Posthatch growth and development of a circadian rhythm for thyroid hormones in chicks incubated at different temperatures.

Rhode Island Red eggs were incubated at 33.8 and 376.8 C from day 17 until hatching under a photoperiod of 12L:12D. After hatching, the same light schedule was maintained, and all chicks were raised at 25 +/- 1 C and 50% relative humidity. Infrared lamps were used up to 7 days. Pipping and hatching occurred significantly later in chicks incubated at 33.8 C, and the interval between events was prolonged. Chicks of the 33.8 C group had a lower body weight on day 3 posthatch, but relative growth was accelerated from the 2nd week on, resulting in equal weights on day 36 for males and day 43 for females. Circadian mean serum levels of triiodothyronine (T3) during a 24-hr period (blood sampling every 4 hrs) were positively correlated with relative growth rate in both temperature groups, whereas thyroxine (T4) correlated with weight and age. Body weight of females remained lower from day 9 (33.8 C) and 22 (37.8 C) on. In the 37.8 C group, a significant fit of serum T4 to a sinusoidal curve (observed during a 24-hr cycle) was present for both sexes from day 16 on; this was true from day 32 on in the lower incubation temperature group. The acrophases of this rhythm occurred at about 0500 hr (1100 hr after onset of the dark period) and did not differ between temperature groups of sexes. A significant fit to a sinusoidal curve for the T3 data was only possible for both sexes and temperature groups on days 112 and 128 with an acrophase at about 1700 hr. Differences in growth rate between temperature groups can be related to differences in mean serum levels of T3. Differences in growth rate between sexes were not parallelled by differences in levels of thyroid hormones or acrophases, but males did not have a higher amplitude in the sinusoidal T4 rhythm compared to females. This difference was more pronounced in the 37.8 C group.