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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