Changes in plasma thyroid hormone concentrations in chronically food-restricted female rats and their offspring during suckling.

To examine the joint effects of lactation and dietary restriction on thyroid function, we studied chronically food-restricted rat dams at conception and dams and their pups during the nursing period compared with controls that had free access to food. As expected, both dietary restriction [plasma thyroxine and 3,3',5-triiodothyronine (T3) values were lower and reverse T3 (rT3) values were higher in food-restricted than in control rats] and stage of lactation (T3 values decreased in both groups) had independent effects on maternal thyroid function. There also were interactive effects: rT3 values decreased during lactation among the food-restricted dams but remained constant among controls. Maternal dietary restriction also did not have a consistent effect on pup thyroid function during the nursing period: in offspring of food-restricted compared with control dams, plasma thyroxine values were lower only at d 14, T3 values were lower at d 7-21, and rT3 values were higher from birth to d 14. These data demonstrate that dietary restriction and lactation have synergistic negative effects on thyroid status in lactating rats and their pups and that thyroid hormone ontogeny is retarded in the offspring of food-restricted rats. These hormonal effects may contribute to the impaired milk production of food-restricted rats and to altered growth characteristics of their offspring.