The regional hypothalamic distribution of type II 5'-monodeiodinase in euthyroid and hypothyroid rats.

The brain topographical distribution of type II 5'-monodeiodinase (5'D-II), which converts thyroxine (T4) to triiodothyronine (T3), was studied in euthyroid and hypothyroid rats. Low levels of 5'D-II activity were detected in the median eminence, but not in any other brain regions of euthyroid rats. The arcuate nucleus and median eminence were also the sites of highest 5'D-II activity in brains of hypothyroid rats. Under these conditions, the paraventricular nucleus contained almost no detectable 5'D-II, while intermediate enzyme activity was present in other medial basal hypothalamic sites.