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The influence of an intravenous injection of ovine prolactin on circulating levels of thyroid hormones was studied in Rhode Island Red embryos and chicks after hatching. In the chick embryo, 2 h after injection of 0·1 µg prolactin (on incubation day 19), serum tri-iodothyronine (T₃) increased threefold; after 10 or 100 µg prolactin (on incubation day 18) serum T₃ increased 15- to 25-fold. These profound increases were not observed in chicks after hatching. Serum concentrations and thyroid content of thyroxine (T₄) in embryos and chicks of all ages studied were not influenced by the prolactin injections. Maximal serum concentrations of reverse T₃ (rT₃) were found on incubation day 18 (110·25 ± 23·36 pmol/l; 71·66 ± 15·18 pg/ml; n = 8), whereas after hatching no rT₃ could be detected. An injection of 10 µg prolactin on day 18 depressed serum rT₃ after 2 h to 5·68 ± 3·20 pmol/l (3·69 ± 2·08 pg/ml; n = 8; P<0·001); the effect of 100 µg prolactin was less pronounced. After hatching, chronic administration of prolactin resulted in decreased serum levels of T₃, but not of T₄, and hypertrophy of the follicles in the thyroid gland. It is concluded that prolactin plays a major role in the maturation of embryonic thyroid metabolism by changing the T₄-5-monodeiodination into a T₄-5'-monodeiodination. The hypertrophy of the thyroid gland observed after hatching following prolonged prolactin administration may be due to decreased negative feedback of T₃ on the hypophysis.

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				G. Bédécarrats, D. Guémené, C. Morvan, U. Kühnlein, and D. Zadworny Quantification of Prolactin Messenger Ribonucleic Acid, Pituitary Content and Plasma Levels of Prolactin, and Detection of Immunoreactive Isoforms of Prolactin in Pituitaries from Turkey Embryos During Ontogeny Biol Reprod, September 1, 1999; 61(3): 757 - 763. [Abstract] [Full Text]