the condition of the newborn. Rarely, gestation occurs with severe endocrine pathologies. Diseases of this kind, as a rule, lead to impaired reproductive function, infertility. Most often, during pregnancy, goiter is diagnosed (an enlarged thyroid gland of a diffuse nature) with preservation of euthyroidism, as well as autoimmune thyroiditis, which provokes changes in the hormonal background. The prenatal period is characterized by a change in the functional state of the gland.
Recommendations
To correctly interpret the results of laboratory tests during pregnancy, some points should be taken into account. The definition of common 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 prenatal period, a normal decrease in TSH is noted (in about 20-30% of patients with a singleton and in all with a multiple pregnancy). In the treatment of thyrotoxicosis, only T4 (free) is examined. Late levels of unbound thyroxine can be marginally reduced. In this case, the concentration of TSH will remain within normal limits.