Effects of TRH injection on hippocampal-hypothalamic-pituitary-thyroid interactions

The first experiment was conducted in an attempt to ascertain the effects of hippocampal lesions on plasma levels of total T₄, total T₃, free T₄, and TSH. Animals with hippocampal lesions had significantly lower levels of total T₄, free T₄ and TSH than cortical control and normal animals. There were no significant differences in total T₃ among the three groups. Since these results indicated that animals with hippocampal lesions manifested a dysfunction in the hypothalamic-pituitary-thyroid axis, a second experiment was undertaken to determine the site of mediation. A TRH injection test demonstrated that the dysfunction occurs at the level of the hypothalamus. It is hypothesized that in the normal animal the hippocampus exerts a facilitating effect on the hypothalamic-pituitary-thyroid axis.