Effect of lactational intensity on extrathyroidal 5'-deiodinase activity in rats

Concentrations of thyroxine and triiodothyronine in serum and 5'-monodeiodination activity in liver and kidney were studied in lactating Sprague-Dawley rats with different litter sizes. Litter sizes were adjusted at birth to 0 (postpartum nonlactating group), 4, 8, 12, and 16 pups per lactating rat. Serum and tissue samples were collected from lactating rats and pups on d 12 of lactation and from 6 nulliparous females. Nulliparous and postpartum nonlactating rats did not differ in serum thyroxine, triiodothyronine concentrations, or in 5'-deiodinase activity in liver and kidney. As litter size increased, maternal serum thyroxine, triiodothyronine concentrations, and 5'-deiodinase activity in liver and kidney decreased. Growth rate of pups was inversely correlated with litter size and positively correlated with concentration of serum thyroxine and liver 5'-deiodinase in mothers. In pups, serum triiodothyronine concentrations decreased as litter size increased, but serum thyroxine concentrations were not affected. Results suggest a relationship between the hypothyroid status of lactating rats and suckling intensity. The thyroid status of the dam may influence thyroid status and growth of offspring.