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. After a 90-min suckling period, serum total and free T4 concentrations of 12-day-old pups increased 1 h postsuckling to levels significantly higher than those in the fasted animals; maximum levels were achieved at 2 h. T3 levels were unchanged over this period. In contrast, elevated total and free T4 and T3 levels and a reciprocal decrease in rT3 concentration, were evident immediately after the 90-min suckling period (0 h) in 15-day-old rat pups and incr...