The Use of X-ray Computed Tomography to Assess the Intrathyroidal Iodine Concentration and its Storage in the Thyroid Gland

Abstract

In this lecture, the need of using X-ray computed tomography (CT) to assess the intrathyroidal iodine concentration and its storage in the thyroid gland has being discussed. Due to the fact that 80 % of intrathyroidal iodine is located in the phenolic ring of thyroid hormones, which are structurally located in colloid-thyroglobulin follicles as a hormonal depot, the parameter of intrathyroidal iodine (PII) is an indicator of the stores of iodine-containing thyroid hormones directly in the organ. A decrease in intrathyroidal iodine indicates a significant functional impairment of storing thyroid hormones in the colloid of thyroglobulin of the thyroid follicles and is an early highly accurate prognostic sign of the formation of gland dysfunction. Due to the compensatory capabilities of the body, this dysfunction may appear late onset (for example, 2 months after detecting a decrease in intrathyroidal iodine). The most convenient and affordable method for determining intrathyroidal iodine is CT with two types of tomographs: 1) standard by which intrathyroidal iodine is determined by the density of the thyroid gland in Hounsfield units (HU); 2) tomographs with the option of assessing the concentration of intrathyroidal iodine (CII) in the most common units of measurement – mg/g or μg/g (used since 2016). If necessary, the conversion of some units of intrathyroidal iodine to others has the formula: CII (in μg/g) = ([density in HU] – 65) / 104. Based on the literature and our own research results, for the first time, we calculated the limits of normal intrathyroidal iodine fluctuations in euthyroid individuals, which are 85–140 HU units or 200–700 μg/g intrathyroidal iodine. Identification of the examined intrathyroidal iodine beyond the indicated fluctuations indicates the functional impairment of storing thyroid hormones, which ultimately will lead to hypothyroidism or hyperthyroidism (except when the patient is taking levothyroxine, mercazole, β-blockers – drugs that reduce intrathyroidal iodine). For the first time, an algorithm is presented for differential diagnosis of iodine-deficient and iodine-induced thyroid dysfunctions, which can only be done using CT: if there is a functional impairment of the thyroid gland with intrathyroidal iodinelevel less than 85 units of HU or 200 μg/g CII, then it is considered iodine deficient; with intrathyroidal iodinelevel more than 140 units of HU or 700 μg/g of CII, it is considered iodine-induced. The algorithm for the prevention of iodine-induced thyroid pathology with iodine prophylaxis is that iodine prophylaxis should not be prescribed or continued when intrathyroidal iodinelevel is 140 units of HU or 700 μg/g of CII or more.