Fig. 30. High magnificationin situhybridization autoradiographs of proTRH mRNA in the paraventricular nucleus (PVN) of a (A) euthyroid and (B) hypothyroid animal. Note marked increase in TRH mRNA when circulating levels of thyroid hormone fall. Low magnificationin situhybridization autoradiographs of proTRH mRNA in the PVN of (C) normal fed and (D) fasting animals. Hybridization signal is markedly reduced in the fasting animals despite low circulating levels of thyroid hormone.