99mTc-MDP uptake in thyroid nodule: Contribution of SPECT-CT and ultrasonography

Abstract

Iodine kinetic studies were performed in seven patients with hot thyroid nodules, three diffuse toxic goiters and one normal control, using a double label technique (131I and 125I-thyroxine). The investigation was complemented by partial kinetic studies in 29 further subjects, including normal control subjects and euthyroid or hyperthyroid patients with diffuse or nodular goiter. Thyroid iodide uptake and hormonal secretion parameters were significantly increased, up to twice control values, in hot nodules without clinical signs of thyrotoxicosis. Values similar to those of Graves' disease were found in toxic nodular goiter. The exchangeable iodine pool was markedly reduced in those glands where the nodule was adenomatous and radioiodide uptake of normal parenchyma was completely suppressed. In two cases of nontoxic hot nodule with histologic diagnosis of multinodular goiter, and incomplete suppression of the “extranodular” thyroid, the exchangeable iodine pool was comparable to that of normals. The T4 disposal rate was increased, mainly due to a significantly higher metabolic deiodination which seemed to keep pace with the increased T4 production rate in nontoxic nodules. This could be the reason why the mean peripheral T4 pool of clinically nontoxic patients with hot nodules is comparable to that of normal persons, and there are no evident signs of hypermetabolism in spite of the greater thyroid hormone production. A significant “non-T4-iodine” release, probably mainly iodide, was found in every subject. The amount of iodine lost by the thyroid through this “iodide leak” did not relate to the T4 secretion rate but seems to be directly related to the thyroid exchangeable iodine pool size.