Short pulses of low dose thyrotropin-releasing hormone do not affect thyroid hormone secretion from perfused dog thyroid lobes

Small amounts of TRH are present in the thyroid, and TRH has been shown in several studies to have a moderate direct inhibitory effect on TSH-stimulated thyroid hormone secretion. In a recent publication, however, data were presented to demonstrate that very low doses of TRH given in short pulses induce immediate increases in T4 secretion from the thyroid. This type of secretion had never been observed previously. We have tried to reproduce these results employing perfused dog thyroid lobes in which small and rapid variations in the release of hormones from the follicular cells are easily detected. TRH was infused in concentrations ranging from 1.7 X 10(-11) to 1.7 X 10(-8) M for 10-min periods. There was no effect on thyroid secretion of T4 and T3, nor did the infusion of 1.7 X 10(-9) M TRH for 160 min alter the T4 secretion induced by 4 microU/ml TSH.