How do hormones released by the hypothalamus enter the bloodstream?

Answer: The hypothalamus is the brain structure that regulates endocrine function via neurohormone release by the pituitary gland.

hypothalamus portal system bloodstream neurohormone

At the base of the brain is a region of cells called the hypothalamus. As the name indicates, it is immediately ventral to the thalamus and forms the bottom part of the diencephalon. Some cells in the hypothalamus are well-defined nuclei, but not all are clearly classified. The hypothalamus plays a role in several functions, including stress, fear, love, the flight or fight response mediated by the sympathetic nervous system, fatigue, body temperature, and thirst.

The brain uses the hypothalamus to communicate to the body via a structure immediately ventral (below) it called the pituitary gland. The hypothalamus communicates with the pituitary gland by releasing a variety of neurohormones. These neurohormones travel locally via the hypothalamo-hypophyseal portal system, a specialized brain structure that allows diffusion of large molecules like neurohormones to the pituitary. From here, these hormones in turn signal the pituitary gland to release the appropriate hormone into the bloodstream. Some of the major neurohormones released by the hypothalamus include:

  • Corticotropin-releasing hormone (CRH). CRH signals for the pituitary gland to release adrenocorticotropic hormone (ACTH) into the bloodstream, which is involved in the stress response. This neurohormonal signal is important in the HPA axis.

  • Gonadotropin-releasing hormone (GRH). Downstream of GRH release ultimately triggers the release of follicle-stimulating hormone and luteinizing hormone from the gonads.

  • Dopamine (DA). Dopamine can act as a neurohormone in addition to a neuromodulator. DA release from the hypothalamus inhibits the release of prolactin from the pituitary gland.