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Changes in Plasma Thyroid Hormone Concentrations in Chronically Food-Restricted Female Rats and Their Offspring During Suckling^1,2,

Division of Cell and Molecular Biology, Florida Institute of Technology, Melbourne, FL 32901 ^* Division of Nutritional Sciences, Cornell University, Ithaca, NY 14853

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.

KEY WORDS: • thyroid hormones • food restriction • lactation • rats

¹ Supported by NIH research grants HD 21227 and 27068 (to L. V. Oberkotter) and HD 14953 (to K. M. Rasmussen).

² A preliminary report of these studies entitled "Changes in Thyroid Hormone Levels of Ad Libitum-Fed and Chronically Food-Restricted Rats and Their Offspring" was presented by L. V. Oberkotter and K. M. Rasmussen at the meeting of the International Society for Research on Human Milk and Lactation, November 11–15, 1990, Pacific Grove, CA.

Manuscript received 28 May 1991. Revision accepted 14 October 1991.