Acute Decrease of Blood Thyroid Hormone by Isovolemic Exchange Transfusion as a Stimulus for Pulse TSH Release and Its Modifications by CNS Lesions

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Abstract
The level of thyroid hormone in blood has been acutely decreased by about 20-30% within 20 min with the aid of isovolemic exchange transfusion (IET) of thyroid hormone free blood cell suspension (THFBCS) in three groups of rats: (1) intact control (C); (2) median eminence (ME)-lesioned; (3) thalamus (TH)-lesioned. The levels of thyroxine (T4) and TSH were measured by specific radioimmunoassay (RIA) before the transfusion and for 180 min after that. The level of T4 increased to the initial value at about 120 min after IET in C and ME-lesioned rats, while in TH it remained decreased until 180 min. Furthermore, at the end of IET the level of TSH in C and ME-lesioned rats was about 2–3 times higher than the initial value and its continuous decrease was observed until 180 min. In contrast, in TH-lesioned rats the increase of TSH level was delayed, being significant at 60 min only. It was concluded that IET of THFBCS acts as a stimulus of acute TSH release which was remarkably inhibited in TH-lesioned animals. In addition, the destruction of most of the ME did not apparently influence this response, as shown by essentially similar data obtained in C and ME-lesioned rats.