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Kathryn A Martin, MD
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journals and other resources. Updates are added as important new
information is published. The literature review for version 13.3 is
current through August 2005; this topic was last changed on March 2,
2004. The next version of UpToDate (14.1) will be released in
February 2006.
INTRODUCTION — The hypothalamus can be
considered the coordinating center of the endocrine system. It
consolidates signals derived from upper cortical inputs, autonomic
function, environmental cues such as light and temperature, and
peripheral endocrine feedback. In turn, the hypothalamus delivers
precise signals to the pituitary gland which then releases hormones
that influence most endocrine systems in the body. Specifically, the
hypothalamic-pituitary axis directly affects the functions of the
thyroid gland, the adrenal gland, and the gonads, as well as
influencing growth, milk production, and water balance (show table 1) [1-4]. The functions of the hypothalamic and
pituitary hormones will be discussed separately. (See
"The normal menstrual cycle" and see
"Physiology of gonadotropin-releasing hormone" and see
"ACTH and cortisol secretion in health and disease" and see
"Adrenal and extraadrenal actions of ACTH" and see
"Physiology of corticotropin-releasing hormone" and see
"Synthesis and secretion of ACTH and related peptides" and see
"Physiology of growth hormone" and see
"Thyroid hormone action" and see
"Thyroid hormone synthesis and physiology"). Although not
discussed here, the hypothalamus is also involved in several
important nonendocrine functions such as temperature regulation, the
activity of the autonomic nervous system, and control of
appetite.
The anatomy and unique blood supply of the hypothalamic-pituitary
axis are essential to its function. The hypothalamic hormones are
small peptides that are generally active only at the relatively high
concentrations achieved in the pituitary portal blood system. Their
small size and lack of known binding proteins results in rapid
degradation and very low concentrations in the peripheral
circulation. However, ectopic production of several of these
hormones has been identified, both by normal white blood cells and
by chromaffin cell tumors. Peripheral hormone receptors have also
been identified, although their physiologic importance is not
known.
ANATOMY
— The hypothalamus is located at the base of the brain, below the
third ventricle and just above the optic chiasm and pituitary gland
(show figure 1) [5,6]. This location can be thought of as the
intersection of the cortex, the cerebellum, and the brainstem. Most
of the cell bodies of the small neurons containing hypothalamic
releasing hormones are located in the tuberal area in the anterior
part of the hypothalamus. Afferent pathways to the hypothalamic
nuclei arise from the brain stem, thalamus, basal ganglia, cerebral
cortex, and olfactory areas. Efferent pathways include the dorsal
longitudinal fasciculus connecting the hypothalamus to the brainstem
reticular centers, connections to the autonomic nervous system and
the thalamus, and the hypothalamo-neurohypophysial tract which
connects the paraventricular and supraoptic nuclei, which produce
antidiuretic hormone, to nerve terminals in the median eminence and
to the posterior pituitary. (See
"Chapter 6B: Antidiuretic hormone and water balance").
The pituitary stalk, which connects the median eminence to the
pituitary gland (show figure 2), passes through an opening in the
dura surrounding the brain. The pituitary gland lies outside the
dura. It rests in the sella turcica below the optic chiasm and is
divided into anterior and posterior portions, each with distinct
embryology, anatomy, and function. The anterior pituitary (or
adenohypophysis) arises from Rathke's pouch from the oral cavity,
and the posterior pituitary (or neurohypophysis) arises from neural
ectoderm at the floor of the forebrain. The pituitary lies almost
directly behind the area between the eyes and can be accessed
surgically through the back of the nose (the sphenoid bone). It is a
distinctly visible structure on MRI scanning (show figure 2).
BLOOD
SUPPLY — Although most hormones circulate systemically
and therefore contact all body tissues, the hypothalamic hormones
have a special circulation so that they reach their target cells in
the pituitary in high concentrations.
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