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X-ray study of thyroid physiology
Last reviewed: 06.07.2025
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The state of iodine metabolism and thyroid function are assessed by radionuclide studies. As is known, the thyroid gland performs three main functions:
- uptake 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 using radiometry of the gland, the third function, as well as the content of hormones in the blood that regulate the activity of the thyroid gland, are studied using radioimmunological analysis.
Iodine enters the human body with food and water. Absorbed in the intestine, inorganic iodine compounds are quickly distributed in all tissues and the body's aqueous environment. The thyroid gland has the ability to capture iodides from circulating blood. In the gland, iodides are oxidized to form atomic iodine. Subsequently, thyroglobulin is iodized, resulting in the formation of thyroid hormones: triiodothyronine (T3) and tetraiodothyronine, or thyroxine (T4).
Thus, the intrathyroid stage of iodine metabolism consists of two phases: inorganic (capture of iodides from the blood) and organic (formation of thyroid hormones). For a summary assessment of this stage, the patient is given a solution of sodium iodide in water on an empty stomach. The radionuclide is 131 I with an activity of 500 kBq. Gamma radiation of iodine absorbed by the thyroid gland is recorded using a radiometer. In this case, the scintillation sensor is located 30 cm from the anterior surface of the neck. With this counting geometry, the results are not affected by the depth of the gland and its unequal thickness in different sections.
Measurement of radiation intensity over the thyroid gland is performed 2, 4 and 24 hours after taking the radiopharmaceutical. The results of the study of the intrathyroid stage of iodine metabolism are significantly affected by the intake of drugs containing iodine (Lugol's solution, radiopaque iodine-containing agents, kelp) and bromine, the use of hormonal (thyroid hormones, hormones of the pituitary gland, adrenal glands, sex glands) and antithyroid (potassium perchlorate, mercazolil, etc.) drugs. In patients who took any of the above drugs, the capture test is performed only 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 (TBG). This prevents the destruction of hormones, but at the same time makes them inactive. Only a small part of 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 cause the biological effect. In peripheral blood, T4 is 50 times more than T3. However, there is more T3 in the tissues, since some of it is formed on the periphery from T4 by splitting off one iodine atom from it.
The removal of thyroid hormones into the blood, their circulation in the body and delivery to tissues constitute the transport-organic stage of iodine metabolism. Its study provides radioimmunological analysis. For this purpose, blood is taken from the patient's elbow vein in the morning on an empty stomach (in women - in the first phase of the menstrual cycle).
All studies are conducted using standard reagent kits, i.e. in vitro. Thanks to this, examination of children, pregnant women, nursing mothers, non-transportable patients, and patients with drug-induced thyroid blockade has become available.
The radioimmune method is used to determine the content of total and free T3, total and free T4, TSH, and antibodies to thyroglobulin in the blood. In addition, the level of thyrotropin and thyroliberin is determined in the same way.
Thyrotropin is a hormone secreted by thyrotropic cells (thyrotropocytes) of the anterior pituitary gland. The release of thyrotropin into the blood leads to increased thyroid function, which is accompanied by an increase in the concentration of T3 and T4. In turn, these thyroid hormones inhibit the production of thyrotropin by the pituitary gland.
Thus, there is a feedback hormonal relationship between the functioning of the thyroid gland and the pituitary gland. At the same time, thyrotropin stimulates the formation of thyroliberin, a hormone produced in the hypothalamus. At the same time, thyroliberin stimulates the thyroid-stimulating function of the pituitary gland.
Thyroglobulin is the main component of the thyroid follicle colloid. Thyroglobulin circulates in small amounts in the blood of healthy people - at a concentration of 7-60 μg/l. The concentration increases with various thyroid diseases: thyroiditis, toxic adenoma, diffuse toxic goiter. However, the determination of this hormone is of greatest importance in patients with thyroid cancer. With undifferentiated cancer, the thyroglobulin content in the blood does not increase, while differentiated forms of tumors have the ability to produce large amounts of thyroglobulin. The concentration of thyroglobulin increases especially significantly with the appearance of metastases of differentiated thyroid cancer.