Reciprocal effects of prolactin and thyroxine on the growth rate of rat pups from weaning to puberty.

Prolactin and thyroxine were added to the daily diet of rat pups from weaning to puberty (days 20--40). While prolactin significantly retarted the pups' growth, thyroxine stimulated growth and even compensated for the depression caused by prolactin. The significant retardation in the growth of the prolactin-treated pups around days 30--35 of age indicates that at that period they are extremely prolactin-dependent, thus confirming previous reports that exogenous prolactin inhibits its own endogenous secretion. On the other hand, the possible growth promotion caused by thyroxine is mild and does not initiate a feedback mechanism. This suggests that prolactin is indispensable for the normal development of the weaned, immature rat.