1. The effect of glucocorticoids on the deiodination of thyroxine (T4) to 3,5,3′-tri-iodothyronine (T3) was studied in rat renal tubules prepared by collagenase digestion.

2. In short-term (6 h) experiments, cortisol and dexamethasone inhibited the conversion of T4 into T3 at concentrations of 2 × 10-4 mol/l and 2 × 10-5 mol/l respectively. The inhibition by cortisol and dexamethasone was time dependent and was prevented by actinomycin D and progesterone, suggesting that the inhibition is mediated by an effect on nuclear transcription dependent on binding to glucocorticoid receptors.

3. In long-term (16 h) experiments, cortisol and dexamethasone inhibited T4 to T3 conversion by the tubules at concentrations of 1 × 10-12 mol/l and above. In addition, physiological concentrations of corticosterone (1 × 10-8 mol/l) were able to decrease T3 generation from T4.

4. Our data provide strong evidence that physiological concentrations of glucocorticoids are able to affect T3 production from T4 directly and suggest that they may be important regulators of T4 deiodination.

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