Thyroid hormones and 5'-deiodinase activity in neonatal undernourished rats.

Abstract

Undernutrition was induced in rats submitted to food restriction from the fetal stage, and malnutrition was continued after birth until 70 days of life. Body weight was decreased to less than 50%. Plasma T4 and T3 and pituitary TSH content were determined between 8-70 days of life. In control rats, plasma T4 and T3 reached a maximum at 14 and 35 days of life, respectively, and TSH pituitary content at 45 days of life. In undernourished rats, after 8 days of life, plasma T4 and T3 and pituitary TSH content were decreased to about 50% or less, and the pattern of sequential changes observed in control rats was absent or modified. T4 and T3 concentrations were measured in heart, liver, and brain in the fetus (22 days old) and 8, 14, and 23 days after birth, as well as liver and brain 5'-deiodinases (5'D). Hepatic 5'D type I was always decreased in undernourished rats from 8-70 days after birth. Liver and heart T4 and T3 concentrations were decreased in 14-day-old undernourished rats as well as brain T3. Brain 5'D type II was decreased at 8 and 14 days, and total brain 5'D activities at 8 days. These changes occurred during the critical period for brain development (7th to 20th day) during which most processes of myelination take place and T3 brain normal levels are required.

Influence of type II 5′ deiodinase on TSH content in diabetic rats

“…A relevant fact to consider in the thyroid axis regulation in undernutrition and diabetes, is the presence oflow circulating TSH levels along with reduced thyroid hormones (1,13,26,30,34) contrary to what occurs in hypothyroidism. Pituitary TSH levels depends mainly on a balance between the stimulatory action of TRH on TSH synthesis and secretion and the inhibitory role of T3 on TSH secretion and TRH synthesis (4,10).…”

“…Among all "euthyroid sick syndromes", undernutrition and diabetes show a common pattern at the cellular level: cell starvation due to the lack of nutrients or to their reduced utilization by the absence of insulin. Both, undernutrition and diabetes, considered as "low T3 syndromes", induce a significant reduction of serum T3 and T4, together with a fall in pituitary and plasma TSH (1,13,26,30,34) as opposed to the usual increase in plasma TSH observed in hypothyroidism. Pituitary a and~TSH and GH mRNA levels are decreased in restricted feeding, fasting and diabetes in rats (29).…”

“…These alterations in the regulation of the thyroid axis are similar in both syndromes, but several differences have also been established. The hepatic T4 to T3 conversion is impaired in diabetes and fasting (1,2,12,24,25,27,34). Diabetes induces a different adaptive pattern at the neuroendocrine level with a reduced hypothalamic TRH content (13) and release (7,30) as compared to undernutrition (26).…”