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X-ray examination of thyroid physiology
Last reviewed: 19.10.2021
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The state of iodine metabolism and thyroid function is assessed by radionuclide studies. As is known, the thyroid gland performs three main functions:
- seizure of iodides from the blood;
- synthesis of iodine-containing thyroid hormones;
- the release of these hormones into the blood.
The first two functions are studied with the help of radiometry of the gland, the third function, as well as the content in the blood of hormones that regulate the activity of the thyroid gland, by radioimmunological analysis.
In the human body, iodine is supplied with food and water. Absorbed in the intestine, inorganic iodine compounds are rapidly distributed in all tissues and the body's aquatic environment. The thyroid gland has the ability to trap iodides from circulating blood. In iron, oxidation of iodides occurs with the formation of atomic iodine. Further iodization of thyroglobulin occurs, resulting in the formation of thyroid hormones: triiodothyronine (T3) and tetraiodothyronine, or thyroxine (T4).
Thus, the intra-thyroid stage of iodine metabolism consists of two phases: inorganic (capture of iodides from the blood) and organic (formation of thyroid hormones). For the total evaluation of this stage, the patient is given an empty stomach solution of sodium iodide in water. The radionuclide is 131 I activity of 500 kBq. Gamma radiation of iodine absorbed by the thyroid gland is recorded with a radiometer. The scintillation sensor is located 30 cm from the front surface of the neck. With this geometry of counting the results are not affected by the depth of the gland and its unequal thickness in different departments.
Measurement of radiation intensity over the thyroid gland is carried out 2, 4 and 24 hours after administration of RFP. The results of the study of the intra-thyroid phase of iodine metabolism are significantly influenced by the intake of iodine-containing drugs (Lugol's solution, radiocontrast iodine-containing sea kale) and bromine, hormonal (thyroidin hormones, pituitary, adrenal, gonadal hormones) and antithyroid drugs (potassium perchlorate, mercazolyl and etc.) of preparations. In patients taking any of these drugs, the seizure test is performed only after 3-6 weeks after their withdrawal.
From the thyroid gland T3 and T4 enter the blood, where they combine with a special transport protein - thyroxine-binding globulin (TSG). This prevents the destruction of hormones, but also makes them inactive. Only a small part of the thyroid hormones (about 0.5%) circulates in the blood in a free, unbound state, but it is these free fractions of T3 and T4 that determine the biological effect. In peripheral blood, T4 is 50 times greater than T3. However, there are more T3 in the tissues, since part of it is formed on the periphery of T4 by cleaving one iodine atom from it.
The excretion of thyroid hormones into the blood, their circulation in the body and the adduction to the tissues form the transport-organic stage of iodine metabolism. Its study provides a radioimmunoassay. To this end, the patient in the morning on an empty stomach is taken from the vein of the elbow fold (in women - in the first phase of the menstrual cycle).
All studies are performed using standard reagent kits, i.e. In vitro. Thanks to this, a survey of children, pregnant, nursing mothers, non-transportable patients, patients with drug blockade of the thyroid gland became available.
Radio-immune method is used to determine the content of total and free T3 in the blood, total and free T4, TSH, antibodies to thyroglobulin. In addition, the level of tirotropine and tyroliberin is set in the same way.
Thyrotropin is a hormone secreted by thyroid-stimulating cells (thyrotropocytes) of the anterior lobe of the pituitary gland. The release of thyrotropin into the blood leads to an increase in thyroid function, which is accompanied by an increase in the concentration of T3 and T4. In turn, these thyroid hormones inhibit the production of pituitary thyrotropin.
Thus, between the functioning of the thyroid gland and the pituitary gland there is an inverse hormonal bond. At the same time, tiotropine stimulates the formation of tyroliberin, a hormone produced in the hypothalamus. At the same time, tiroliberin stimulates the thyrotropic function of the pituitary gland.
Thyroglobulin is the main component of the colloid of follicles of the thyroid gland. In the blood of healthy people, thyroglobulin circulates in small amounts - in a concentration of 7-60 μg / l. Concentration increases with various thyroid diseases: thyroiditis, toxic adenoma, diffuse toxic goiter. However, the most important is the determination of this hormone in patients with cancer of the gland. With undifferentiated cancer, the thyroglobulin content in the blood does not increase, whereas the differentiated forms of the tumor have the ability to produce a large amount of thyroglobulin. Especially significantly increases the concentration of thyroglobulin when metastases of differentiated thyroid cancer appear.