Abstract
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 the dam may influence thyroid status and growth of offspring.
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