Precocious decrease of chymotrypsinogen by thyroxine in the pancreas of suckling mice

Chymotrypsinogen activity in mouse pancreas gradually decreased from birth, reaching the adult level on day 20 after birth. In suckling mice the enzyme activity was decreased to 1/9 of the control value by injection of thyroxine. The activity was not affected by insulin, and was slightly increased, rather than decreased, by daily injection of hydrocortisone. The effect of thyroxine seemed to be direct, not due to modification of adrenal function.