Time-dependent effects of oral thyrotropin-releasing hormone in hyperthyroid female rats

We have studied the thyrotropin (TSH) and thyroid hormone (T3) response to acute thyrotropin-releasing hormone (TRH) administration in rats previously rendered hyperthyroid by daily ip injection of 25 micrograms L-T4/100 g bw for 30 days. Animals were thereafter put on distilled water or TRH (2 mg/10 ml) as drinking solution for another 18 days, continuing T4 administration. On days 3 and 10 on oral TRH, the serum TSH and T3 response to TRH was studied by administering 2 micrograms of the tripeptide iv. After 3 days, neither the animals on oral DW, nor those on oral TRH showed any response. Nevertheless, after 10 days, animals on oral TRH had a measurable, significant thyrotropin response. At sacrifice (18 days), animals on oral TRH had increased pituitary TSH content. These results confirm and extend a recent report from our laboratory (Iglesias, Llobera and Montoya 1985). We conclude that TRH-mediated thyrotropin accumulation in pituitary is the primary cause for the presence of this response.