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Postnatal development of pituitary-thyroid function in male and female rats: comparison of plasma and thyroid T3 and
Insights
Thyroid hormone levels (T4, T3) and thyroid-stimulating hormone (TSH) in young rats show distinct developmental patterns. These thyroid hormone profiles, including T4 and T3 concentrations, change significantly during the first 45 days of life.
Area of Science:
- Endocrinology
- Developmental Biology
- Thyroid Research
Background:
- Thyroid hormones are crucial for mammalian development and metabolism.
- Understanding the postnatal development of thyroid function is essential for identifying potential disruptions.
Purpose of the Study:
- To characterize the developmental changes in plasma and intrathyroidal thyroid hormones (T4, T3) and TSH in rats from birth to 45 days.
- To investigate potential sex differences in these hormonal profiles during early life.
Main Methods:
- Measurement of plasma T4, T3, and TSH in male and female rats from birth to 45 days.
- Analysis of total and radiolabeled T4 and T3 in thyroid digests.
- Rats were maintained on a high-iodine diet throughout the study.
Main Results:
- Plasma T4 increased linearly postnatally, peaking around day 15.
- Plasma T3 levels rose significantly after day 10.
- Plasma TSH exhibited a bimodal pattern with early and later peaks, and sex differences emerged by day 30.
- Intrathyroidal T3 and T4 concentrations and ratios increased postnatally, with labeled T3 detectable after day 7.
Conclusions:
- Rat thyroid hormone and TSH levels undergo significant postnatal development with distinct temporal patterns.
- Sex differences in TSH emerge by postnatal day 30.
- Intrathyroidal T3/T4 ratios correlate with plasma ratios, indicating coordinated thyroid hormone synthesis and release during development.
Abstract:
Plasma T4, T3, and TSH, and total and 131l-labeled T4 and T3 in thyroid digests were determined in male and female rats from birth to 30--45 days postnatally. Mothers and offspring were fed a high-iodine Purina diet throughout. In both sexes, plasma T4 was less than 1 microgram/100ml at birth but rose linearly to a peak of 6--7 micrograms/100 ml at day 15. Plasma T3 remained low at 30--40 ng/100 ml until day 10, then rose steeply to 80--100 ng/100 ml at day 20--30. Plasma TSH had a bimodal distribution with an early peak in the intial 1--2 weeks of life and a second rise after day 30. Adult pattern sex differences in plasma TSH appeared by day 30. Labeled T3 was undetectable in thyroid digests until after day 7. Both labeled and stable T3 and T4 concentration and T3/T4 ratios rose progressively during the first 2 weeks of life. After day 7, stable and labeled intrathyroidal T3/T4 ratios were significantly correlated, but the latter was always higher than the former for any given age.