Serum concentrations of T4, T3, and rT3 as well as liver and kidney 5′-deiodinase activity, have been examined in rats stressed by restraint. After immobilization, serum concentrations of T3 decreased significantly (6 hr, -33 +/- 1%; 8 h, -42 +/- 3%), while serum rT3 increased (6 h, +55 +/- 3%; 8 h, +75 +/- 5%). In the same or similarly treated animals, there was a time-dependent reduction in T4 5′-deiodinase activity in both liver (4 h, -23 +/- 2%; 8 h, -43 +/- 3%) and kidney (4 h, -18 +/- 1%; 8 h, -42 +/- 3%) homogenates. The reduction in hepatic and renal T3 production was due to reduced enzyme activity and not to reduced substrate availability. In spite of reductions in serum TSH (4 h, -9 +/- 1%; 8 h, -51 +/- 5%), the serum T4 concentration did not fall. The serum concentration of corticosterone reached 30 times the basal level after 8 h of restraint. Either adrenalectomy or metyrapone treatment, followed by replacement with nonstress doses of B, completely prevented the alterations of iodothyronine metabolism induced by restraint. These results indicate that the stress-induced elevation of plasma glucocorticoids plays a key role in the pathogenesis of the low T3 syndrome in this model. The reduction in serum T3 may be accounted for by a reduction in T3 production by liver and kidney, adding support to the concept that these organs are an important source of plasma T3 in the rat.

The role of glucocorticoids in the stress-induced reduction of extrathyroidal 3,5,3′-triiodothyronine generation in rats.

Bianco AC, Nunes MT, Hell NS, Maciel RM. Endocrinology. March, 1987

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