Effect of thyroactive materials upon plasma corticoids, pituitary prolactin, and mammary oxidative phosphorylation of lactating rats.

Abstract Approximately 10 times more thyroxine and triiodothyronine were fed to lactating rats than the normal secretion rate of thyroxine. The phosphorus toxygen ratio, an index of the efficiency of oxidative phosphorylation, of the mammary gland and liver mitochondria was not changed by triiodothyronine-thyroxine feeding. The mammary gland nucleic acid contents of triiodothyronine-thyroxine fed rats were reduced at Days 10, 15, and 20 of lactation. These levels of thyroactive materials did not affect the pituitary gland weight, but the adrenal size was greatly increased. The pituitary prolactin concentration appeared reduced on Day 10 of lactation, with no appreciable difference on Day 15 and an increase over the controls on Day 20 of lactation. In a subsequent experiment, feeding triiodothyronine and thyroxine increased concentration of plasma corticosteroids of the rats. The results indicate that hyperthyroidism impairs lactation by causing a hormonal imbalance.