Thyroid scintigraphy is a method of radiological examination of the functional activity of thyroid tissue and nodules, based on the assessment of the accumulation of a radioindicator. Scintigraphy allows us to judge the morphology, topography and size of the thyroid gland, to identify its focal and diffuse changes, to identify and differentiate “hot” (hormonally active) and “cold” (functionally inactive) nodes of the gland. Before thyroid scintigraphy, the patient is injected with a microdose of a radiopharmaceutical (iodine isotope I131, I123 or technetium Tc99) capable of accumulating in thyroid tissue and in nodes, then its distribution is evaluated using a gamma camera and a series of scintigrams.
Thyroid scintigraphy serves as a method of clarifying diagnostics and, unlike CT, ultrasound and MRI, has a lower resolution and gives a less clear image of the organ. The advantage of thyroid scintigraphy is the possibility of visual assessment of the level of hormonal activity of normal thyroid tissue and foci of compaction. The technique helps to detect ectopia or possible fragments of thyroid tissue after removal of the gland. The study cannot accurately diagnose the goodness or malignancy of the node, although it suggests the presence of oncological alertness.
Scintigraphy of the thyroid gland with Tc 99 preparation allows to detect metastatic lesion of regional (submandibular, cervical) lymph nodes. The procedure has a low radiation load: the radiation dose is lower compared to other methods (in particular, X-ray), and the radioisotopes used are quickly washed out of the body.
After thyroid ultrasound and hormonal examination, scintigraphy is indicated in case of detection of anatomical and functional pathology – focal and diffuse changes of the gland, ectopia of thyroid tissue, hyper- and hypothyroidism. Thyroid scintigraphy may be prescribed by an endocrinologist to monitor the effectiveness of drug treatment, identify possible changes in the thyroid gland and lymph nodes.
The study is performed at the preoperative stage to determine the scope of surgical intervention; in the postoperative period – to assess the results of surgery (hemithireectomy, thyroidectomy or subtotal resection of the thyroid gland). Thyroid scintigraphy is performed in patients under dispensary supervision after thyroidectomy for medullary or differentiated thyroid cancer in order to detect residual tumor tissues and metastases.
Methodology of conducting
Before thyroid scintigraphy, it is required to stop taking any iodine-containing drugs: L-thyroxine 3 weeks before the study, mercaptisol and propylthiuracil – 5 days before. Thyroid scintigraphy should not be performed earlier than three weeks after the CT scan using a contrast agent containing iodine. If the patient is a nursing mother, it is necessary to inform the radiologist conducting thyroid scintigraphy about this.
Scintigraphy is a non-invasive examination that does not give side effects. 20-30 minutes before thyroid scintigraphy, a radiopharmaceutical containing microdoses of iodine (I) or technetium (Tc) isotopes is administered intravenously to the patient. Then the patient is placed in a supine position in a special gamma camera to scan the radiation emitted by radioisotopes absorbed by thyroid tissues. Snapshots are taken within 5-15 minutes. Distribution of the radioactive drug in the thyroid gland tissues (i.e. functional activity of tissues) is evaluated in the image by the density of the background (dark, light) and by color.
Interpretation of results
Normally, all parts of the thyroid gland accumulate the injected radioisotope evenly, so on the scan they look like two dark symmetrical oval sections resembling a butterfly in shape. According to thyroid scintigraphy, lighter areas in certain areas of the thyroid gland are considered as insufficiently saturated with radioisotope, therefore, not producing hormones, and are called “cold” foci. Darkened places, as intensely saturated and hormonally active, are called “hot” foci.
With diffuse toxic goiter, thyroid scintigraphy reveals an increase in all parts of the gland with a uniform distribution of radioisotope in them, while the cumulative function is significantly increased. Scintigraphy of the thyroid gland in acute thyroiditis shows the absence of inclusion of radioactive iodine in the affected area, in the subacute form of granulomatous thyroiditis – a noticeable decrease in the cumulative function.
Toxic adenoma on scintigrams is visualized as a “hot” node, while the surrounding tissue is very weak or not colored at all (a symptom of “stealing” – the node absorbs almost the entire drug). A picture of a “hot” node in thyroid scintigraphy is also given by a nodular thyrotoxic goiter. “Cold” nodes can be detected in thyroiditis, cysts, involution and fibrosis, as well as thyroid cancer.
This method of diagnostic cannot provide accurate information about the morphology of nodular formations. The only reliable criterion for the diagnosis of thyroid cancer are the results of a fine needle biopsy. In order to detect metastases (papillary, follicular cancer) in which iodine does not accumulate, endocrinology uses whole-body scintigraphy or other imaging studies, for example, positron emission tomography (PET).
Risks of research
During scintigraphy, a certain dose of radiation is injected into the body together with a radiopharmaceutical. However, its power does not exceed the level of radioactive radiation that accompanies chest X-ray or CT. Microdoses of radioisotopes used in scintigraphy are soon removed from the thyroid gland by natural routes, for which the use of an increased volume of fluid is recommended. Complications of thyroid scintigraphy associated with a damaging effect on the body were not noted.