Overview of the endocrine system
A hormone is a chemical messenger secreted by specialised endocrine cells or glands into
the bloodstream. Hormones travel in the blood and bind to specific receptors on target cells,
where they alter cellular activity.
The endocrine system is tightly regulated and usually works through feedback loops,
especially negative feedback, to maintain homeostasis.
Primary endocrine glands Secondary endocrine glands
hypothalamus Liver
Pituitary glands Kidney
Thyroid glands Intestines
Adrenal glands Skin
pancreas
Reproductive glands
Main actions of the endocrine system
The endocrine system helps regulate:
• Metabolism
• Cardiovascular function
• Skeletal function
, • Reproductive function
• Immune function
• Central nervous system function
• Renal function
• Cell growth and cancer-related processes
Examples of hormonal effects
Hormones can have major physiological and psychological effects. For example:
• During pregnancy, hormone levels change significantly and then fall after childbirth.
• High-dose steroid treatment can sometimes cause mood changes or steroid-induced
psychosis.
Hormone structure and synthesis
Hormones can be grouped based on their chemical structure and mechanism of action.
• Hormones
• Neurotransmitters
• Growth factor and cytokines
Hormone type Key features Examples
Peptide Water-soluble, stored in vesicles, act via Insulin, oxytocin, ADH,
hormones membrane receptors and second growth hormone
messengers
Steroid Lipid-soluble, synthesised from Cortisol, aldosterone,
hormones cholesterol, not stored in vesicles, act testosterone, oestrogen,
via intracellular receptors progesterone
Amino acid- Derived from amino acids; some are Adrenaline, noradrenaline,
derived water-soluble and some are lipid- thyroid hormones
hormones soluble
Peptide hormones pathway
Peptide hormones are water-soluble, so they cannot easily cross the lipid bilayer. Instead,
they bind to receptors on the cell surface and activate intracellular signalling pathways.
Gs pathway Gq pathway
Hormone binds GPCR Hormone binds GPCR
↓ ↓
Conformational change in receptor Gq protein activated
↓ ↓
Gs protein activated Phospholipase C activated
↓ ↓
GDP is replaced by GTP PIP2 split into IP3 and DAG
↓ ↓
Adenylyl cyclase activated IP3 increases intracellular Ca²⁺
↓ DAG activates PKC
, ATP converted into cAMP ↓
↓ Protein phosphorylation
Protein kinase A activated ↓
↓ Cellular response
Phosphorylation of target proteins
↓
Cellular response
This can change: This causes:
• Membrane excitability IP3 binds to receptors on the smooth
• Metabolism endoplasmic reticulum or sarcoplasmic
• Gene expression reticulum, causing Ca²⁺ release.
• Cell growth The rise in intracellular Ca²⁺ activates
proteins such as calmodulin, which can
then activate kinases and produce the final
response.
Steroid hormones
Steroid hormones are lipid-soluble hormones synthesised from cholesterol. Because they are
lipid-soluble, they can diffuse directly through the plasma membrane and bind to
intracellular receptors.
Unlike peptide hormones, steroid hormones are generally not stored in vesicles. They are
synthesised on demand and released immediately after production.
Examples include:
, • Cortisol
• Aldosterone
• Testosterone
• Oestrogen
• Progesterone
• Vitamin D (steroid-like hormone)
Mechanism of steroid hormone action
𝑆𝑡𝑒𝑟𝑜𝑖𝑑 ℎ𝑜𝑟𝑚𝑜𝑛𝑒 → 𝐼𝑛𝑡𝑟𝑎𝑐𝑒𝑙𝑙𝑢𝑙𝑎𝑟 𝑟𝑒𝑐𝑒𝑝𝑡𝑜𝑟 → 𝐻𝑜𝑟𝑚𝑜𝑛𝑒 𝑅𝑒𝑠𝑝𝑜𝑛𝑠𝑒 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 (𝐻𝑅𝐸)
→ 𝐺𝑒𝑛𝑒 𝑡𝑟𝑎𝑛𝑠𝑐𝑟𝑖𝑝𝑡𝑖𝑜𝑛 → 𝑃𝑟𝑜𝑡𝑒𝑖𝑛 𝑠𝑦𝑛𝑡ℎ𝑒𝑠𝑖𝑠
Example: Testosterone
1. Testosterone diffuses through the plasma membrane.
2. It binds to an intracellular receptor in the cytoplasm or nucleus.
3. Heat shock proteins (HSPs) dissociate from the receptor.
4. The activated hormone–receptor complex binds to specific DNA sequences called
Hormone Response Elements (HREs).
5. This stimulates gene transcription and protein synthesis.
Effects include:
• Structural protein production
• Functional protein synthesis
• Cell growth and differentiation
• DNA replication and mitosis
Hypothalamus: posterior pituitary gland
The hypothalamus is a
collection of grey matter nuclei
located inferior and anterior to
the thalamus. It links the
nervous system with the
endocrine system.
The pituitary gland consists of:
• Posterior pituitary
(neurohypophysis)
• Anterior pituitary
(adenohypophysis)
A hormone is a chemical messenger secreted by specialised endocrine cells or glands into
the bloodstream. Hormones travel in the blood and bind to specific receptors on target cells,
where they alter cellular activity.
The endocrine system is tightly regulated and usually works through feedback loops,
especially negative feedback, to maintain homeostasis.
Primary endocrine glands Secondary endocrine glands
hypothalamus Liver
Pituitary glands Kidney
Thyroid glands Intestines
Adrenal glands Skin
pancreas
Reproductive glands
Main actions of the endocrine system
The endocrine system helps regulate:
• Metabolism
• Cardiovascular function
• Skeletal function
, • Reproductive function
• Immune function
• Central nervous system function
• Renal function
• Cell growth and cancer-related processes
Examples of hormonal effects
Hormones can have major physiological and psychological effects. For example:
• During pregnancy, hormone levels change significantly and then fall after childbirth.
• High-dose steroid treatment can sometimes cause mood changes or steroid-induced
psychosis.
Hormone structure and synthesis
Hormones can be grouped based on their chemical structure and mechanism of action.
• Hormones
• Neurotransmitters
• Growth factor and cytokines
Hormone type Key features Examples
Peptide Water-soluble, stored in vesicles, act via Insulin, oxytocin, ADH,
hormones membrane receptors and second growth hormone
messengers
Steroid Lipid-soluble, synthesised from Cortisol, aldosterone,
hormones cholesterol, not stored in vesicles, act testosterone, oestrogen,
via intracellular receptors progesterone
Amino acid- Derived from amino acids; some are Adrenaline, noradrenaline,
derived water-soluble and some are lipid- thyroid hormones
hormones soluble
Peptide hormones pathway
Peptide hormones are water-soluble, so they cannot easily cross the lipid bilayer. Instead,
they bind to receptors on the cell surface and activate intracellular signalling pathways.
Gs pathway Gq pathway
Hormone binds GPCR Hormone binds GPCR
↓ ↓
Conformational change in receptor Gq protein activated
↓ ↓
Gs protein activated Phospholipase C activated
↓ ↓
GDP is replaced by GTP PIP2 split into IP3 and DAG
↓ ↓
Adenylyl cyclase activated IP3 increases intracellular Ca²⁺
↓ DAG activates PKC
, ATP converted into cAMP ↓
↓ Protein phosphorylation
Protein kinase A activated ↓
↓ Cellular response
Phosphorylation of target proteins
↓
Cellular response
This can change: This causes:
• Membrane excitability IP3 binds to receptors on the smooth
• Metabolism endoplasmic reticulum or sarcoplasmic
• Gene expression reticulum, causing Ca²⁺ release.
• Cell growth The rise in intracellular Ca²⁺ activates
proteins such as calmodulin, which can
then activate kinases and produce the final
response.
Steroid hormones
Steroid hormones are lipid-soluble hormones synthesised from cholesterol. Because they are
lipid-soluble, they can diffuse directly through the plasma membrane and bind to
intracellular receptors.
Unlike peptide hormones, steroid hormones are generally not stored in vesicles. They are
synthesised on demand and released immediately after production.
Examples include:
, • Cortisol
• Aldosterone
• Testosterone
• Oestrogen
• Progesterone
• Vitamin D (steroid-like hormone)
Mechanism of steroid hormone action
𝑆𝑡𝑒𝑟𝑜𝑖𝑑 ℎ𝑜𝑟𝑚𝑜𝑛𝑒 → 𝐼𝑛𝑡𝑟𝑎𝑐𝑒𝑙𝑙𝑢𝑙𝑎𝑟 𝑟𝑒𝑐𝑒𝑝𝑡𝑜𝑟 → 𝐻𝑜𝑟𝑚𝑜𝑛𝑒 𝑅𝑒𝑠𝑝𝑜𝑛𝑠𝑒 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 (𝐻𝑅𝐸)
→ 𝐺𝑒𝑛𝑒 𝑡𝑟𝑎𝑛𝑠𝑐𝑟𝑖𝑝𝑡𝑖𝑜𝑛 → 𝑃𝑟𝑜𝑡𝑒𝑖𝑛 𝑠𝑦𝑛𝑡ℎ𝑒𝑠𝑖𝑠
Example: Testosterone
1. Testosterone diffuses through the plasma membrane.
2. It binds to an intracellular receptor in the cytoplasm or nucleus.
3. Heat shock proteins (HSPs) dissociate from the receptor.
4. The activated hormone–receptor complex binds to specific DNA sequences called
Hormone Response Elements (HREs).
5. This stimulates gene transcription and protein synthesis.
Effects include:
• Structural protein production
• Functional protein synthesis
• Cell growth and differentiation
• DNA replication and mitosis
Hypothalamus: posterior pituitary gland
The hypothalamus is a
collection of grey matter nuclei
located inferior and anterior to
the thalamus. It links the
nervous system with the
endocrine system.
The pituitary gland consists of:
• Posterior pituitary
(neurohypophysis)
• Anterior pituitary
(adenohypophysis)
