AJP - Endocrinology and Metabolism, Vol 251, Issue 2 151-E155, Copyright © 1986 by American Physiological Society
Serum thyroid hormones and tissue 5'-monodeiodinase activity in acutely thyroidectomized newborn lambs
D. H. Polk, S. Y. Wu and D. A. Fisher
After either total thyroidectomy or sham operation in full-term fetal
sheep, fetuses were delivered and serial blood samples were obtained for
measurements of thyroxine (T4), triiodothyronine (T3), and catecholamines.
Despite comparable serum T4 values (T4 means +/- SE, sham 7.1 +/- 0.6
micrograms/dl, thyroidectomized 6.8 +/- 0.7 micrograms/dl at 60 min after
birth), serum T3 values were lower in the thyroidectomized animals (T3
means +/- SE, thyroidectomized 39 +/- 4.8 ng/dl, sham 153 +/- 20.1 ng/dl at
60 min after birth). Four hours after birth, the animals were killed with
an intravenous overdose of barbiturate. Brain, thyroid, liver, kidney, and
brown adipose tissues were dissected and analyzed for thyroxine
5'-monodeiodinase (5'-MDI) activity in vitro. 5'-MDI activity was
comparable in all tissues from sham-operated and thyroidectomized lambs.
Plasma epinephrine and norepinephrine concentrations, mean arterial
pressure, mean pulse, rectal temperature, and arterial blood gas values
were similar in the two groups of animals. These data support the
hypothesis that the thyroid gland is the major source of T3 for the T3
surge in the immediate newborn period. They also indicate that the neonatal
T3 surge has limited immediate metabolic significance in euthyroid
newborns.
