**2. Three types of deiodinase enzymes are involved in thyroid hormone activation and inactivation**

Three types of deiodinase enzymes (DIO1, DIO2, DIO3) are responsible for the activation and inactivation of thyroxine (T4) and triiodothyronine (T3) thyroid hormones [21]. Deiodinase enzymes demonstrate tissue-specific localization. DIO1 enzyme is expressed in the liver, kidney and thyroid parenchymal cells localized in the plasma membrane [22]. Its active center is found in the cytosol. T4 plays as a prohormone for the active T3 hormone. T4 has four iodine bindings at the 3,3′, 5 and 5′ positions. DIO1 enzyme is able to cleave iodine from 5 (inner ring deiodination, step of T4 inactivation) or 5′ position (outer ring deiodination, step of active T3 hormone production). The dual effect of DIO1 enzyme plays a crucial role in the excessive thyroid hormone production, called hyperthyroidism. DIO2 enzyme is a widespread 5′-deiodinase expressed in thyroid, skeletal muscle and adipose tissues, hypothalamus, pituitary, skin, osteoblast, astroglia, retina, cochlea, placenta and endothelial cells localized in the endoplasmic reticulum [23]. Its active center is found in the cytosol. DIO1 expression can be induced transcriptionally by T3 and TSH receptor stimulating antibodies. Fasting and chronic illnesses decrease DIO1 activity. The inhibitory effect on thyroidal DIO1 and DIO2 activities was demonstrated *in vitro* in the presence of proinflammatory cytokines (IL-1, IL-6 and TNFα) [24]. The inhibitory rate was higher on DIO2 than on DIO1 activities. The inhibitory degree was dose-dependent. Thyroidal DIO1 activity is responsible for only 6% of the daily T3 production [25]. Propylthiouracil (PTU) inhibits its activity. DIO2 plays a crucial role in the maintenance of intracellular T3 levels via 5′-deiodination, converting T4 to T3. Its increased activity is partly present in hyperthyroidism; however, its activity is decreased in nonthyroidal illness [26]. DIO2 is a posttranslationally T4-dependent enzyme, which accelerates its proteasomal degradation. DIO2 enzyme is involved in the feedback mechanism of hypothalamic–pituitary-thyroid axis [27]. The normal development and regeneration of skeletal muscle requires DIO2 activity [28].

DIO3 is an enzyme located in the plasma membrane. It has both extra- and intracellular activity [29]. DIO3 plays a crucial role in fetal development and tissue-repair. It is expressed in placenta, uterus, neurons, skin, alveolar cells, glial cells, urothelium, gastrointestinal tract, hypothalamus and skeletal muscle [30]. DIO3 inactivates T3 through inner ring 5-deiodination. Its increased activity is responsible for the consumptive hypothyroidism observated in hepatic hemangiomas [31]. Nether DIO2 nor DIO3 are PTU sensitive enzymes. Hypothyroidism is connected to an increase in DIO1 and DIO2 activities, but DIO3 activities are decreased [22]. Hyperthyroidism is connected to an increase in both thyroidal DIO1 and DIO2, but to a decrease in extrathyroidal DIO2 activities. Iopodic acid, the contrast material with high iodine content decreases the activities of all deiodinase enzymes. The alterations in T4 and T3 levels according to thyroid function have a different effect on the deiodinase enzyme activities in the living cells vs. sonicated cells [23]. No protein synthesis can happen in sonicated cells. Therefore, the sonicated cell content could be regarded as a deiodinase enzyme solution.
