Thyroxine in the Early Postnatal Period: Minimal Changes in Thyroid Function

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Abstract
Newborn rats were injected at the age of 3, 4, and 5 days with 30 or 100 /ig thyroxine and were studied at the age of 75-90 days. Slight growth impairment, as described by Eayrs and Holmes (1964), resulted. The impairment of hypophyseal and thyroid growth was small and inconsistent and only minimal changes in thyroid function were found. The accumulation of radioiodine, when calculated per mg thyroid, was only slightly lower in thyroxine-injected animals and the goitrogenic response to methylthiouracil feeding was the same in males, and nearly the same in females, injected in the early postnatal period with thyroxine, as in their control littermates. The experimental animals did not prefer food containing dried thyroid to normal food and dried thyroid feeding (self-selection) did not augment their weight gains. It is concluded that only a small and relatively unimportant decrease in TSH [thyroid-stimulating hormone]-thyroid function was produced by the given dose and number of injections of thyroxine.

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