Serum concentrations of thyroid hormones and extrathyroidal thyroxine-5'-monodeiodinase activity during lactation in the rat.

Serum thyroxine (T4) and triiodothyronine (T3) concentrations and T4-5'-monodeiodinase activity in liver and kidney homogenates were studied in Sprague-Dawley rats during lactation. Blood and tissue samples were collected from nulliparous and pregnant rats 2 days before delivery and from lactating rats 0, 2, 7, 12, 19, and 26 days after delivery. Litters were removed from half of the mothers immediately after delivery to create a postpartum nonlactating group for study at the same times. Pregnant rats had lower serum T4 and T3 concentrations and higher liver T4-5'-monodeiodinase activity than nulliparous females. Low serum T4 persisted throughout lactation but further decrease in serum T3 was observed. Activity of T4-5'-monodeiodinase in liver and kidney homogenates was significantly reduced during lactation as compared to nonlactating rats. Serum concentration of T4 and T3 and T4-5'-monodeiodinase activity in liver and kidney returned toward control values 5 days after weaning (Postpartum Day 26). Our findings suggest that the relative hypothyroid state observed during lactation in rats is associated with a significant decrease in T4 to T3 conversion in the liver and kidneys.