Serum thyroid hormones and tissue 5'-monodeiodinase activity in acutely thyroidectomized newborn lambs

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.