Research Article| Volume 73, ISSUE 1, P135-143, January 1969

Accelerated maturation and persistent growth impairment in the rat resulting from thyroxine administration in the neonatal period

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      The immediate and long-term effects of thyroxine administration to neonatal rats have been studied. As compared with control animals the rats given 216 μg thyroxine during the first 2 weeks of life showed accelerated maturation as manifested by earlier eye opening and greater early locomotor activity. The treated animals displayed a diminished growth rate which persisted for the 20 weeks of observation, and the growth rate was not altered by replacement thyroxine. Sexual maturity was demonstrated by the experimental animals at approximately the same age but body weight was significantly lower than that of control animals. Reproductive performance differed only in that the rats given thyroxine in infancy had fewer pups per litter. Evidence of a permanent alteration of thyroid status was provided by a decrease in the relative weight of the thyroid gland persistent for at least 18 weeks after the last thyroxine injection. Eight weeks after completion of thyroxine treatment the experimental animals had identical 18 hour thyroidal uptake of 131I, decreased serum protein-bound 131I, and decreased rebound in thyroidal 131I uptake following methimazole treatment, as compared with control animals.
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