EFFECT OF THYROXINE ON THE INACTIVATION OF THYROTROPHIN‐RELEASING HORMONE BY RAT AND HUMAN PLASMA

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Abstract
The inactivation of synthetic [3H]thyrotrophin‐releasing hormone (TRH) by plasma was studied in rats treated with propylthiouracil (PTU) alone or with PTU and thyroxine. 48 h after the onset of treatment with thyroxine, the capacity of rat plasma to inactivate [3H]TRH was significantly increased. The percentage of deamidation of TRH to TRH‐free acid was increased 2‐fold after 4 days of administration of thyroid hormone. The inactivation of TRH by plasma from hypothyroid patients was compared to that obtained from hyperthyroid patients. Extraction of human plasma incubated with [3H]TRH, followed by thin‐layer electrophoresis, showed that transformation of [3H]TRH into TRH‐free acid was 44% higher in plasma from hyperthyroid than from hypothyroid patients (P < 0.05). These data suggest that the inactivation process of TRH by blood proteins could be an important factor in the regulation of the hypothalamo‐hypophyseal‐thyroid axis in rat and man.

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