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.

Effect of iopanoic acid on basal and thyrotropin-stimulated thyroid hormone levels in suckling rat pups.

We proposed that basal and thyrotropin (TSH)-stimulated thyroid hormone levels of rat pups would be altered in the presence of iopanoic acid (IA), a radiographic contrast agent which competitively inhibits T4-to-T3 conversion, and that the nature of these changes would further depend upon the route of TSH administration in a manner distinct from that reported in adults. To test this hypothesis, litters from 24 Sprague-Dawley female rats were adjusted to 8 pups each. On day 5, 80 pups received IA (2.

Suckling, but not formula feeding, induces a transient hyperthyroxinemia in rat pups.

To assess the effect of breast feeding on circulating thyroid hormone levels in the suckling neonate, we examined serum concentrations of T4, T3, and rT3 by RIA in 12- and 15-day-old rat pups fasted for 3 or 6 h before, at, and hourly for 3 or 4 h after suckling. In both age groups, serum T4 levels decreased significantly in response to the 6-h fast; no significant differences were observed after 3 h. T3 levels in 15-day-old (but not 12-day-old) rats were likewise significantly depressed, and rT3 levels were higher relative to those in the prefast group.

Developmental changes in rat thyroid responsiveness to thyrotropin administered by the subcutaneous and peroral route.

It has been demonstrated that orally administered thyrotropin (bovine, bTSH) evokes an increase in circulating T4 and T3 levels in 15-day-old suckling rat pups, but not in weaned animals. Because the feedback mechanisms of the hypothalamo-pituitary-thyroid axis change dramatically during the neonatal period, we chose to examine the efficacy of exogenous bTSH in eliciting a thyrostimulatory response via the subcutaneous (sc) or peroral (po) route in rat pups at 5, 8, 12, and 15 days postpartum. Suckling pups were divided into four groups and received one of the following: (i) 2 IU bTSH/100 g body wt administered sc; (ii) distilled H2O (dH2O) sc; (iii) 2 IU bTSH/100 g body wt given po; (iv) dH2O po.