Summary
Iodothyronine-5′-deiodinase activity (I-5′-DA) was measured in the progenies of control
rats, hypothyroid (Tx) rats, and hypothyroid treated with ovine GH (Tx + GH) during
gestation. The enzyme was measured in cerebral cortex and cerebellum at 22 days gestation
and at 5, 10, 30 and 60 days postpartum. In addition, the pituitary I-5′-DA was assessed
in the postnatal animals. The experiments were undertaken because the tissues of the
progenies of rats that were hypothyroid during pregnancy appeared in many ways to
resemble those of hypothyroid animals, even at ages when serum thyroxine (T4) and triiodothyronine (T3) levels were normal. It was found that the progenies of Tx mothers had low liver
5′-deiodinase activities. This is a likely cause of the low serum T3 levels with normal T4 levels seen in these progenies in the neonatal period. Cerebral and cerebellar 5'-deiodinase
activities were low in these progenies during the thyroid hormone-dependent perinated
period of brain development. The progenies of GH-treated Tx dams had higher enzyme
activities than the progenies of untreated Tx dams. These pups from GH-treated Tx
mothers have been shown previously to have significantly less neurological impairment
than the progenies of untreated Tx mothers. As most of the brain intracellular T3 is produced in situ, a functional thyroid deficiency could result from such a 5′-deiodinase deficiency.
As the deiodinase deficiency was still seen in the progenies of Tx mothers at 60 days
of age, such a deficiency could explain why, even though serum T4 and T3 levels were normal, brain metabolism was in many ways characteristic of hypothyroidism.
Pituitary I-5′-DA of the progenies of Tx mothers was not significantly different from
that of controls. These results do not explain the observation that serum TSH levels
are high and T4 and T3 levels are normal in the 60 day old progenies of Tx mothers.
Key-Words
Hypothyroid Rats
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Iodothyronine-5′-Deiodinase
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Cerebrum
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Cerebellum
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Liver
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Pituitary
