Correlation between plasma thyroxine levels and corticosterone-binding activity in intact, ovariectomized, and...

Plasma levels of total thyroxine (T4) and corticosteroid-binding activity (CBA) were determined in intact, ovariectomized, or ovariectomized, estrogen-treated rats during treatment with propylthiouracil (PTU) and following recovery from PTU. Propylthiouracil treatment reduced the mean plasma T4 concentration to 9 ng/ml and the mean plasma CBA to 39% of normal, while following recovery from PTU, the mean values for plasma T4 and CBA were 56 ng/ml and 108%, respectively. There was a strong (r equals plus +0.89) relationship between plasma levels of T4 and CBA as determined by linear regression analysis. Ovariectomy had no effect on either plasma T4 or CBA in euthyroid animals but significantly decreased CBA in PTU-treated, hypothyroid animals. The results indicate a robust influence of thyroid activity on plasma CBA and suggest a possible interaction between the thyroid and ovary in control of CBA.