Deiodinase 1 both activates T4 to produce T3 and inactivates T4. Besides its increased function in producing extrathyroid T3 in patients with hyperthyroidism, its function is less well understood than D2 or D3 Deiodinase 2, located in the ER membrane, converts T4 into T3 and is a major source of the cytoplasmic T3 pool. Deiodinase 3 prevents T4 activation and inactivates T3. D2 and D3 are important in homeostatic regulation in maintaining T3 levels at the plasma and cellular levels. In hyperthyroidism D2 is down regulated and D3 is upregulated to clear extra T3, while in hypothyroidism D2 is upregulated and D3 is downregulated to increase cytoplasmic T3 levels.
deiodinase 1 both activates t4 to produce t3 and inactivates t4 besides its increased function in producing extrathyroid t3 in patients with hyperthyroidism its function is less well understood than d2 or d3 deiodinase 2 located in the er membrane converts t4 into t3 and is a major source of the cytoplasmic t3 pool deiodinase 3 prevents t4 activation and inactivates t3 d2 and d3 are important in homeostatic regulation in maintaining t3 levels at the plasma and cellular levels in hyperthyroidism d2 is down regulated and d3 is upregulated to clear extra t3 while in hypothyroidism d2 is upregulated and d3 is downregulated to increase cytoplasmic t3 levels