T₄ (0.26 μm) was incubated in 0.1 m phosphate buffer (pH 7.4) containing 10 mm EDTA with homogenates (3–5 mg protein) of various rat tissues and up to 400 mm dithiothreitol (DTT) for 1 h at 37 C; the rT₃ generated was measured by RIA of ethanol extracts of the incubation mixture. Among the various tissues of the male rat, homogenates of skin and cerebral cortex were very active in the conversion of T₄ to rT₃; other tissues demonstrated little or no T₄ 5-monodeiodinating activity (MA). The tissue content of rT₃ was also greatest in these two tissues. The MA in skin increased linearly with incubation period (up to 4 h) and with increasing concentration of protein (up to 5 mg), substrate (up to 10 μm) and DTT (up to 400 mm); its optimal pH was 7.4, and optimal temperature was 37 C. Its K_m and maximum velocity approximated 0.29 μm and 9.6 pmol/h · mg protein, respectively, in the presence of 400 mm DTT. There was no appreciable difference in T₄ to rT₃ MA of skin from different parts of the body. The MA was most abundant in microsomes and least in cytosol. The MA was unaffected by propylthiouracil (up to 25 μm), methimazole (up to 100 μm), sodium salicylate (up to 80 μm), or 8-anilino-1-naphthalene sulfonic acid (up to 75 μm). Ipodate (up to 80 μm) weakly inhibited the MA. T₃ and 3,5-diiodothyronine inhibited dermal T₄ to rT₃ MA in a dose-dependent manner; T₃ was 3–12 times more potent than 3,5-diiodothyronine on a molar basis in different experiments. Treatment of euthyroid rats with 3,5-dimethyl-3′isopropylthyronine (45 μg/day, ip) for 3 or 5 days significantly increased dermal T₄ to rT₃ MA. Similar treatment of rats with T₄ (100 μg/day, ip) or T₃ (20 or 80 μg/day, ip) did not change with MA appreciably. Hypothyroidism markedly inhibited the MA, and fasting inhibited it modestly. Pregnancy was associated with marked reduction in the MA of skin in the mother [0.30 ± 0.11 (±SE) vs. 7.2 ± 2.2 ng/h·mg protein; P < 0.02] and fetus (0.67 ± 0.075; P < 0.025).

The various data suggest that 1) skin is a major site for monodeiodination of T₄ to rT₃; 2) monodeiodination of T₄ to rT₃ in the skin is enzymic in nature; 3) inhibitors of outer ring monodeiodination of T₄ are poor inhibitors of inner ring monodeiodination of T₄; and 4) reduction in dermal conversion of T₄ to rT₃ in pregnancy, during which the placenta is an active source of MA, suggests the possibility that the metabolism of T₄ to rT₃ is physiologically regulated. (Endocrinology117: 2106–2113, 1985)