01. General Endocrinology

Coordination of body functions by chemical messengers Neural Neurotransmitters at synaptic junction ...

Coordination of body functions by chemical messengers

Neural	Neurotransmitters at synaptic junction
Endocrine	Glands/Specialized cells secreate “hormones”
Neuroendocrine	Neurons secreta “neurohormones”
Paracrine	cells secrete substcnces to ECF-affect adjacent cells
Autocrine	cells secrete substance affecting the same cell

Endocrine glands

secrete “hormones “ : Internal secretions that are transported in blood and control metabolism in organs and tissues

Has widespread interrelated effects

• long term , Slow acting effects : physical and sexual growth
• Short term ,Rapid acting adaptive function : e.g. Insulin , adrenalin

Function of hormones

1. Homeostasis

• Via metabolism : Insulin , Glucogan , Thyroxine , Cortisol
• Via antistress action : Thyroxine , Cortisol , Adrenalin , ATCH , ADH
• Via water & mineral regulation : ADH, Aldosterone , PTH

2. Growth

• IGF
• Thyroxine
• Testosterone

3. Reproduction

• Testicular / Ovarian steroids
• Steroid
• Gonadotropins

4. Combating emergency

• Adrenaline
• Noradrenalin

Endocrine system

Physically separated but functionally related glands

• Normal endocrine state depend on a balance functioning of endocrine glands
• hormone may have widespread effects (e.g. Growth hormone ,Thyroxin ) ,Self effects (e.g. ACTH on renal cortex )
• Usually more general manifestation than local effects seen in endocrine disorders

A. Hormone synthesis

1. Peptide

• usually synthesized as longer precursors
• stored in cells
• secreted by exocytosis on stimulation
• act on cell membrane receptor

2. Steroid

• usually synthesized from cholesterol
• very little storage

3. Amine

• derived from Tyrosine

B. Hormone Secretion

1. Different hormones has different onset of secretion after stimulus also different duration of action
e.g.

• Norepinephrine/Epinephrine : Secrete within second of stimulation ,full action within second / minutes
• Thyroxin / GH : Slower action / may need months for full action 

2. Control hormone secretion  by

Feedback control

(+) Feed Back	(-) Feed Back
Increase in target tissue activity leads to further increase in [ Hormone ] E.g. LH surge due to oestrogen	Prevent over activation [ Hormone ] Regulation can occur at all form gene transcription to release

Biological Rhythms

• Circadian rhythm : Cortisol
• Menstrual Cycle : Oestrogen, Progesteron ,LH,FSH

Other Stimuli

• Stress : GH,Prolactin
• Sexual activity : Prolactin, oestrogen
• Sucking : Prolactin, Oxytocin
• Fasting Feeding : Insulin ,Glucagon
• Sleep : Prolactin

C. Hormone Transport in Blood

[ Hormone ] = ( Rate of secretion <-----> Rate of removal )

• Water Soluble hormones , Usually dissolved : peptide ,Catecholamine
• Steroids & Thyroid hormones : mainly bound to plasma proteins
• Protein bound hormones >>> Free hormones : e.g 99% of thyroxine in blood is protein bound


• Free hormones = active  hormones
• Protein bound hormones = Reservoir /Slow clearance

D. Mechanisms of action

• Steroid hormones -----> Cytoplasmic  Receptors-----> Activate protein transcription & Stimulate Protein Synthesis 
• Thyroid hormone ----> Nuclear Receptors-----> Activate protein transcription & Stimulate Protein Synthesis 
• Peptide hormone ----> Cell surface Receptors-----> 2 type of action


• via 2nd messenger ( e.g. ACTH,FSH,GnRH,HCG,TSH,Adrenalin,Calcitonin)
• Direct (  Tyrosine kinase activity e.g. Insulin , IGF)

E. Clearance

Several Mechanism

1. Destruction by enzymes in blood /Tissues
2. Tissue binding
3. Excretion by liver
4. Excretion by liver 

• Most peptide hormones & catecholamines : rapidly declared
• Protein bound hormones : Slow clearance