Module 4a: GROWTH HORMONE Notes (Endocrine Control of Growth and Metabolism)
Growth Hormone (GH): Basic Facts
GH is a protein consisting of 191 amino acids and is the most abundant anterior pituitary hormone.
Most GH circulates free in plasma; its plasma half-life is approximately .
A significant fraction (about 40%) is bound to Growth Hormone Binding Protein (GHBP), which increases GH’s half-life.
GH actions are mediated primarily by an indirect pathway via IGF-1, but GH also has direct effects.
GH Structure, Transport, and Core Concepts
GH exerts its effects through binding to GH receptors on target cells, triggering intracellular signalling cascades.
A key mediator is Insulin-like Growth Factor 1 (IGF-1), produced mainly in the liver but also in other tissues; many GH actions are IGF-1–dependent.
IGFs circulate bound to a family of binding proteins; GH effects can be modulated by these IGF-binding proteins (IGFBPs).
A notable clinical example: short stature in some African pygmies is linked to an inability to synthesize significant amounts of IGF-1 despite normal GH levels.
Daily and Lifespan Patterns of GH Secretion
GH secretion is highest in mid-fetal life with little effect on fetal growth.
In childhood, GH secretion increases and peaks at puberty with circulating levels around 5–10 ng/mL.
In young adults, GH levels are typically 2–5 ng/mL.
GH secretion is very low in old age.
Overall, GH secretion varies with life stage and physiological state (sleep, nutrition, exercise, stress, etc.).
Regulation of GH Secretion: Key Pathways
Hypothalamic regulation:
GHRH (Growth Hormone-Releasing Hormone) stimulates GH release via a Gs protein → adenylyl cyclase → cyclic AMP (cAMP) → PKA → phosphorylation of CREB → transcriptional activation of GH gene (via PIT1 and other factors).
Somatostatin inhibits GH release via Gi coupling, reducing cAMP.
Transcriptional control:
PIT1 (also known as POU1F1) is a pituitary transcription factor essential for GH gene expression; other transcription factors (e.g., FOS family) participate in regulation.
Signalling cascade (GH receptor action):
GH binding to its receptor activates JAK2 and Src family kinases, initiating intracellular signalling that culminates in transcription of genes required for growth and metabolism.
Overall concept:
GH release is governed by a balance of GHRH stimulation and somatostatin inhibition, integrated with metabolic and physiological cues.
GH’s effects are typically indirectly mediated by IGF-1 (major postnatal mediator), though GH also has direct effects on various tissues.
Triggers and Modulators of GH Secretion
Secretion is enhanced by:
Sleep
Exercise
Stress
Postprandial decline in glucose
Increase in specific amino acids
Postprandial hyperglycemia
Elevated free fatty acids
Elevated IGF-1
Aging
Secretion is inhibited by:
(Text lists inhibitors alongside enhancers; clinically, known inhibitors include high blood glucose and somatostatin activity.)
Notes on the regulatory pattern:
GH displays pulsatile secretion with pronounced peaks during sleep and in response to fasting or metabolic stress, and suppression after meals when glucose rises.
Figures in the source illustrate GH peaks around sleep periods and suppressions after meals, adapted from classic studies (e.g., SASSIN et al., Science 1972; KanaLEY et al., J Appl Physiol 1997).
Effects of Meals and Sleep on GH Secretion (Patterns)
Postprandial state effects:
Meals produce declines in GH secretion, reflecting glucose and nutrient signals that suppress GH release.
The pattern shows reduced GH during waking hours with meals; higher GH when meals are not suppressing it (e.g., during sleep).
Example timepoints in the figure: 8 AM vs 8 PM, meal vs sleep periods (adapted from Sassin et al., 1972).
Exercise effects:
Physical activity markedly increases GH secretion across time blocks, with multiple exercise stimuli producing repeated GH elevations (adapted from Kanaley et al., 1997).
Mechanisms of GH Action: Direct and Indirect Pathways
GH receptor signalling:
GH binds its receptor on target cells, activating JAK2 and Src family kinases.
This initiates intracellular signalling cascades that regulate transcription of genes involved in growth and metabolism.
Indirect pathways via IGF-1:
GH stimulates IGF-1 production (primarily in liver) which then mediates many growth-promoting effects.
IGF-1 provides the majority of the postnatal growth effects through endocrine, paracrine, and autocrine actions.
IGFs and binding proteins:
IGFs are bound in the circulation to one of six IGF-binding proteins (IGFBPs), which modulate their bioavailability and activity.
Clinical note:
Defective IGF-1 synthesis (even with normal GH) can cause growth deficiencies, as seen in certain populations.
Key references in the field: Brooks & Waters (Nat Rev Endocrinol, 2010); Guyton & Hall (Textbook, 10th ed., Fig. 75-5).
IGF-1: Central Mediator of GH Actions
GH’s postnatal growth effects are largely indirect via IGF-1.
IGF-1 is produced in liver and other tissues in response to GH stimulation.
Binding proteins (IGFBPs) regulate IGF-1’s bioavailability and actions.
GH Actions on Tissues and Metabolism
Indirect effects (via IGF-1):
Cartilage formation and skeletal growth (bone lengthening and mineralization) – especially during development.
Increased protein synthesis and cellular growth/proliferation in multiple tissues.
Organ growth (growth of most internal organs except the brain).
Lipolysis (breakdown of lipids).
Direct effects of GH (some are IGF-1–independent):
Anti-insulin/anti-glucose uptake actions in muscle and adipose tissue.
Increased gluconeogenesis in the liver.
Net metabolic effects:
GH increases blood glucose levels and exerts anti-insulin effects; it shifts metabolism toward nutrient mobilization (proteins and fats) to support growth.
Summary of mechanisms (adapted from Guyton & Hall, Fig. 75-5):
GH acts via indirect IGF-1–mediated pathways and via direct actions on target tissues.
Indirect effects include cartilage formation, skeletal growth, protein synthesis, cell growth, and lipolysis.
Direct effects include anti-insulin actions affecting carbohydrate metabolism and lipolysis; IGF-1 amplifies growth-related outcomes.
Growth Hormone and IGF-1: Integrated Summary
Major outcomes of GH/IGF-1 axis:
In children and adolescents, GH promotes height growth via bone lengthening and mineralization.
GH increases lean body mass by stimulating protein synthesis.
GH stimulates growth of most internal organs (except the brain).
GH promotes lipolysis, contributing to mobilization of fatty acids for energy.
GH tends to reduce glucose uptake by muscle and fat and increases hepatic gluconeogenesis, contributing to higher blood glucose levels (anti-insulin effects).
Mechanistic distinction:
GH actions can be IGF-1–dependent (indirect) or IGF-1–independent (direct effects).
Clinical relevance:
Dysregulation of GH/IGF-1 axis underpins conditions such as GH deficiency (short stature) or GH excess (acromegaly); therapies often target this axis for growth disorders and metabolic health considerations.
Connections to Foundational Principles
The GH/IGF-1 axis exemplifies classic endocrine feedback loops: hypothalamic signals (GHRH, somatostatin) regulate pituitary GH, which stimulates hepatic and extrahepatic IGF-1 production, which in turn modulates GH release (negative feedback) and tissue growth/metabolism.
Distinction between direct hormone effects and hormone actions mediated by a secondary effector (IGF-1) highlights the complexity of endocrine signaling and tissue-specific responsiveness.
The balance between anabolic (growth-promoting) processes and metabolic effects (anti-insulin actions) underscores the integrated nature of growth and energy homeostasis.
Key Formulas and Numerical References
GH amino-acid length: 191 amino acids.
GH half-life: .
Puberty circulating GH levels: .
Young adult GH levels: .
IGF-1 is produced in response to GH and is the primary mediator of postnatal GH actions.
IGFs circulate bound to IGFBPs (six binding proteins), regulating bioavailability.
Practical and Ethical Considerations (Real-World Relevance)
GH therapy is used for diagnosed GH deficiency and certain growth disorders, but it must be carefully managed due to metabolic effects (e.g., insulin resistance) and potential adverse events.
Understanding GH’s indirect (IGF-1–mediated) actions is essential for interpreting growth patterns, metabolic health, and the effects of aging on body composition.
The GH/IGF-1 axis has implications for pediatric and adult endocrinology, pediatrics, metabolism, and geriatric health.