Novel trends for the use of X-ray computed tomography for the etiological diagnosis of iodine metabolism disorders

Abstract

In the diagnosis of thyroid pathology, it is not enough to determine only functional disorder like, euthyroidism, hypothyroidism or hyperthyroidism; it is also necessary to rull out the cause of the identified disorders - is this the primary thyroid disease or caused by external influences, for example, iodine-induced (iodine-deficient) or iatrogenic. There are numbers of drugs, which can affect the metabolism of iodine, the synthesis of thyroid hormones and their storage in the thyroid gland. The thyroid gland differs from the surrounding tissues by a high level of absorption of X-rays due to the presence of iodine, which is reflected in the density index[1]. Despite the fact that X-ray computed tomography (CT) has long been used in clinical practice, the potential of the method in assessing disorders of iodine metabolism is far from not being used. Purpose: To develop a method of etiological and differential diagnosis of the genesis of iodine metabolism disorders based on the determination of the density of the thyroid gland by the X-ray CT method. Materials and Methods. A retrospective analysis of computed tomography data was carried out on 289 patients examined at the Center of Nuclear Medicine and Positron Emission Tomography of the Central Clinical Hospital of Russian Railways No. 2 (Moscow). The study was carried out on a single-photon emission computed tomograph combined with an X-ray computed tomography (SPECT/CT) "Symbia T16" (Siemens). The study time was 7-10 minutes, the local radiation exposure was 0.4-1 mSv. The density of the gland was determined by the intensity of the light image of the tissue in Hounsfield units (HU)[2]. Structural changes in the thyroid gland were studied according to ultrasonography, the functional state - according to the level of hormones TSH, f.T4 (enzyme-linked immunosorbent assay, analyzers from Abbott, Ortho-Clinical Diagnostics). Antibodies to thyroid peroxidase (AT-TPO) and thyroglobulin (AT-TG) were determined. The ethical norms were confirmed by Ethics Committee of RUDN University (Protocol No. 30 of February 22, 2018)Results: In the course of the study, the boundary values were determined for determining the norm (86-140 HU units), decreasing (less than 85 HU units) and increasing (above 140 HU units) density. The novelty of the obtained data is confirmed by a patent [3]. The majority of patients, when referred for CT examination, had thyroid disorders - 66.4% (192 people) and only 33.6% (97 people) had euthyroidism. However, a subsequent thorough examination made it possible to clarify the etiology of thyroid disorder (thyroidal or non-thyroidal genesis). In the group of patients with a reduced level of thyroid density, more than 20% were patients without thyroid pathology, but taking medications (10.1%) or having AT-TPO (13.2%) without changing the level of hormones and the structure of the thyroid gland. Among patients with a high index of thyroid density, there were two groups - with iodine-induced hypothyroidism (caused by taking potassium iodide at a dose of 200 μg / day for 1.5-2 years) and amiodarone-induced hyperthyroidism (using amiodarone 8-12 months). The low concentration of intrathyroidal iodine in patients with hyperthyroidism of autoimmune genesis indicates its rapid consumption for the synthesis of an excess amount of thyroxine, but in amiodarone-induced hyperthyroidism, the reason was its excessive intake (density increased). A poorly functioning organ that produces little of its own hormones always has a low density of 85 HU or less. In iodine-induced pathological conditions of the thyroid gland (iodine-induced hypothyroidism or hyperthyroidism), the density of the thyroid gland increases, and is expressed by an increase in the Hounsfield unit to 140 HU.Conclusion: The use of the X-ray CT technique allowed us, already at the stage of primary diagnosis, to identify the main etiological factors - thyroidal (thyroid disease) and non-thyroidal (exogenous or endogenous) causes of iodine storage disorders in the thyroid gland. Assessment of the density of the thyroid gland is effective in the differential diagnosis of iodine-deficient or iodine-induced disorders of the hormone-forming function of the thyroid gland.