Changes in Thyrotropin Messenger RNA and Circulating Hormone Levels during Induced Molting of the Hen (Gallus domesticus)

Levels of pituitary thyrotropin messenger RNA and circulating hormones during molting induced by feed and water deprivation were determined by reverse-transcription polymerase chain reaction and radioimmunoassays. Plasma levels of luteinizing hormone and estradiol decreased from the next day (Day 1), and progesterone from Day 3, of deprivation treatment, and were associated with the cessation of egg laying on Day 4. The plasma thyroxine level of experimental hens increased on Day 5 and remained high until Day 17, whereas the plasma concentration of triiodothyronine was lower than the control level throughout the experimental period. The messenger RNA level of the pituitary thyrotropin β subunit of experimental hens decreased markedly at first, but then increased showing a peak on Day 3 followed by a gradual decrease until Day 17. The serum prolactin level decreased on Day 3 and remained low until Day 17. These results suggest that (1) a decline in hormones associated with egg laying is followed by activation of the pituitary-thyroid axis, (2) thyrotropin triggers the increase in thyroxine but is not necessarily responsible for the maintenance of the high thyroxine level, and (3) prolactin might be involved in molting induced by feed and water deprivation.