**5. Recent research with tissue-specific DIO2 activities in Graves' hyperthyroidims**

## **5.1 Measurement and evaluation of DIO2 enzyme activities: DIO2 enzyme activities were measured after adding patient sera with Graves' ophthalmopathy to thyroidal, skeletal and eye muscle cytosol fractions, all containing DIO2 enzymes. The results were then evaluated according to thyroid functional stages**

Fifty-two patients with Graves' Disease, of whom 37 had ophthalmopathy, were investigated [42]. The difference in the disease duration, the ratio of FT3 to FT4 and the serum levels of TSH receptor antibodies was significant between the Graves' patients with and without ophthalmopathy. The difference in DIO2 activities was relevant and constant among thyroidal, skeletal and eye muscle cytosol fractions in hyper-, eu- and hypothyroidism in Graves' ophthalmopathy. The effect of increased serum FT4 levels was 1.9 times greater on eye muscle DIO2 than thyroidal DIO2 activity. The findings demonstrated that, the tissue-specific DIO2 activities also play a crucial role in the T3 content of peripheral tissues in hyperthyroidism. The skeletal muscle and thyroidal DIO2 activities were lower by 27% and 47%, respectively in hyperthyroidism, as well as were lower by 27% and 87%, respectively in hypothyroidism compared to eye muscle DIO2 activity. DIO2 activities of all

cytosol fractions were 6.3 times lower in hyperthyroidism and 3.5 times greater in hypothyroidism compared to those in euthyroidism (**Figure 2**). In hyperthyroidism, the thyroidal DIO2 activity was better inhibited than that of peripheral tissues. In hypothyroidism, increased thyroidal DIO2 activity could be found together with increased peripheral tissue DIO2 activities.

The effects of FT3 hyperthyroidism were identical on DIO2 activities in all cytosol fractions, but their activities were 2 times higher in euthyroidism compared to those found by increased FT4 levels. No increase in any DIO2 activities could be detected with respect to FT3 levels in hypothyroidism compared to those in euthyroidism. The decrease in all DIO2 activities was the consequence of the increased FT4 levels, which demonstrated a substrate-mediated inhibitory effect in hyperthyroidism. Note, however, that the inhibitory effect of proinflammatory cytokines (IL-6, IL-1 and TNFα) and the therapy cannot be excluded in some cases. Our previous study confirmed the role of IL-6 in Graves' ophthalmopathy with active eye signs [43]. The presence of inflammatory orbital events and a longer manifestation of ophthalmopathy were associated with increased serum IL-6 levels. Therefore, the autoimmune features of Graves' Disease can modify DIO2 activities. The increased DIO2 activities in all cytosol fractions in FT4 hypothyroidism could be explained by the concomitantly increased serum levels of TSH receptor antibodies compared to those in FT3 hypothyroidism. Nevertheless, serum TSH levels were not suppressed by increased serum FT4 levels, which could be explained by the pituitary resistance to T4 [44]. No similar results could be demonstrated for increased T3 levels. Contrary to FT4 hypothyroidism, the lack of increased DIO2 activities in FT3 hypothyroidism support that in this condition the active protein synthesis of DIO2 enzyme is needed for increasing their activities. The partly increased skeletal and eye muscle DIO2 activities in both FT4 and FT3 hyperthyroidism excluded a relevant inactivating role of DIO3 in muscle cytosol fractions.

DIO2 activities in all cytosol fractions were significantly lower in Graves' ophthalmopathy with increased serum FT3 levels keeping the proportional discrepancies constantly among thyroidal, skeletal and eye muscle DIO2 activities. However, Graves' sera without ophthalmopathy resulted in a 5-fold increase in all

#### **Figure 2.**

*The effect of patient sera with Graves' Disease on thyroidal, skeletal and eye muscle DIO2 activities with respect to thyroid functional stages based on FT4 levels. DIO2: Type 2 deiodinase.*

*Deiodinase Enzymes and Their Activities in Graves' Hyperthyroidism DOI: http://dx.doi.org/10.5772/intechopen.97007*

DIO2 activities still keeping the proportional discrepancies constant among thyroidal, skeletal and eye muscle DIO2 activities. The elevation in TSH receptor antibody levels did not associated with DIO2 elevation, but it did with TSH suppression and with a lower ratio of FT3 to FT4. Note that due to the small number of patients without ophthalmopathy, the conclusions can be limited.

#### **5.2 Effects of antithyroid and antiextraocular muscle autoantibodies on thyroidal, skeletal and eye muscle DIO2 activities**
