3,5, 3'-Tri-iodothyronine(T3)is produced by the thyroid and locally, by monodeiodination of thyroxine (T4), in the peripheral tissues. During pregnancy the thyroid hormone status in rats is altered: plasma and tissue levels of T4 and T3 are decreased. We investigated the effects of pregnancy on the contribution of T3 produced locally in the maternal tissues by administering a continuous infusion of [125I]T4 and [131I]T3. The transport of T4 to almost all maternal organs diminished. Less T was transported from the plasma to brown adipose tissue (BAT), liver, kidney and pituitary, in contrast to the ovary where an increase was found. In BAT and brain the amount of locally produced T3 decreased, despite the known increase in deiodinase type II activity in the brain while in liver the contribution of locally produced T3 remained constant, despite a known increase in deiodinase type I activity during pregnancy. This discrepancy between deiodinase activities and locally produced T3 can be explained by an insufficient availability of T4. Thus, we conclude that in the rat a decrease in maternal T4 concentration, together with the transport of T4 to the feto-placental compartment, results indirectly in a diminished availability of T3 in the maternal organs.
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