[Correlation between the thyroxine, 17-beta-estradiol and luteinizing hormones in the systemic circulation and the rectal and vaginal temperature in heifers and cows after administration of cloprostenol].

Changes in the concentration of 17beta-oestradiol (E2) and luteinizing hormone (LH) and in the rectal and vaginal temperatures (RT and VT) were studied along with the changes in thyroxine concentration (T4) in three cows and three heifers in the luteal stage of the cycle; the animals had been intramuscularly treated with 2 ml Oestrophan (cloprostenol). The closeness of the correlation between T4 and the remaining parameters under study was determined by the calculation of the correlation coefficient and statistical significance. The concentrations of T4 before and during cloprostenol administration were high in comparison with the post-treatment levels. E2 concentrations at cloprostenol administration time were much higher than those recorded before administration. After treatment the concentrations of T4 and E2 sank. The first E2 peak, recorded in the 44th hour, was immediately followed by a marked drop of E2 as well as T4, the lowest values of both being recorded in the 52nd and 56th hour. The second peak of E2 in the 60th hour was followed by a slow but steady decrease. The rise of the concentration of T4 after the 56th hour was slow and reached the peak in the 74th hour; after a partial decrease no further marked changes in concentrations were recorded. LH concentrations rose at a slow rate to reach the peak in the 64th hour. After a rapid decline they reached the pre-peak value in the 70th hour. The lowest RT and VT levels were recorded in the 54th and 94th hour. It can be assumed on the basis of the behaviour of the hormones and the evaluation of their correlations that thyroidal hormones are involved in the stimulation of the synthesis of ovarial oestrogens which tend, after their synthesis, to eliminate from circulation the T4 as well as their own levels and thereby to influence (as feedback) the stimulation of T4 synthesis, their own synthesis, and LH.