Changes in thyroxine monodeiodination in rat liver, kidney and placenta during pregnancy

Monodeiodination of thyroxine (T4) was studied in the liver, kidney and placenta of pregnant rats. Age matched female non-pregnant and pregnant Sprague-Dawley rats on the 7th, 14th, 17th and 21st days of gestation were used. The 800 X g supernatants of tissue homogenates (protein 1 mg/tube) were incubated with 1 microgram of stable T4 in the presence of 5 mM dithiothreitol (DTT) at 37 degrees C for 60 min at pH 7.5. Net triiodothyronine (T3) generation from T4 in rat liver homogenates on the 7th day of gestation was significantly lower than that in the non-pregnant rat. Thereafter it increased, but values on the 14th, 17th and 21st days of gestation were not significantly different from those obtained in the non-pregnant rat. Net renal T3 generation from T4 on the 14th day was significantly lower than that in the non-pregnant rat. It was increased thereafter and the values at the 17th and 21st days of gestation were not significantly different from those in the non-pregnant rat. Net reverse T3 (rT3) generation from T4 in the placenta rose from the 14th to the 17th day and then dropped by the 21st day and the value at the 17th day was significantly higher than those at the 14th and 21st days of gestation. These results indicate that 1) both T4 outer-ring monodeiodination in the pregnant rat liver and kidney, and T4 inner-ring monodeiodination in the placenta show significant variation with the progress of gestation; 2) the time course of the T4 outer-ring monodeiodination in pregnant rat liver and kidney is completely different from T4 inner-ring monodeiodination in the placenta.