Maturation of renal and hepatic monodeiodination of thyroxine to triiodothyronine and post-natal changes of serum thyroid hormones in young rats

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Abstract
Maturational changes of renal and hepatic 5''-monodeiodination of thyroxine (T4) and post-natal changes of serum thyroid hormone levels were investigated in young rats under 35 days of age. Renal T3 [triiodothyronine] generation in the 1-day-old rats was low, rose progressively to a level of more than 200% of the adult rats on days 21 and 28 and declined thereafter. In contrast, hepatic T3 generation increased from an initial low activity to a plateau after 7 days of age, which was 1 1/2 times higher than that of adult rats. Because of the extremely low value of serum T4 in the neonatal period, T3/T4 and r[reverse]T3/T4 ratios were elevated on day 7. The ratio of rT3/T4 decreased gradually and became stable after 21 days of age, while the T3/T4 ratio increased reciprocally to a peak on days 21 and 28, corresponding to the period of maximal activity of renal T4 monodeiodination. In addition to elevated hepatic T4 monodeiodination, renal conversion of T4 to T3 may play a significant physiological role during the period of enhanced T3 requirement for maturation.