MODULE – 5
Introduction to endocrinology
• The endocrine system consists of glands widely separated from each other with
no physical connection and hormones.
• Endocrine glands are groups of secretory cells surrounded by an extensive
network of capillaries that facilitates diffusion of hormones (chemical
messengers) from the secretory cells into the bloodstream.
• They also referred to as ductless glands beca hormones diffuse directly into the
bloodstream. Then carried to target tissues and organs and influence cell
growth and metabolism.
HYPOTHALAMUS
• Major integrating centre of the brain
• Hypothalamus is the basal part of diencephalon and it is located below the
thalamus
• It is situated in the center of the brain, just above the pituitary gland
• Pituitary gland attached to the hypothalamus by a stalk called infundibulum
• It has both neural and endocrine functions
• It contain several groups of neurosecretory cells called nuclei ( preoptic
nucleus, supraoptic nucleus, ventromedial and dorsomedial nucleus) formed of
masses of grey matter, which produce hormones. These hormones regulate the
synthesis and secretion of pituitary hormones.
• Two types of hormones
1. Releasing hormones: stimulate secretion of pituitary hormones
2. Inhibiting hormones: inhibit secretion of pituitary hormones
• The hormones originates in the hypothalamic neurons, pass through axons and
are released from their nerve endings. These hormones reach the pituitary gland
through a portal circulatory system.
• PRH – the hypothalamus produces prolactin releasing hormone, which
stimulate the pituitary gland to produce prolactin. While PIH (prolactin
inhibiting hormone) which is dopamine, acts on pituitary gland to inhibit
prolactin release.
• TRH – the hypothalamus produces thyroid stimulating hormone, which
stimulate the pituitary gland to release thyroid-stimulating hormone (TSH).
Thus, in turn , stimulates the thyroid gland to produce thyroid hormones.
• CRH – Corticotropin releasing hormones- stimulates the pituitary gland to
release adrenocorticotropin hormone (ACTH)
• GHRH – Growth hormone- releasing hormone – stimulates the pituitary gland
to release growth hormone
• GHIH – Somatostatin - inhibits the release of growth hormone from the
pituitary.
• GnRH- stimulates the pituitary synthesis and release of gonadotrophin
• Other two hormones synthesised by the hypothalamus are oxytocin and
vasopressin. Then it is transported to posterior pituitary which stores and
releases these hormones.
• Oxytocin- stimulate uterine contractions during childbirth and milk letdown
during lactation
Vasopressin – It is also called antidiuretic hormone. It is produced by
hypothalamus, which is then transported to posterior pituitary gland for storage
and release. It regulate water balance and blood pressure
Functions
• It regulate temperature- the hypothalamus acts as the body’s thermostat,
maintaining a stable temperature.
• Control sleep-wake cycle
• It is involved in emotional responses, such as fear, anger and aggression.
• Lateral hypothalamus - feeding centre. Stimulation of this area leads to food
intake
Ventromedial hypothalamus – satiety centre
THYROID GLAND
• It is a butterfly shaped gland with two wings wrapping the trachea. found in the neck
below the larynx and thyroid cartilage.( Cartilage is a strong, flexible connective
tissue that protects your joints and bones).
• It consists of two lobes. The two lobes of the human thyroid are connected by a
bridge of connective tissue called the thyroid isthmus.
• Thyroid gland is composed of large number of closed follicles lined with single layer
of cuboidal epithelial cells, called follicular cells.
• The cavities of follicles are filled by colloidal material(thick), with
major constituent of glycoprotein called thyroglobulin produced by cuboidal epithelial
cells. Thyroglobulin contains the amino-acid tyrosine that forms the raw material for
the formation of thyroid hormone.
• Follicles are surrounded by blood vessels to provide nourishment and iodine which
help the production of thyroid hormones.
Hormones
• Store , relase when need arises
• When hypothalamus produce TRH (thyrotropin releasing hormone), pituitary gland
produce TSH ( thyroid stimulating hormone) which stimulate thyroid gland and
secrete thyroid hormones .
• Your hypothalamus releases thyrotropin-releasing hormone (TRH), which triggers
your pituitary gland to release thyroid-stimulating hormone (TSH), which stimulates
your thyroid to release T3 and T4.
• Once your thyroid releases thyroxine (T4) into your bloodstream, certain cells in
your body transform it into triiodothyronine (T3) through a process called de-
iodination.
• T3 – Triiodothyronine (20 %) & Thyroxine-T4 (80%)
❖ Brain development
❖ Enhances basal metabolic rate
❖ Maintenance of bones
❖ Heart and digestive function
❖ Support the process of RBC formation ❖ Increases protein synthesis
❖ Increases heat production in the body
❖ Muscle control
❖ Maintenance of water and electrolyte balance
• Calcitonin
❖ Maintenance of calcium level in the blood
❖ Store in bones
• Hypo-secretion causes hypothyroidism – slow metabolism
Cretinism in children – mental retardation, stunted growth, swelling of face, low IQ
Myxoedema in adults
• Hyperthyroidism( Grave’s disease) too much hormones –fever ,sweating, bulging of
eyeball, anxiety, fatigue, breast enlargement, anger, irregular menstrual period
• Goitre- enlargement of thyroid gland., due to iodine deficiency, the thyroid cannot
make sufficient hormones
