Transport Capacity of Median Eminence

The capacity for and mechanism of transport of thyroxine (T4) by the median eminence has been examined in vivo. Following an intravenous injection of 125I-thyroxine, the median eminence accumulates more hormone than either the anterior or posterior pituitary, although it does not achieve a tissue:plasma ratio greater than 1. The uptake of T₄ by the median eminence is greater than normal in rats treated with methimazole and in animals (normal or hypophysectomized) treated with exogenous TSH. In reserpine-treated animals, the uptake of T₄ by the median eminence is reduced. Following the infusion of T₄ into the lateral ventricle, a 5-fold greater uptake is seen in the median eminence; thyroxine accumulates in the anterior pituitary and achieves a tissue:plasma ratio 2.5 times greater than when the hormone is administered intravenously. When ouabain is infused intravenously immediately before the T4, uptake of the hormone by the median eminence and delivery to the anterior pituitary are both markedly reduced. Autoradiographic studies also reveal the high accumulation of thyroxine in the median eminence; the grain concentration in this tissue was greater than any other region of the basal hypothalamus. Grain localization clearly associates thyroxine with ependyma of the median eminence.