Suckling, but not formula feeding, induces a transient hyperthyroxinemia in rat pups
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1Department of Biological Sciences, Florida Institute of Technology, Melbourne 32901.
Insights
Neonatal rat pups experience decreased thyroid hormones (T4, T3) with fasting. Breastfeeding rapidly restores these levels, unlike formula feeding, highlighting the importance of milk intake for thyroid health in young rats.
Area of Science:
- Endocrinology
- Neonatal Physiology
- Comparative Medicine
Background:
- Thyroid hormones are crucial for neonatal development and metabolism.
- Suckling and milk intake significantly influence neonatal physiological parameters.
- Understanding the impact of feeding on thyroid hormone status is vital for neonatal care and research.
Purpose of the Study:
- To investigate the effect of breastfeeding on circulating thyroid hormone levels (T4, T3, rT3) in neonatal rat pups.
- To compare the hormonal response to breastfeeding versus formula feeding after a period of fasting.
- To determine the influence of fasting duration on thyroid hormone concentrations in suckling rats.
Main Methods:
- Serum concentrations of T4, T3, and rT3 were measured using radioimmunoassay (RIA).
- 12- and 15-day-old rat pups were subjected to fasting periods of 3 or 6 hours.
- Hormone levels were assessed before, during, and at hourly intervals after suckling or formula feeding.
Main Results:
- A 6-hour fast significantly decreased serum T4 levels in both age groups; T3 levels decreased in 15-day-old pups.
- Breastfeeding rapidly increased T4 and T3 levels, leading to transient hyperthyroxinemia in 15-day-old pups.
- Formula feeding did not restore thyroid hormone levels to those observed in pre-fasted or suckled pups.
Conclusions:
- Neonatal rat pups aged 12-15 days can become hypothyroxinemic after 3-6 hours of milk deprivation.
- Breastfeeding effectively reverses hypothyroxinemia and induces transient hyperthyroxinemia.
- The study underscores the critical role of maternal milk in maintaining thyroid hormone homeostasis in suckling neonates and highlights the importance of study design regarding fasting periods.
Abstract:
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 increased through 1 h to levels significantly higher than those in the prefast group. After refeeding (0 h) of 3-h fasted pups, serum T4 levels increased significantly over fasted levels in suckled 12- and 15-day-old rats, as did T3 levels in 15-day-old pups, through 2 h postsuckling. In contrast, thyroid hormone levels of pups gavage-fed a commercial formula preparation at no time exceeded those of the prefast groups at either age, despite highly comparable stomach weights for both formula-fed and suckled pups at the time of refeeding. We conclude that 1) rat pups between the ages of 12 and 15 days postpartum become hypothyroxinemic between 3 and 6 h of milk deprivation, and 2) this condition is reversed by significant increases in T4 and T3, rendering the animal transiently hyperthyroxinemic after suckling, but not in response to formula feeding. Our data further demonstrate the critical nature of study designs involving fasts (either planned or incidental) of suckling rats in light of the observed pronounced variations in their thyroid hormone status relative to time postsuckling.