CONTRIBUTION OF 3,5,3'-TRI-IODOTHYRONINE PRODUCED LOCALLY FROM THYROXINE IN SEVERAL MATERNAL TISSUES OF THE NEAR-TERM PREGNANT RAT

3,5,3'-Tri-iodothyronine (T-3) is produced by the thyroid and locally by monodeiodination of thyroxine (T-4), in the peripheral tissues. Dur ing pregnancy the thyroid hormone status in rats is altered: plasma an d tissue levels of T-4 and T-3 are decreased. We investigated the effe cts of pregnancy on the contribution of T-3 produced locally in the ma ternal tissues by administering a continuous infusion of [I-125]T-4 an d [I-131]T-3. The transport of T-4 to almost all maternal organs dimin ished. Less T-3 was transported from the plasma to brown adipose tissu e (BAT), liver, kidney and pituitary, in contrast to the ovary where a n increase was found. In BAT and brain the amount of locally produced T-3 decreased, despite the known increase in deiodinase type II activi ty in the brain while in liver the contribution of locally produced T- 3 remained constant, despite a known increase in deiodinase type I act ivity during pregnancy This discrepancy between deiodinase activities and locally produced T-3 tan be explained by an insufficient availabil ity of T-4. Thus, we conclude that in the rat a decrease in maternal T -4, concentration, together with the transport of T-4 to the feto-plac ental compartment, results indirectly in a diminished availability of T-4 in the maternal organs.