Medical School · Year 1 · Physiology · includes a quiz and discussion video

Lecture 7: Endocrine Principles

Unit 1.6: Physiology Foundations

Learning Objectives

By the end of this lecture, students will be able to:

1. Compare the mechanisms of endocrine, paracrine, and autocrine signaling
2. Classify hormones by chemical structure and describe their synthesis
3. Explain hormone transport and receptor mechanisms
4. Describe the hypothalamic-pituitary axis and feedback regulation
5. Explain general principles of hormone action and signal transduction
6. Apply endocrine physiology to clinical conditions

Overview of Endocrine Signaling

The endocrine system communicates through hormones—chemical messengers secreted by specialized cells that travel through the bloodstream to act on distant target cells. This signaling mode contrasts with neural communication, which operates locally at synapses with neurotransmitters, but the principles of ligand-receptor interaction and signal transduction apply to both systems.

Several modes of chemical signaling exist beyond classical endocrine communication. Paracrine signaling involves local diffusion of signaling molecules to affect nearby cells without entering the bloodstream; histamine released from mast cells affecting adjacent blood vessels exemplifies this mode. Autocrine signaling occurs when a cell releases signals that act on receptors on its own surface, as when cancer cells produce growth factors that stimulate their own proliferation. Neurocrine signaling describes classical neurotransmitter action at synapses. Neuroendocrine signaling represents a hybrid mode in which neurons release hormones into the bloodstream, exemplified by hypothalamic neurons releasing oxytocin and antidiuretic hormone.

Hormones share several defining characteristics. They are synthesized and secreted by specialized endocrine cells or tissues. They travel through the blood to reach target cells throughout the body. They act only on cells expressing the appropriate receptors. They produce specific physiological effects at very low concentrations (picomolar to nanomolar range), reflecting high receptor affinity.

The endocrine system regulates virtually every physiological process. Metabolism of carbohydrates, fats, and proteins depends on insulin, glucagon, thyroid hormones, and cortisol. Growth and development require growth hormone, thyroid hormones, and sex steroids. Reproduction depends on the hypothalamic-pituitary-gonadal axis. Homeostasis of calcium, water, and electrolytes requires parathyroid hormone, calcitonin, vitamin D, aldosterone, and antidiuretic hormone. Even mood and behavior are influenced by endocrine signals.

<image>Panel A: Endocrine signaling with hormones released into bloodstream traveling to distant target cells with specific receptors. Panel B: Paracrine signaling with local diffusion of molecules affecting adjacent cells without entering circulation. Panel C: Autocrine signaling with cells releasing molecules that bind receptors on their own surface. Panel D: Neurocrine and neuroendocrine signaling comparing synaptic transmission with neuronal hormone release into blood vessels.</image>

Hormone Classification

Hormones fall into several chemical classes that determine their synthesis, transport, receptor mechanisms, and duration of action. Understanding these categories provides a framework for predicting hormone behavior.

Peptide and protein hormones range from small peptides like TRH (three amino acids) to large glycoproteins like FSH. They are synthesized on ribosomes as preprohormones, processed through the secretory pathway, and stored in secretory vesicles until release. Being water-soluble, they circulate freely in plasma without carrier proteins, bind to cell-surface receptors, and signal through second messenger cascades. Their effects are typically rapid (seconds to minutes) because they modulate existing proteins rather than requiring new gene expression.

Steroid hormones—including glucocorticoids, mineralocorticoids, androgens, estrogens, and progestins—are synthesized from cholesterol through a series of enzymatic modifications. Different enzymes in different endocrine tissues produce the various steroid classes. Being lipid-soluble, steroids require carrier proteins for transport in the aqueous bloodstream. They can cross cell membranes readily to bind intracellular receptors, forming hormone-receptor complexes that act as transcription factors. Their effects are slow (hours to days) because they require gene transcription and new protein synthesis.

Amine hormones derive from the amino acids tyrosine or tryptophan. The catecholamines—epinephrine, norepinephrine, and dopamine—are water-soluble tyrosine derivatives that behave like peptide hormones: they circulate freely, bind cell-surface receptors, and signal through second messengers. Thyroid hormones (T3 and T4), though also derived from tyrosine, are lipophilic due to their iodinated ring structures. They circulate bound to carrier proteins, cross cell membranes, and act through intracellular receptors like steroid hormones. Melatonin derives from tryptophan.

Eicosanoids—prostaglandins, thromboxanes, and leukotrienes—derive from arachidonic acid and typically act as paracrine or autocrine signals rather than classical hormones. They are synthesized on demand rather than stored and have very short half-lives.

<image>Panel A: Hormone classification by chemical class showing water solubility, transport mechanisms, and receptor locations. Panel B: Peptide hormone synthesis pathway from gene through preprohormone processing to vesicular storage. Panel C: Steroid hormone synthesis from cholesterol through branching pathways to different steroid classes. Panel D: Amine hormone synthesis including catecholamines from tyrosine and thyroid hormones through iodination.</image>

Peptide Hormone Synthesis

Peptide hormones are synthesized through the regulated secretory pathway, allowing precise control of their release in response to physiological signals. The synthesis follows the general pattern for secreted proteins, with modifications specific to hormone processing.

Gene transcription produces mRNA encoding a preprohormone, which includes a signal sequence directing the nascent peptide to the rough endoplasmic reticulum. Co-translational cleavage removes this signal peptide, yielding a prohormone that enters the secretory pathway. In the Golgi apparatus, additional processing converts prohormone to the mature hormone through specific enzymatic cleavages and modifications including glycosylation, phosphorylation, or amidation.

Insulin synthesis exemplifies this pathway. The initial translation product, preproinsulin, contains a signal peptide, B chain, C peptide, and A chain in sequence. Signal peptide cleavage in the ER produces proinsulin. In secretory granules, proteolytic enzymes cleave out the C peptide, leaving the A and B chains connected by disulfide bonds as mature insulin. The C peptide is co-secreted with insulin, providing a clinical marker of endogenous insulin production that is not affected by exogenous insulin administration.

Storage in secretory granules (dense-core vesicles) allows accumulation of hormone for rapid release when needed. Regulated exocytosis releases vesicle contents in response to specific stimuli. For insulin, elevated blood glucose depolarizes pancreatic beta cells through ATP-sensitive potassium channels, triggering calcium influx and granule fusion. This stimulus-secretion coupling ensures hormone release matches physiological needs.

<image>Panel A: Peptide hormone synthesis pathway from gene transcription through preprohormone and prohormone processing to mature hormone storage. Panel B: Insulin processing showing signal peptide cleavage, C peptide removal, and disulfide-linked A and B chains. Panel C: Stimulus-secretion coupling with glucose metabolism, ATP-sensitive potassium channel closure, and calcium-triggered exocytosis. Panel D: Co-secretion of insulin and C peptide into the bloodstream as clinical marker of endogenous insulin production.</image>

Steroid Hormone Synthesis

All steroid hormones derive from cholesterol through enzymatic modifications occurring in the mitochondria and smooth endoplasmic reticulum of steroidogenic cells. Cholesterol itself comes from dietary sources, hepatic synthesis transported to endocrine tissues, or local synthesis from acetyl-CoA. The rate-limiting step in steroid synthesis is not any particular enzyme but rather the transport of cholesterol from the outer to inner mitochondrial membrane, mediated by the steroidogenic acute regulatory (StAR) protein.

Once cholesterol reaches the inner mitochondrial membrane, cytochrome P450 side-chain cleavage enzyme (CYP11A1) converts it to pregnenolone, the common precursor for all steroid hormones. From pregnenolone, the pathway diverges depending on which enzymes are expressed in a particular cell type. The adrenal cortex zona glomerulosa produces aldosterone, the zona fasciculata produces cortisol, and the zona reticularis produces androgens (DHEA and androstenedione). Ovarian theca and granulosa cells produce estrogens and progesterone. Testicular Leydig cells produce testosterone.

Steroids are not stored in vesicles but are synthesized on demand and diffuse out of cells immediately due to their lipophilicity. This means steroid hormone levels depend on synthesis rates rather than release from stores. Regulation occurs primarily through control of enzyme expression and StAR protein activity, often mediated by trophic hormones from the pituitary (ACTH for adrenal steroids, LH and FSH for gonadal steroids).

Because steroids are lipophilic, they require carrier proteins for transport through the aqueous plasma. Specific carriers include corticosteroid-binding globulin (CBG or transcortin) for cortisol, and sex hormone-binding globulin (SHBG) for testosterone and estradiol. Albumin provides additional nonspecific binding capacity. Carrier binding prolongs hormone half-life, creates a circulating reservoir, and restricts hormone access to target tissues. Only the free (unbound) fraction is biologically active—the free hormone hypothesis.

<image>Panel A: Cholesterol transport to mitochondria via StAR protein as the rate-limiting step in steroidogenesis. Panel B: Common conversion of cholesterol to pregnenolone followed by diverging pathways to specific steroids in different tissues. Panel C: Key steroidogenic enzymes including CYP11A1, CYP17, CYP21, and aromatase in their respective pathways. Panel D: Steroid transport showing equilibrium between free active hormone and carrier-bound reservoir with CBG, SHBG, and albumin.</image>

Hormone Transport and the Free Hormone Hypothesis

Hormone transport through the bloodstream differs between water-soluble and lipid-soluble hormones, with important implications for hormone availability and half-life.

Water-soluble hormones (peptides, catecholamines) circulate freely dissolved in plasma without carrier proteins. They are readily accessible to target tissues but also susceptible to enzymatic degradation and renal clearance, resulting in short half-lives (minutes). Their concentrations can change rapidly, enabling quick physiological responses.

Lipid-soluble hormones (steroids, thyroid hormones) bind to carrier proteins in plasma, with a small fraction remaining free. Specific high-affinity carriers include corticosteroid-binding globulin for cortisol, sex hormone-binding globulin for sex steroids, and thyroxine-binding globulin for thyroid hormones. Albumin provides additional lower-affinity but high-capacity binding for all lipophilic hormones.

Carrier protein binding serves several functions. It prolongs hormone half-life by protecting from degradation and reducing renal filtration—bound hormones are too large for glomerular filtration. It creates a circulating reservoir that buffers hormone levels against rapid fluctuations. It restricts access to target tissues because protein-bound hormone cannot readily cross capillary walls.

The free hormone hypothesis states that only unbound hormone is biologically active. Free hormone can diffuse from capillaries, cross cell membranes, and bind intracellular receptors. The equilibrium between free and bound hormone means that as free hormone is consumed, it is replenished from the bound pool. Changes in binding protein concentrations affect total hormone levels but not necessarily free (biologically active) levels.

Clinical implications arise when binding protein levels change. Pregnancy increases TBG and CBG levels, raising total T4 and cortisol while free levels remain normal. Nephrotic syndrome decreases albumin, affecting total but not free hormone levels. Medications and genetic variants can alter binding protein levels. When interpreting hormone measurements, distinguishing total from free hormone levels is often essential.

<image>Panel A: Protein-bound hormone complexes restricted from crossing capillary endothelium while free hormone diffuses to target tissues. Panel B: Dynamic equilibrium between free and carrier-bound hormone with replenishment from bound pool as free hormone is consumed. Panel C: Clinical examples showing pregnancy and nephrotic syndrome affecting total but not free hormone levels. Panel D: Typical free hormone fractions for cortisol, thyroxine, and testosterone demonstrating extensive protein binding.</image>

Hormone Receptors and Signal Transduction

Hormone receptors translate extracellular hormone signals into intracellular responses through two fundamentally different mechanisms depending on the hormone's chemical properties and the receptor's location.

Membrane receptors serve water-soluble hormones that cannot cross the lipid bilayer. G-protein coupled receptors (GPCRs) represent the largest family, with seven transmembrane domains and coupling to heterotrimeric G proteins (Gα, Gβ, Gγ). Different Gα subtypes produce different effects: Gαs stimulates adenylyl cyclase, increasing cAMP (examples: ACTH, TSH, LH, FSH, glucagon); Gαi inhibits adenylyl cyclase, decreasing cAMP (example: somatostatin); Gαq activates phospholipase C, generating IP3 and DAG, which increase intracellular calcium and activate protein kinase C (examples: GnRH, TRH, oxytocin).

Receptor tyrosine kinases have intrinsic enzymatic activity—hormone binding causes dimerization and autophosphorylation, creating docking sites for signaling proteins. The insulin receptor exemplifies this family, activating the PI3K-Akt pathway for metabolic effects and the Ras-MAPK pathway for growth effects. Growth factor receptors (EGF, IGF-1, PDGF) also belong to this family.

The JAK-STAT pathway serves cytokine and hormone receptors that lack intrinsic kinase activity. Hormone binding causes receptor dimerization and activation of associated Janus kinases (JAKs), which phosphorylate STAT transcription factors that translocate to the nucleus. Growth hormone and prolactin signal through this pathway.

Intracellular receptors serve lipophilic hormones (steroids, thyroid hormones, vitamin D) that can cross the plasma membrane. These receptors are ligand-activated transcription factors with characteristic domains: a hormone-binding domain, a DNA-binding domain (with zinc fingers), and a transcription activation domain. Hormone binding causes conformational changes that expose nuclear localization signals (for cytoplasmic receptors) or release corepressors and recruit coactivators (for nuclear receptors). The hormone-receptor complex binds specific DNA sequences called hormone response elements (HREs), modulating transcription of target genes. Effects are slow (hours to days) because they require mRNA synthesis and protein translation.

<image>Panel A: G-protein coupled receptor signaling through Gs, Gi, and Gq pathways producing cAMP changes or calcium and DAG release. Panel B: Receptor tyrosine kinase signaling with dimerization, autophosphorylation, and activation of PI3K-Akt and Ras-MAPK pathways. Panel C: Cytoplasmic steroid receptor activation with nuclear translocation and hormone response element binding. Panel D: Nuclear thyroid hormone receptor signaling with direct transcriptional regulation.</image>

Hypothalamic-Pituitary Axis

The hypothalamus and pituitary gland form the central command structure for most endocrine systems, integrating neural and endocrine signals to coordinate body-wide hormonal responses. This axis exemplifies neuroendocrine integration—the translation of neural information into hormonal output.

The hypothalamus contains neurosecretory neurons that synthesize releasing and inhibiting hormones, which control anterior pituitary function. These neurons project to the median eminence, where their axon terminals release hormones into the hypothalamic-hypophyseal portal system. This specialized vascular arrangement carries hypothalamic hormones directly to the anterior pituitary, achieving high local concentrations without systemic distribution.

Thyrotropin-releasing hormone (TRH) stimulates TSH release. Corticotropin-releasing hormone (CRH) stimulates ACTH release. Gonadotropin-releasing hormone (GnRH) stimulates LH and FSH release. Growth hormone-releasing hormone (GHRH) stimulates GH release. Somatostatin inhibits GH (and TSH) release. Dopamine inhibits prolactin release—prolactin is unique in being under tonic inhibitory control, explaining why pituitary stalk transection causes hyperprolactinemia.

The anterior pituitary (adenohypophysis) contains multiple endocrine cell types, each producing specific hormones in response to hypothalamic signals. Thyroid-stimulating hormone (TSH) stimulates thyroid hormone synthesis. Adrenocorticotropic hormone (ACTH) stimulates adrenal cortisol production. Luteinizing hormone (LH) triggers ovulation and testosterone production. Follicle-stimulating hormone (FSH) stimulates follicle development and spermatogenesis. Growth hormone (GH) promotes growth and has metabolic effects. Prolactin stimulates milk production.

The posterior pituitary (neurohypophysis) differs fundamentally—it is not a gland but a neural structure containing axon terminals of hypothalamic neurons. The magnocellular neurons of the supraoptic and paraventricular nuclei synthesize oxytocin and antidiuretic hormone (ADH, vasopressin), transport them down their axons, and release them directly into the systemic circulation. Oxytocin causes uterine contraction during labor and milk ejection during lactation. ADH promotes water retention in the kidney collecting ducts.

<image>Panel A: Hypothalamic releasing and inhibiting hormones transported via portal system to control anterior pituitary function. Panel B: Anterior pituitary cell types producing TSH, ACTH, LH, FSH, GH, and prolactin with their target organs. Panel C: Posterior pituitary as neural extension with magnocellular neurons releasing ADH and oxytocin directly into systemic circulation. Panel D: Target tissue effects including thyroid, adrenal cortex, gonads, liver, kidney, uterus, and breast.</image>

Feedback Regulation

Feedback regulation maintains hormone levels within appropriate ranges, preventing both deficiency and excess. Negative feedback is the predominant mechanism, with the end product of an endocrine axis inhibiting earlier steps in the pathway.

Long-loop negative feedback occurs when a target gland hormone inhibits the hypothalamus and/or pituitary. Cortisol inhibits CRH release from the hypothalamus and ACTH release from the anterior pituitary, completing the hypothalamic-pituitary-adrenal (HPA) axis. Similarly, thyroid hormones inhibit TRH and TSH, and sex steroids inhibit GnRH, LH, and FSH.

Short-loop feedback occurs when a pituitary hormone inhibits its own hypothalamic releasing hormone—ACTH may inhibit CRH, for example. Ultra-short-loop feedback occurs when a hormone inhibits its own release from the cell that secreted it, through autocrine mechanisms.

Positive feedback, though less common, plays essential roles in specific circumstances. The LH surge that triggers ovulation demonstrates positive feedback: rising estrogen levels from the dominant follicle, once exceeding a threshold concentration for a sufficient duration, switch from inhibiting to stimulating LH release. This positive feedback produces the explosive LH surge required for ovulation. After ovulation, the corpus luteum produces progesterone, which reinstates negative feedback. Oxytocin during labor provides another example: uterine contractions stimulate stretch receptors that promote oxytocin release, which intensifies contractions in a positive feedback loop terminated by delivery.

Many hormones are secreted in pulsatile patterns rather than continuously. GnRH exemplifies the importance of pulsatile secretion—normal gonadal function requires GnRH pulses approximately every 60-90 minutes. Continuous GnRH exposure paradoxically suppresses LH and FSH through receptor downregulation, a phenomenon exploited therapeutically with GnRH agonists to treat prostate cancer and endometriosis.

Circadian rhythms impose 24-hour patterns on hormone secretion. Cortisol peaks in early morning and reaches its nadir around midnight, reflecting hypothalamic clock regulation. Growth hormone secretion peaks during deep sleep. Melatonin rises in darkness and suppresses during light exposure. These rhythms have clinical implications—cortisol should be measured in the morning, and shift work can disrupt endocrine function.

<image>Panel A: Negative feedback in the HPA axis with cortisol inhibiting both hypothalamic CRH and pituitary ACTH release. Panel B: Positive feedback during ovulation with estrogen switching from inhibition to stimulation of LH surge. Panel C: Pulsatile secretion patterns showing importance of GnRH pulse frequency for normal gonadotropin release. Panel D: Circadian rhythms in cortisol, growth hormone, and melatonin secretion over 24 hours.</image>

Hormone Measurement and Clinical Testing

Clinical assessment of endocrine function requires understanding the principles of hormone measurement and the rationale for dynamic testing. Static measurements of hormone levels, while useful, often provide incomplete information.

Immunoassays—including radioimmunoassay (RIA), enzyme-linked immunosorbent assay (ELISA), and chemiluminescent immunoassay—detect hormones using antibodies that bind specifically to the hormone of interest. These methods offer excellent sensitivity but may cross-react with structurally similar molecules or hormone precursors. Mass spectrometry provides definitive identification and quantification without antibody-related limitations. Bioassays measure the actual biological activity of hormones but are more complex and less commonly used clinically.

Baseline hormone levels must be interpreted considering several factors. Time of collection matters for hormones with circadian variation—cortisol should be measured in the morning. Fasting state affects metabolic hormones—glucose and insulin measurements require fasting. Pulsatile secretion causes moment-to-moment variation—a single measurement may catch a peak or trough.

The distinction between total and free hormone levels is critical for protein-bound hormones. Total T4 reflects both bound and free hormone; free T4 measures only the biologically active fraction. Changes in binding proteins affect total but not free levels. When binding protein changes are suspected (pregnancy, estrogen therapy, liver disease), free hormone measurements provide more accurate assessment.

Dynamic testing evaluates the functional integrity of endocrine axes by measuring responses to stimulation or suppression. Stimulation tests assess the capacity of a gland to respond when challenged. The ACTH (cosyntropin) stimulation test assesses adrenal cortex function—failure to raise cortisol indicates adrenal insufficiency. The insulin tolerance test (gold standard for HPA axis) causes hypoglycemia that should stimulate cortisol and GH release. Suppression tests assess whether negative feedback mechanisms are intact. The dexamethasone suppression test determines whether exogenous glucocorticoid appropriately suppresses ACTH and cortisol—failure to suppress suggests Cushing syndrome with autonomous cortisol production.

<image>Panel A: Immunoassay measurement principles with antibody-hormone binding and competitive labeled hormone techniques. Panel B: Free versus total hormone distinction with clinical conditions affecting binding protein levels. Panel C: Stimulation testing such as ACTH stimulation to assess gland reserve and response capacity. Panel D: Suppression testing such as dexamethasone suppression to assess feedback regulation and autonomous secretion.</image>

Clinical Applications

Endocrine disorders follow predictable patterns based on whether dysfunction originates in the endocrine gland itself (primary disorder) or in the regulatory system controlling that gland (secondary or tertiary disorder).

Primary hypofunction occurs when the target endocrine gland fails. Target hormone levels are low, and trophic hormone levels rise due to loss of negative feedback. Primary hypothyroidism produces low T4 with elevated TSH as the pituitary attempts to stimulate the failing thyroid. Primary adrenal insufficiency (Addison disease) produces low cortisol with elevated ACTH. Treatment requires replacement of the deficient target hormone.

Secondary hypofunction occurs when pituitary production of trophic hormone is inadequate. Both trophic and target hormone levels are low. Pituitary tumors, surgery, radiation, or infarction (Sheehan syndrome) can cause secondary adrenal insufficiency with low ACTH and low cortisol. The pattern of low trophic hormone distinguishes secondary from primary hypofunction.

Primary hyperfunction occurs when a target gland produces excess hormone autonomously, usually due to adenoma, hyperplasia, or malignancy. Target hormone levels are high, and trophic hormone levels are suppressed by negative feedback. Adrenal adenomas producing cortisol autonomously cause Cushing syndrome with high cortisol and suppressed ACTH. Primary hyperaldosteronism (Conn syndrome) produces high aldosterone with suppressed renin.

Secondary hyperfunction occurs when pituitary overproduction of trophic hormone drives excess target hormone production. Both levels are elevated. TSH-secreting pituitary adenomas cause hyperthyroidism with elevated T4 and nonsuppressed TSH—contrasting with the suppressed TSH of primary hyperthyroidism (Graves disease).

Diabetes mellitus, the most common endocrine disorder, involves glucose homeostasis failure. Type 1 diabetes results from autoimmune destruction of pancreatic beta cells, causing insulin deficiency. Type 2 diabetes involves insulin resistance followed eventually by relative insulin deficiency as beta cells cannot maintain compensatory hyperinsulinemia.

Cushing syndrome encompasses all causes of cortisol excess—ACTH-secreting pituitary adenomas (Cushing disease), adrenal adenomas or carcinomas, ectopic ACTH production, and exogenous glucocorticoid administration. Clinical features include central obesity, muscle wasting, skin fragility with striae, hypertension, glucose intolerance, and osteoporosis.

<image>Panel A: Primary hypofunction diagnostic pattern with low target hormone and elevated trophic hormone from loss of feedback inhibition. Panel B: Secondary hypofunction pattern with low target hormone and low or inappropriately normal trophic hormone. Panel C: Primary hyperfunction pattern with high target hormone and suppressed trophic hormone from intact feedback. Panel D: Secondary hyperfunction pattern with elevated target and trophic hormones plus Cushing syndrome clinical features.</image>

Summary

The endocrine system uses hormones traveling through the bloodstream to regulate distant target cells, contrasting with paracrine (local) and autocrine (self) signaling. Hormone classes differ in their synthesis, transport, and signaling mechanisms. Peptide hormones are synthesized on ribosomes, stored in vesicles, circulate freely, bind membrane receptors, and act rapidly through second messengers. Steroid hormones derive from cholesterol, are synthesized on demand, require carrier proteins in blood, bind intracellular receptors, and act slowly through gene transcription.

The free hormone hypothesis states that only unbound hormone is biologically active—carrier proteins extend half-life and create a reservoir while restricting access to tissues. Membrane receptors include GPCRs (coupled to Gs, Gi, or Gq), receptor tyrosine kinases (insulin receptor), and cytokine receptors using the JAK-STAT pathway. Intracellular receptors for steroids and thyroid hormones are ligand-activated transcription factors.

The hypothalamic-pituitary axis coordinates endocrine function through releasing and inhibiting hormones delivered via the portal system to the anterior pituitary, which produces trophic hormones regulating peripheral glands. The posterior pituitary releases oxytocin and ADH synthesized by hypothalamic neurons.

Negative feedback predominates, with target hormones inhibiting hypothalamic and pituitary hormone release. Positive feedback (LH surge, oxytocin during labor) amplifies specific physiological processes. Pulsatile and circadian secretion patterns add temporal organization.

Clinical endocrine disorders are classified as primary (target gland dysfunction) or secondary (pituitary/hypothalamic dysfunction), with characteristic patterns of target and trophic hormone levels that guide diagnosis. Dynamic stimulation and suppression tests evaluate the functional integrity of endocrine axes.

Key Terms

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