Introduction

The hypothalamus is a small almond-sized endocrine gland in the central part of the brain. This part is known as the diencephalon in neuroanatomy. Hypothalamus is found below the thalamus of the brain. Hypothalamus consists of several nuclei. The hypothalamus by its hormones functions as a chemical coordinator of the human body. It bridges the nervous system with the endocrine system.

The pituitary is another endocrine gland in the human brain placed just below the hypothalamus. The pituitary consists of the anterior pituitary and posterior pituitary. The pituitary gland is also known as hypophysis. Therefore, the anterior pituitary is known as adenohypophysis and the posterior pituitary is known as neurohypophysis.

The HP axis

Hypothalamus carries out its functions via the pituitary gland. That is because some hormones secreted by the hypothalamus acts on the pituitary gland to secrete pituitary hormones. Some other hypothalamic hormones are stored in the pituitary and secreted from the pituitary according to the needs of the human body.

This explains why these hormones act in an axis known as the hypothalamic-pituitary axis (HP axis) or else the hypothalamic hypophyseal axis.

Hormones secreted by the hypothalamus

• TRH – Thyrotropin-releasing hormone
• CRH – Corticotropin-releasing hormone
• Dopamine / PIH – Prolactin-inhibiting hormone
• GHRH – Growth-hormone-releasing hormone
• GnRH – Gonadotropin-releasing hormone
• Somatostatin / GHIH -Growth-hormone-inhibiting hormone

• Oxytocin
• Vasopressin / ADH – Antidiuretic hormone

TRH acts on the anterior pituitary to stimulate the secretion of TSH.

CRH acts on the anterior pituitary to stimulate the secretion of ACTH.

Dopamine inhibits prolactin release from the anterior pituitary. That is why dopamine is also known as PIH (prolactin-inhibiting hormone)

Secretion of Growth Hormone is done by the anterior pituitary. It is stimulated by Growth-hormone-releasing hormone (GHRH) whereas it is inhibited by Growth-hormone-inhibiting hormone (GHIH) / Somatostatin

Gonadotropin-releasing hormone (GnRH) stimulates the secretion of gonadotrophs from the anterior pituitary. The gonadotrophs are FSH and LH. FSH is the abbreviation for follicle-stimulating hormone. LH is the abbreviation for luteinizing hormone.

Oxytocin and vasopressin are made in the hypothalamus and transferred to the posterior pituitary. The posterior pituitary, which is also known as the neurohypophysis is an anatomical extension of the hypothalamus. The function of oxytocin and vasopressin is discussed in the posterior pituitary hormones section

Hormones secreted by the anterior pituitary

• TSH – Thyroid-stimulating hormone
• ACTH – Adrenocorticotropic hormone
• Prolactin
• GH – Growth Hormone
• FSH – Follicle-stimulating hormone
• LH – Luteinizing hormone

TSH from the anterior pituitary acts on the thyroid gland to increase the secretion of thyroid hormones. The thyroid hormones are essential for the regulation of metabolic reactions in the human body.

ACTH from the anterior pituitary acts on the adrenal cortex. Adrenal cortex secrets cortisol in response to ACTH. Cortisol acts as a stress hormone in the human body.

Prolactin has a role in milk production and breast mammary gland development in breastfeeding mothers.

Growth hormone has major functions in the human body related to the growth of body tissues. The functions of GH are carried out by a peptide known as IGF (Insulin-like growth factor). GH also plays a role in sleep and memory.

In females, FSH acts on ovaries and stimulates the growth of eggs in the ovaries. In males, FSH stimulates sperm production by acting on cells of the testes.

In females, LH stimulates ovulation whereas in males LH promotes testosterone production.

Both FSH and LH vary in levels, in a distinct pattern, throughout the menstrual cycle in females.

Hormones secreted by the posterior pituitary

• Oxytocin
• Vasopressin / ADH – Antidiuretic hormone

When pituitary hormones are produced in excess it is known as hyperpituitarism. When some of the pituitary hormones are at low levels it is called hypopituitarism. When most of the pituitary hormones are not produced to desired levels it is called panhypopituitarism.