Published in Thyroid. January 2014, 24(1): 88-168. DOI: 10.1089/thy.2013.0109 © Mary Ann Liebert, Inc. FIG. 9. Strategies to measure individualized deiodinase activity in the presence of other deiodinases. (A) Two strategies to assess type II deiodinase (D2) activity in the presence of type I deiodinase (D1; e.g., in the human thyroid): (i) use 1 mM 6-n-propyl-2-thiouracil (PTU) to inhibit D1 or (ii) use 1–2 nM [125I]T4 in the presence or absence of 100 nM cold thyroxine (T4). (B) To measure D2 activity in D2/D3 co-expressing tissues (e.g., the brain), use 1000 nM cold T3 to saturate D3. (C) To measure D2 activity in D1/D2/D3 co-expressing tissues (e.g., rodent gonads, placenta, cerebrum, and skin), inhibit D1 with 1 mM PTU and saturate D3 with 1000 nM T3.