Thyroid scintigraphy is one of the examination methods used in nuclear medicine. In this procedure, the thyroid gland is imaged with the help of a radioactive agent through a gamma camera. The aim of thyroid scintigraphy is to check the functioning of the organ, to examine the structure of the tissue and, if necessary, to differentiate between hot and cold nodules.
What is thyroid scintigraphy?
Thyroid scintigraphy is one of the examination methods used in nuclear medicine. In this procedure, the thyroid gland is imaged using a radioactive agent through a gamma camera. Image shows location of the thyroid gland in the body. Thyroid scintigraphy is one of the nuclear medicine examinations because it uses a radioactive agent to image the thyroid gland. It is one of the classic thyroid examinations, along with palpation (palpation), ultrasound (sonography) and any tissue sampling that may be necessary (fine needle aspiration). The substance used in scintigraphy to visualize the thyroid gland and its physiological processes is called a tracer. In most cases, the chemical element technetium is used; for certain questions, the use of iodine is also possible. As the radionuclide accumulates in the thyroid cells, the gamma radiation is detected by the corresponding camera and converted into two- or three-dimensional images. The resulting image is called a scintigram. A special form of thyroid scintigraphy is the so-called suppression scintigraphy, in which the normal hormone metabolism of the thyroid gland is brought out of balance with medication in order to look for certain clinical pictures. When it is necessary to assess whether a thyroid nodule is benign or malignant, MIBI scintigraphy can also be used to complement classical diagnostics.
Function, effect, and goals
The main application of thyroid scintigraphy is the clarification of nodules – especially if they exceed a size of 1 cm. Scintigraphy can be used to determine whether a nodule is hot or cold. This is important because cold nodules have a low risk of malignancy, while hot nodules rarely conceal carcinoma. The designation cold or hot nodules is due to the fact that the radionuclide behaves like iodine, which the thyroid gland needs for its hormone metabolism. An increase in storage indicates increased function and appears as a red area (“hot”) on the scintigram, while an area that does not store iodine appears blue and thus “cold”. Uptake of the tracer in the thyroid gland is called uptake. In order to make this storage in the thyroid gland visible with the gamma camera, a waiting period of approximately 20 minutes is observed after the tracer has been administered into the vein, until the exposure, which lasts about five minutes, so that the substance can accumulate well in the thyroid gland. Thyroid scintigraphy is also used as standard when a previous blood test has revealed hyperthyroidism. In this case, nuclear medicine examination is used to look for autonomy of the thyroid gland. In this case, an area of the organ has become encapsulated to produce thyroid hormones on its own – and often too much. These so-called autonomous adenomas may present as single nodules, but may also be diffusely distributed throughout the thyroid gland. Suppression scintigraphy is particularly suitable for confirming the diagnosis of autonomy. Through the preparation, in which thyroid hormones are taken, one achieves that normally working thyroid areas no longer take up any tracer due to saturation: The autonomous area then appears quite clearly. The diagnosis of so-called Hashimoto’s thyroiditis can also be confirmed by thyroid scintigraphy: In this inflammatory autoimmune disease of the thyroid gland, the tissue destroys itself, which can also be visualized in the scintigram. Often thyroid diseases are already visible by the typical goiter. Sometimes, however, the tissue grows behind the breastbone (retrosternal strumen) or settles away from the thyroid gland. These special forms can also be detected by thyroid scintigraphy.In addition, the proven method of nuclear medicine is also suitable as a therapy control, for example after surgery or radioiodine therapy, but also during drug treatment.
Risks, side effects and dangers
Because of the insertion of a radioactive tracer, thyroid scintigraphy is associated with fear of radiation in many patients. Nevertheless, it is a very low-risk diagnostic procedure because, even compared with other nuclear medicine examinations, only a small amount of the tracer needs to be used to obtain meaningful imaging of the thyroid gland. The radiation exposure is well below the level to which one is exposed in a year by natural radiation on earth. The half-life of the radionuclide is also very short at six hours. However, thyroid scintigraphy is contraindicated in pregnant women. Breastfeeding mothers must not breastfeed for 48 hours after the examination. As a precaution, it is also recommended not to have too close contact with pregnant women or young children on the day of the scintigraphy. There should be an interval of at least three months between two scintigraphies. The technetium most commonly used is usually tolerated by patients without any problems. It is not comparable with the contrast medium used for computer tomography (CT), for example, so that allergic reactions are not to be feared. To ensure undisturbed absorption of the tracer in the thyroid gland, the patient must not have consumed excessive amounts of iodine prior to the scintigraphy. For example, no CT must have been performed until about two months before the thyroid scintigraphy, because the iodine-containing contrast medium could falsify the scintigraphy result. In consultation with the physician, various thyroid medications must also be discontinued a certain period of time before the examination.
