TRH is secreted by neurosecretory neurons of the hypothalamus and released into the hypothalamic-pituitary portal blood to act on thyrotroph cells of the anterior pituitary. This is neurocrine (neuroendocrine) signalling — a neuron secretes a chemical messenger that travels in blood to act on a distant endocrine target. This differs from classical paracrine (local) or autocrine (self) signalling.