Effects of Thyroxine and Thiouracil on Hypothalamic PIF and Pituitary Prolactin Levels ∗

Summary

The effects of thyroxine or thiouracil on pituitary prolactin and hypothalamic prolactin inhibiting factor (PIF) levels were studied in rats. Thyroxine injected in doses of 5 or 25 μg/100 g of body weight daily for 21 days significantly increased AP prolactin content and concentration despite a significant decrease in AP weight. Thiouracil feeding (0.1%) for 21 days reduced pituitary prolactin content and concentration although these decreases were not significant. Neither thyroxine nor thiouracil had any effect on hypothalamic PIF content. These results suggest that thyroxine stimulates and thiouracil depresses prolactin secretion by a direct action of thyroxine or of the lack thereof on the AP. They do not provide any evidence that thyroxine or thiouracil influence prolactin secretion via the hypothalamus.