Publications No publications reported this period. Progress to Outputs Studies in several species have demonstrated that thyroid hormones play a major role seasonal reproductive activity. In sheep, thyroidectomy or suppression of thyroxine concentrations by chemical thyroidectomy at specific times of the year lead to continuation of reproductive activity during the non-breeding season.
In view of this observation, supplemental administration of thyroxine may not modify the timing of the onset of the breeding season. This suggestion is by observation from published studies and suggests that thyroxine supplementation is an economic burden and an ineffective method to modify the timing of the breeding season.
Impacts This study will impact the broodmare industry in two ways. First, the study will determine whether thyroid hormones, and specifically the widespread supplementation of thyroxine, are beneficial to the onset of reproductive activity.
However, one recent study demonstrated that in the mare thyroidectomy during the summer and fall months did not result in the continuation of reproductive activity during the winter. The latter observation might suggest that thyroid hormones are not involved in the timing of the onset of seasonal anestrous.
T 3 seems to be only a minor source of T 2 ' production. The molar ratio of the two T 4 deiodination pathways T 4 to T 3 /T 4 to T 2 ' under these conditions is about.
Potential signals mediating the maintenance of reproductive activity during the non-breeding season of the mare. Reproduction. Suppl 59: 115-129. McManus CJ, Davison LA and Fitzgerald BP (2002). Effect of 2-deoxy-d-glucose on gonadotropins, prolactin and serum glucose concentrations in the mare.
The study was terminated in early April since both treated and control mares initiated the breeding season. The result demonstrates that hypothyroidism does not modify reproductive activity and at the time of year when treatment was initiated, did not advance the timing of the breeding season.
An effective dose of PTU to suppress thyroxine to low values was established and a. by a decrease in the rate of T4 deiodination to the.
To investigate whether thyroid hormones similarly play a role in the timing of the onset of the breeding season a study is in progress to evaluate whether suppression of thyroxine during late winter advances the onset of the breeding season.
Progress to Outputs This project examined the role of thyroxine in the timing of the onset of the breeding season in the mare. Specifically, the study investigated the hypothesis that chemical thyroidectomy during late winter and early spring would lead to an advancement of the onset of reproductive activity during the spring months.
The apparent K M of the reaction rT3 to T 2' is 2 10 8 M. The activities catalyzing the reactions T4 to T3, T4 to T2' and rT3 to T2' are found in the g pellet.
Thyroid parameters and deiodination were investigated in healthy dogs (group C) and in dogs with cutaneous lesions associated with hypothyroidism (group H) or with a low-T3 syndrome (group LT). The ability of the skin to convert T4 to T3 was impaired in both groups H and LT but not in the controls.
Compared to unrestricted mares, the proportion of mares exhibiting estrous cycles during the winter months was unaffected by treatment. However, feed restriction was accompanied by several adaptive mechanisms, notably elevated circulating concentrations of thyroxine and development of insulin resistance.
In this regard, in several species feed restriction leads to a cessation of reproductive activity. To investigate whether an energy surplus plays a role in the continuation of estrous cycles during the winter months a group of mares were subjected to feed restriction for 12 months.
Investigations on the deiodination of thyroxine (T4) to 3,3'-diiodothyronine (3,3'-T 2) in rat liver homogenate on ResearchGate, the professional network for scientists.
Furthermore, using this dose it is anticipated that circulating concentrations of thyroxine will steadily decline beginning within two weeks of initiation of treatment and reach nadir values within 3 weeks. This time period will permit adequate time to test the hypothesis that treatment with PTU advances the onset of the breeding season, since at this.
One explanation to account for these and similar observations is that the annual change in photoperiod is not the primary signal that leads to termination of the breeding season. To investigate the role of metabolic fuels as a modulator of seasonal rhythms of reproductive activity we examined the timing of the onset of anestrus in.
Anim Reprod Sci. Powell DM, Reedy SE, Sessions DR, Fitzgerald BP (2002). Effect of short-term exercise training on insulin sensitivity in obese and lean mares. Equine Veterinary Journal, Suppl. McManus CJ and Fitzgerald BP.