We have found thyroxine 5’-deiodinase in rat posterior pituitary. The isozyme pattern was different in intermediate and neural lobes. The former contained only type I enzyme. In the neural lobe, in euthyroidism only type I was found while in hypothyroidism type II was a prominent form. Since type II 5’-deiodinase has a high affinity for thyroxine and is a more tissue-specific isozyme than type I, our results suggest there may be a previously unrecognized role of thyroid hormone in posterior pituitary physiol