The effect of perinatal thyroxine treatment on the development of the plasma corticosterone diurnal rhythm.

Thyroxine (10 microgram/g b.w., s.c.), injected either on days 2, 3 and 4 or days 8, 9 and 10, resulted in a higher a.m.-p.m. plasma corticosterone difference on day 23, compared to the vehicle-treated controls. In a separate experiment the same dose of thyroxine on days 8, 9 and 10 caused an earlier appearance of significantly higher p.m. than a.m. plasma corticosterone levels (day 15 vs day 18 in vehicle-treated controls), and earlier eye-opening (12.38 +/- 0.11 thyroxine vs 15.36 +/- 0.14 days, vehicle). These results suggest that thyroxine influences the maturation of those neural structures which participate in the regulation of the diurnal rhythm of ACTH secretion and, as a consequence, in that of corticosterone production.