Sources and quantity of 3,5,3'- triiodothyronine in several tissues of the rat

which is an important source of intracellular 3,5,3'-triiodothyronine (T3) in several rat tissues, has been subject of recent investigations. In the present study the regulation of this phenomenon in vivo was inves-tigated in various peripheral tissues of the rat. Intact euthyroid and radiothyroidectomized (Tx) rats received a continuous intravenous infusion of [125I]T4 and ['31I]T3 until isotope equilibrium was attained. In addition to the labeled iodothyronines, Tx rats received a continuous intravenous infusion of 0.2 or 1.0,g carrier T4/100 g body wt per d, to create hypothyroid or slightly hypothyroid conditions, respectively. After the animals were bled and perfused the contribution of T3 derived from local conversion of T4 to T3 [Lc T3(T4)] to the total T3 in homogenates from several tissues an