Effect of neonatal hyperthyroidism on maturation and learning in the rat

Abstract

1. 1. Rats made hyperthyroid by injections of l-triiodothyronine (T3) given during the first 24 days of life grew more slowly but matured more rapidly than normal litter-mates. The eyes opened earlier and the age at which the righting and placing reactions first appeared was accelerated. The effects of combining growth hormones with T3 were almost identical with those of giving T3 alone; those of giving T3 alone over the 2nd to 4th days of life were similar in trend but, except in the case of eye opening, did not attain statistical significance. 2. 2. Despite this accelerated maturation the animals given all three treatments performed significantly less well, when adult, on the Hebb-Williams closed field test. 3. 3. Maturation of innately organized behaviour was not advanced in animals in which treatment was delayed until the 14th day and the performance of these rats on the closed field test was equal to that of the controls. 4. 4. These findings are discussed in the light of growth and endocrine changes reported elsewhere.