Instrumental visualization of glands
Assessing the anatomical structure of the endocrine system requires the use of modern technologies. Conventional radiography has the lowest informativeness in this field and has practically given way to more advanced technologies.
The "gold standard" of diagnostics today is recognized to be ultrasonography (ultrasound), computed tomography (CT), and magnetic resonance imaging (MRI).
These methods are routinely used for a detailed study of the following organs:
- Pituitary gland;
- Thymus gland;
- Adrenal glands;
- Pancreas;
- Parathyroid glands.
The main clinical goal of using ultrasound, CT, and MRI in endocrinology is the timely detection of tumor processes that alter the normal anatomy and topography of the organs.
Radioisotope scanning (scintigraphy)
Scintigraphy is a method that allows assessing not only the shape and size, but also the functional activity of tissues. Radioisotope scanning is most widely used in thyroidology (study of the thyroid gland), as well as for diagnosing adrenal pathologies.
Essence of the method:
- The patient is injected with a specific radioactive isotope (most commonly isotopes of iodine $^{131}\text{I}$ or technetium $^{99}\text{Tc}$).
- The gland tissues capture the isotope from the bloodstream.
- A special gamma camera registers the $\gamma$-radiation emitted by the accumulated preparation and captures the picture on light-sensitive paper.
The diagnostic value of the method lies in the ability to visualize so-called "hot nodules" — local areas of glandular tissue that possess excessive functional activity and therefore accumulate the isotope much more intensively than surrounding healthy cells.
Radioimmunoassay (RIA) for hormone determination
RIA is a crucial tool of laboratory diagnostics with extremely high sensitivity. It allows detecting vanishingly small concentrations of substances in the blood, such as insulin, pituitary trophic hormones, or thyroglobulin. The method is based on the "antigen-antibody" immunological reaction and the principle of competitive binding.
Stages of competitive binding:
- Preparation. An initial mixture is created in a test tube: a standard, strictly measured amount of specific antibodies (antisera) is mixed with a known amount of radioactively labeled antigen (a hormone carrying a $^{125}\text{I}$ or $^{131}\text{I}$ label).
- Interaction. The patient's serum is added to this ready mixture. It contains the sought-after "natural" hormone, which acts as an unlabeled antigen.
- Competition. The hormone from the patient's blood begins to compete with the laboratory-labeled hormone for the right to bind to a limited pool of antibodies. The natural hormone literally displaces the radioactive analogue from the formed complexes.
Measurement and calculations: After the reaction is completed, it is necessary to separate the formed "antigen-antibody" complex (precipitate) from the remaining free hormone. This is done using absorption or precipitation. Then, the obtained pellet is placed into a gamma counter to measure radioactivity.
Key pattern of RIA: the more of the patient's own hormone was in the blood, the stronger it displaced the radioactive label into the free fraction. Consequently, the radioactivity of the final precipitate is inversely proportional to the real concentration of the hormone in the sample. High pellet activity means a hormone deficiency in the patient, and vice versa.
Difficulties and limitations of the method: Despite its precision, RIA has its specificity nuances. First, the method is capable of reacting to the hormone bound to transport proteins, rather than only to its biologically active free fraction. Second, cross-reactions with chemical precursors or metabolites of the hormone are possible: they may have a similar antigenic structure and bind to antibodies while lacking actual hormonal activity in the body.
Additional tests and biochemical markers
In addition to direct hormone measurements, evaluating metabolic effects and functional reserves is important in endocrinology:
- Loading tests. Involve artificial stimulation or suppression of endocrine gland function followed by measurement of hormone levels. This allows for an accurate assessment of the organ's reserve function, which is hidden in a state of rest.
- Biochemical markers. Reflect the consequences of hormonal shifts. For example, blood and urine glucose levels serve as the main and basic marker of the course of diabetes mellitus, while calcium metabolism indicators are key markers for identifying parathyroid gland pathologies.
Briefly: answers to topical questions
What specific loading tests and trials are used to diagnose pituitary and adrenal disorders?
Low-dose and high-dose dexamethasone suppression tests are used to diagnose pituitary and adrenal pathologies.
- Dexamethasone test — is the main method for differentiating Cushing's disease (pituitary genesis) from Cushing's syndrome (adrenal genesis).
Dexamethasone acts via a negative feedback mechanism. In Cushing's disease, the drug suppresses excessive ACTH secretion by the pituitary gland, leading to a decrease in hormone production by the adrenal cortex (positive test). In a hormone-active adrenal tumor (corticosteroma), secretion is autonomous, so the hormone level does not change.
What specific calcium metabolism parameters are investigated to identify parathyroid gland pathologies?
To identify parathyroid pathologies, calcium metabolism parameters are evaluated.
- Blood calcium — parathyroid hormone increases the concentration of calcium in plasma; in secondary hyperparathyroidism, calcium levels are low, while in tertiary, they are high.
- Phosphates — in chronic kidney disease, high phosphate levels stimulate the parathyroid glands and increase parathyroid hormone secretion.
- Parathyroid hormone (PTH) — synthesized by the parathyroid glands; its secretion increases upon stimulation of the parathyroid glands.
Sources also indicate that calcium metabolism is a biochemical marker of parathyroid pathology.
What are the indications for fine-needle aspiration biopsy (FNAB) of thyroid nodules?
Fine-needle aspiration biopsy is performed primarily for the differential diagnosis of thyroid nodular lesions.
- Nodular goiter — FNAB is performed as part of the primary diagnostic workup.
- Medullary thyroid cancer — biopsy with washout testing for calcitonin is indicated in primary preoperative diagnosis (especially with moderately elevated basal calcitonin).
The procedure is performed under ultrasound guidance. At the same time, it is worth noting that a puncture biopsy is not indicated to confirm autoimmune thyroiditis (AIT).
To avoid getting confused by RIA results, remember the "see-saw" rule: the MORE hormone is in the patient's blood, the LESS radioactivity will remain in the pellet (precipitate). The relationship is always inversely proportional.
Class and exam questions
Why is radiography rarely used?
Among all imaging modalities, radiography has the lowest diagnostic value for assessing the structure of endocrine glands. It has been replaced by ultrasound, CT, and MRI.
What does the term «hot nodule» mean?
This is a tissue area (e.g., in the thyroid gland) detected during radioisotope scanning. It is characterized by high functional activity and intensively absorbs the radioactive isotope.
What is the main drawback of RIA specificity?
Antibodies in RIA may cross-react with inactive metabolites or hormone precursors due to their similar structure, and also react with protein-bound hormones.
What else to study on this topic
- Mechanisms of performing stimulation and suppression loading tests
- Features of evaluating carbohydrate metabolism (blood and urine glucose)
- Markers of calcium metabolism in parathyroid pathologies
- Physical operating principles of the gamma camera in scintigraphy
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