L-thyroxine, cortisol, and diet affect the postnatal development of the facial part of the skull in developing rats.

In rats, the effects of L-thyroxine (T4; 0.2 micrograms/g body weight), hydrocortisone (HC; 5 micrograms/g body weight) and mastication on the postnatal development of the facial part of the skull were examined. Administration of T4 and/or HC to rats for 5 consecutive days from day 5 after birth induced a precocious increase in the ratio of the facial length to the head length. The effects of these hormones decreased with the progression of weaning, and after the 20th postnatal day, the hormones had no effects on the postnatal development of either (facial and head) part of the skull. Moreover, these hormones played an additive role in the growth of facial part. On the other hand, continuous suckling from birth to the 30th postnatal day caused a significant decrease in the ratio of the facial length to the head length. These results suggest that increases in the circulating levels of T4 and HC in suckling rats and mastication in the weaning period are involved in development of the postnatal development of the facial part of the skull.