Perinatal thyroid hormones and hepatic 5'deiodinase in relation to hatching time in weight-selected lines of chickens

Plasma thyroid hormone concentrations, liver weight, body weight, and hepatic capability for triiodothyronine (T3) production were measured at 8-h intervals on Days 19 through 21 in embryos and chicks from lines selected for high (HW) and low (LW) body weight at 8 wk of age. Embryos were categorized as not having entered the perinatal period (nonpipped, NP) or as being in one of three perinatal stages: embryos that had pipped into the air cell (AC), embryos pipped through the shell (TS), and chicks within 24 h of hatching (H). Both plasma thryoxine (T4) and T3 peaked at the TS stage in embryos of both lines. Embryos had higher concentrations of both hormones and hatched earlier in the LW than in the HW line. This association and evidence from other studies is suggestive that T3 may be playing a role in line differences in hatching time. Hepatic 5' deiodinase (5'D) activity was higher in LW than HW embryos and chicks at the AC, TS, and H stages although the opposite had been the case prior to the perinatal period (NP stage). Hepatic 5'D increased at each stage from AC to TS to H. When the patterns of liver growth and body weight were considered, this pattern and the differences between the lines were exaggerated further. Comparison of the plasma and 5'D patterns during the perinatal period suggests that substrate (T4) supply may be more important than 5'D capability of the liver in determining the pattern of T3 supply during some of these stages