It has been reported that T4 secretion by the thyroid gland may be involved in photoresponses in birds and in mammals. T4 could be necessary for the onset and continuation of photorefractoriness, since increased levels of T4 were associated with occurrence of photorefractoriness in starlings and Japanese quail. Sustained high levels of T4 in immature turkeys prior to 23 wk of age were observed in the LL group in trial 2 in the present study. After LD photostimulation of SL birds, T4 concentrations decreased as the birds matured. These data suggest that T4 might somehow be involved in regulation of reproduction in male turkeys, with high T4 levels prior to 23 wk of age associated with maintenance of the juvenile state. buy birth control online
In conclusion, age-related increases in both LH and T occurred independently of photoperiods at about 18 wk of age in male turkeys. Sexual maturation was delayed in birds maintained under SD, but these birds did respond to LD photostimulation at 29 wk by rapid increases in LH and T. Pulsatile secretory patterns of LH and T were detectable in sexually immature turkeys as well as in mature turkeys. The baseline levels of LH and T were much higher in sexually mature turkeys. The physiological importance of differing pulsatile secretory patterns of LH and T on regulation of reproduction needs to be further studied. Photostimulation of turkeys at a very early age was associated with enhanced secretion of LH and T after 18 wk of age and with precocious semen production. The plasma T4 concentration may be related to the onset of sexual maturation.