text: SPINA-GD correlates to the T4-T3 conversion rate in slow tissue pools, as determined with isotope-based measurements in healthy volunteers. It was also shown that GD correlates with resting energy expenditure, body mass index and thyrotropin levels in humans, and that it is reduced in nonthyroidal illness with hypodeiodination. Multiple studies demonstrated SPINA-GD to rise after initiation of substitution therapy with selenium, a trace element that is essential for the synthesis of deiodinases. Conversely, it was observed that SPINA-GD is reduced in persons positive for autoantibodies to selenoprotein P, which is assumed to be involved in transport and storage of selenium.