Variations in thyroidal activity during estrous cycle and natural breeding season in Markhoz goat breeds

The aim of this study was to determine the profile of changes in T3, T4 and TSH levels during breeding season and estrous cycle in Markhoz (Angora) Goats. Whereas the peaks of T3 were recorded in January, concentrations of T4 and TSH were highest in October. Variations in T3 and TSH concentrations among the different months of experiments period were not significant, although T4 concentration was significantly higher during September, October and November in comparison to December and January. Weekly variations in serum T4 and TSH concentrations were directly correlated to the changes in photoperiod and temperature. Monthly variation in serum T3 and TSH did not have a significant (p>0.05) relationship with the changes in photoperiod and temperature, but there was a highly significant positive relationship between serum T4 and temperature. This study showed that T4 concentration was high in the early phase but decreased in the late phase of the breeding season, but T3 and TSH concentrations varied markedly from week to week. However, it appears that weekly rhythms are controlled by photoperiod and temperature because, changes in these factors resulted in different profiles of both T4 and TSH, but there was not any correlation between T3 and those factors.