We have used in situ hybridization histochemistry to determine the effect of hippocampal-hypothalamic disconnection on hypothalamic thyrotrophin-releasing hormone (TRH) and anterior pituitary thyrotrophin beta subunit (TSH beta) transcripts in adult male CFY rats. Electrothermal lesions of the fornix pathway significantly increased TRH and TSH transcripts and increased circulating levels of triiodothyronine (T3). Fornix transection did not, however, prevent feedback regulation of TRH and TSH transcripts during exogenous T3-induced hyperthyroidism or propylthiouracil-induced hypothyroidism. Hippocampal inputs to the hypothalamus contribute to setting the basal activity of the thyroid axis, but do not mediate the feedback effects of T3.
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Shi et al. (2008) studied this question.