# Lecture 24: The Endocrine System — Pituitary Gland and Hypothalamus

## Anatomy and Physiology I

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## Learning Objectives

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

1. Compare the nervous and endocrine systems as mechanisms of intercellular communication
2. Define hormone and classify hormones by chemical structure
3. Describe the general mechanisms of hormone action including receptor binding, signal transduction, and cellular response
4. Explain the mechanisms of hormone secretion regulation including negative feedback, positive feedback, and neural control
5. Describe the anatomy of the hypothalamus and pituitary gland and their vascular connections
6. List the hormones of the anterior and posterior pituitary, their target organs, and their actions
7. Describe the hypothalamic-pituitary axes and their regulation

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## Lecture Content

### I. Overview of the Endocrine System

The endocrine system communicates via **hormones**, chemical messengers secreted into the blood by endocrine glands and transported to distant target cells. It works alongside the nervous system to maintain homeostasis but differs in several important ways. Compared to the nervous system, the endocrine system has a slower onset of action (seconds to hours versus milliseconds), produces longer-lasting effects (hours to weeks versus milliseconds), exerts broader effects (since hormones reach all cells via blood, with target specificity depending on receptor expression), and uses chemical messengers in the blood rather than electrochemical signals along neurons.

#### Types of Chemical Signaling

Chemical signaling occurs through several distinct modes. **Endocrine** signaling involves a hormone released into the blood that acts on distant target cells. **Paracrine** signaling involves chemical messengers acting on nearby cells, as exemplified by prostaglandins and growth factors. **Autocrine** signaling involves a chemical messenger acting on the same cell that secreted it. **Neuroendocrine** signaling occurs when neurons release hormones into the blood, as when hypothalamic neurons release ADH and oxytocin.

### II. Hormone Chemistry

#### Chemical Classification

Hormones are classified into two broad categories based on their chemical structure. **Amino acid-based hormones** include the amines, which are derived from single amino acids. Tyrosine derivatives include catecholamines (epinephrine, norepinephrine, dopamine) and thyroid hormones (T3 and T4), while tryptophan derivatives include melatonin and serotonin. This category also includes peptides and proteins, which are chains of amino acids ranging from short peptides like ADH, oxytocin, TRH, and GnRH to longer proteins like insulin, growth hormone, prolactin, PTH, FSH, and LH (some of which carry carbohydrate groups as glycoproteins).

**Lipid-based hormones** include steroids, which are derived from cholesterol and encompass cortisol, aldosterone, testosterone, estrogen, progesterone, and calcitriol (active vitamin D). Eicosanoids, derived from arachidonic acid, include prostaglandins, leukotrienes, and thromboxanes that function primarily in paracrine and autocrine signaling.

#### Hormone Transport in the Blood

**Water-soluble hormones** (amines except thyroid hormones, peptides, and proteins) dissolve freely in plasma, have a short half-life measured in minutes, cannot cross cell membranes, and bind to cell-surface receptors. **Lipid-soluble hormones** (steroids and thyroid hormones) must bind to transport proteins in the blood (such as thyroxine-binding globulin, sex hormone-binding globulin, or albumin), have a longer half-life measured in hours to days, can cross cell membranes, and bind to intracellular receptors. Only the **free (unbound)** fraction of a hormone is biologically active, while the bound fraction serves as a circulating reservoir.

### III. Mechanisms of Hormone Action

#### Water-Soluble Hormones — Cell-Surface Receptors

Water-soluble hormones cannot cross the plasma membrane and therefore bind to receptors on the cell surface, activating **second messenger systems** inside the cell. The **cAMP pathway** involves hormone binding to a receptor, activation of a Gs protein, activation of adenylyl cyclase, conversion of ATP to cAMP, activation of protein kinase A (PKA), and phosphorylation of target proteins to produce the cellular response. Examples include epinephrine (at beta receptors), glucagon, TSH, ACTH, PTH, FSH, and LH.

The **PIP2-IP3/DAG pathway** involves hormone binding to a receptor, activation of a Gq protein, activation of phospholipase C, and cleavage of PIP2 into IP3 (which releases calcium from the ER) and DAG (which activates protein kinase C). Both calcium and PKC phosphorylate target proteins. Examples include oxytocin, GnRH, and epinephrine (at alpha-1 receptors).

The **tyrosine kinase pathway** involves hormone binding to a receptor tyrosine kinase, receptor autophosphorylation, and activation of intracellular signaling cascades including the Ras-MAPK and PI3K-Akt pathways. Examples include insulin and growth factors such as IGF, EGF, and PDGF.

These cell-surface receptor pathways produce rapid effects (seconds to minutes) that are generally short-lived. A key feature is the amplification cascade, in which each step amplifies the signal so that one hormone molecule can ultimately activate thousands of enzyme molecules.

#### Lipid-Soluble Hormones — Intracellular Receptors

Lipid-soluble hormones cross the plasma membrane and bind to **intracellular receptors** located either in the cytoplasm or the nucleus. The hormone-receptor complex acts as a **transcription factor**, binding to specific DNA sequences called hormone response elements (HREs) in the nucleus to activate or repress gene transcription. New mRNA is transcribed and new proteins are synthesized. The effects are slower (hours) but longer lasting (days to weeks) because they involve changes in gene expression. Examples include all steroid hormones (cortisol, aldosterone, testosterone, estrogen, progesterone), thyroid hormones, and calcitriol.

### IV. Regulation of Hormone Secretion

#### Negative Feedback

Negative feedback is the **primary** mechanism regulating most hormone levels. Rising hormone levels or the hormone's downstream effects inhibit further release of that hormone. For example, rising blood T3/T4 inhibits TRH release from the hypothalamus and TSH release from the anterior pituitary, reducing further thyroid hormone production. This mechanism maintains hormones within a narrow physiological range.

#### Positive Feedback

Positive feedback is rare and occurs when a hormone's effects amplify its own secretion. Examples include oxytocin during labor (uterine contractions stimulate more oxytocin release, which stimulates stronger contractions until delivery ends the loop) and the LH surge during ovulation (rising estrogen stimulates more LH release). Positive feedback loops require an external event to terminate them.

#### Neural Control

The nervous system can override or modulate hormonal feedback. For example, the sympathetic nervous system stimulates epinephrine release from the adrenal medulla during stress regardless of blood epinephrine levels.

#### Humoral Stimuli

Changes in blood levels of specific ions or nutrients directly stimulate hormone release. Rising blood glucose stimulates insulin release from pancreatic beta cells, and falling blood calcium stimulates PTH release.

#### Hormonal Stimuli

One hormone can stimulate the release of another hormone, as when TSH from the anterior pituitary stimulates T3/T4 release from the thyroid.

### V. The Hypothalamus — The Neuroendocrine Link

The hypothalamus serves as the master regulator of the endocrine system. Located in the diencephalon, forming the floor and lateral walls of the third ventricle, it functions as the critical bridge between the nervous system and the endocrine system. It receives neural input from the limbic system, cerebral cortex, reticular formation, and visceral afferents, and converts these neural signals into hormonal signals.

The hypothalamus controls the pituitary gland through two distinct mechanisms. First, it exerts **neural control** of the posterior pituitary, in which hypothalamic neurons send axons directly to the posterior pituitary. Second, it exerts **hormonal control** of the anterior pituitary, in which hypothalamic neurons release regulatory hormones into a portal blood system.

### VI. The Pituitary Gland (Hypophysis)

The pituitary gland is a small (approximately 1.2 cm diameter), pea-sized gland located in the **hypophyseal fossa (sella turcica)** of the sphenoid bone. It is connected to the hypothalamus by the **infundibulum (pituitary stalk)** and consists of two functional lobes with different embryological origins. The **anterior pituitary (adenohypophysis)** is derived from oral ectoderm (Rathke pouch) and consists of glandular tissue. The **posterior pituitary (neurohypophysis)** is derived from neural ectoderm and is essentially an extension of the hypothalamus, containing axon terminals of hypothalamic neurons.

### VII. The Posterior Pituitary (Neurohypophysis)

The posterior pituitary does not synthesize hormones; rather, it stores and releases hormones produced by hypothalamic neurons. The **supraoptic nucleus** primarily produces **ADH (antidiuretic hormone, also called vasopressin)**, while the **paraventricular nucleus** primarily produces **oxytocin** (and also some ADH). These hormones are synthesized in the hypothalamic neuron cell bodies, packaged into vesicles, and transported down the axons through the infundibulum to the posterior pituitary via the **hypothalamo-hypophyseal tract**. They are stored in axon terminals called **Herring bodies** until released into the blood by exocytosis.

#### ADH (Antidiuretic Hormone / Vasopressin)

ADH targets the kidneys, specifically the collecting ducts, where it increases water reabsorption by inserting **aquaporin-2 (AQP2)** water channels into the apical membrane. This produces concentrated, low-volume urine. At high concentrations, ADH also causes vasoconstriction (hence "vasopressin"), raising blood pressure. ADH release is stimulated by increased blood osmolarity (detected by osmoreceptors in the hypothalamus), decreased blood volume or pressure (detected by baroreceptors), pain, nausea, and stress. It is inhibited by decreased blood osmolarity, increased blood volume, and alcohol (which explains the diuretic effect of alcohol).

Clinically, **diabetes insipidus** results from deficiency of ADH (central type) or kidney insensitivity to ADH (nephrogenic type), producing large volumes of dilute urine and intense thirst. Conversely, **SIADH (syndrome of inappropriate ADH secretion)** involves excess ADH, causing water retention, hyponatremia, and concentrated urine.

#### Oxytocin

Oxytocin targets the uterus and mammary glands. It stimulates **uterine smooth muscle contraction** during labor through a positive feedback mechanism (contractions stimulate more oxytocin release until delivery) and stimulates **milk ejection (the let-down reflex)** by contracting myoepithelial cells around mammary alveoli during breastfeeding. Oxytocin also plays roles in social bonding, trust, and pair bonding, earning it the nickname "love hormone." Its release is stimulated by cervical and uterine stretch (Ferguson reflex), suckling (a neuroendocrine reflex), and emotional bonding. Clinically, synthetic oxytocin (Pitocin) is used to induce or augment labor.

<image>A midsagittal diagram of the hypothalamus and pituitary gland showing the two control systems. On the left side, the hypothalamo-hypophyseal portal system is shown: neurosecretory cells in the hypothalamus release regulatory hormones (releasing and inhibiting hormones) into the primary capillary plexus in the median eminence. These hormones travel via portal veins down the infundibulum to the secondary capillary plexus in the anterior pituitary, where they stimulate or inhibit hormone-producing cells. The six anterior pituitary hormones (GH, TSH, ACTH, FSH, LH, PRL) are shown being released into the systemic circulation. On the right side, the hypothalamo-hypophyseal tract is shown: large neurosecretory neurons in the supraoptic and paraventricular nuclei send axons through the infundibulum to the posterior pituitary. ADH and oxytocin are stored in axon terminals (Herring bodies) and released directly into the blood. The sella turcica of the sphenoid bone is shown cradling the pituitary gland. An inset highlights the portal system concept: hypothalamic capillary bed to portal veins to anterior pituitary capillary bed, emphasizing that this direct vascular connection allows very small amounts of hypothalamic hormones to reach the anterior pituitary at high concentrations.</image>

### VIII. The Anterior Pituitary (Adenohypophysis)

#### Hypothalamic-Hypophyseal Portal System

A portal system is defined as two capillary beds connected by portal veins without an intervening heart pump. In this system, the **primary capillary plexus** lies in the median eminence of the hypothalamus, where neurosecretory neurons release releasing and inhibiting hormones. **Hypophyseal portal veins** carry these hormones down the infundibulum to the **secondary capillary plexus** in the anterior pituitary, where the hormones exit and act on anterior pituitary cells. This arrangement allows hypothalamic hormones to reach the anterior pituitary at high concentrations without dilution in the general circulation.

#### Cell Types and Hormones of the Anterior Pituitary

**1. Somatotrophs (approximately 40-50% of cells) — Growth Hormone (GH / Somatotropin)**

Somatotrophs are regulated by GHRH (growth hormone-releasing hormone, which stimulates GH release), somatostatin (GHIH, which inhibits GH release), and ghrelin from the stomach (which also stimulates GH release). Growth hormone promotes **linear growth** during childhood and adolescence by stimulating epiphyseal plate growth. It also stimulates the liver to produce **insulin-like growth factor 1 (IGF-1 / somatomedin C)**, which mediates many of GH's growth-promoting effects. Metabolically, GH increases lipolysis, increases blood glucose (an anti-insulin or diabetogenic effect), increases protein synthesis, and stimulates amino acid uptake by cells. Regulation occurs through negative feedback: IGF-1 inhibits GH and GHRH release, while GH stimulates somatostatin release.

Clinically, hypersecretion in children causes **gigantism** (excessive linear growth), while hypersecretion in adults after epiphyseal closure causes **acromegaly** (enlargement of hands, feet, jaw, facial bones, and soft tissues). Hyposecretion in children produces **pituitary dwarfism** (proportional short stature), which is treatable with recombinant GH.

**2. Thyrotrophs (approximately 5%) — Thyroid-Stimulating Hormone (TSH / Thyrotropin)**

TSH release is stimulated by TRH (thyrotropin-releasing hormone) from the hypothalamus. TSH targets the thyroid gland, where it stimulates thyroid hormone (T3/T4) synthesis and secretion and promotes thyroid gland growth. Regulation occurs through negative feedback: T3/T4 inhibit both TSH and TRH release.

**3. Corticotrophs (approximately 15-20%) — Adrenocorticotropic Hormone (ACTH / Corticotropin)**

ACTH release is stimulated by CRH (corticotropin-releasing hormone), with stress and circadian rhythms serving as major drivers. ACTH targets the adrenal cortex (zona fasciculata and zona reticularis), where it stimulates cortisol secretion and, to a lesser extent, adrenal androgens. ACTH is cleaved from a larger precursor molecule called **proopiomelanocortin (POMC)**, which also yields **melanocyte-stimulating hormone (MSH)**. This shared precursor explains the hyperpigmentation seen in conditions with excess ACTH, such as Addison disease. Regulation is through negative feedback: cortisol inhibits both CRH and ACTH release. Cortisol secretion follows a **circadian (diurnal) rhythm**, being highest in the early morning and lowest at midnight.

**4. Gonadotrophs (approximately 10-15%) — Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH)**

Both gonadotropins are stimulated by GnRH (gonadotropin-releasing hormone), which is released in pulses. This pulsatile secretion is critical for normal gonadotropin release. FSH and LH target the gonads (ovaries and testes). In females, FSH stimulates ovarian follicle development and estrogen production, while LH triggers ovulation, stimulates corpus luteum formation, and stimulates progesterone and estrogen secretion. In males, FSH stimulates spermatogenesis by acting on Sertoli cells, while LH stimulates testosterone production by acting on Leydig cells. Regulation involves negative feedback from sex steroids (estrogen, progesterone, testosterone) and inhibin, which inhibit GnRH, FSH, and LH. A notable exception is the positive feedback by estrogen that triggers the LH surge at midcycle, inducing ovulation.

**5. Lactotrophs (approximately 15-20%) — Prolactin (PRL)**

Prolactin regulation is unique among anterior pituitary hormones in that its primary regulator is inhibitory. **Dopamine (prolactin-inhibiting hormone / PIH)** tonically inhibits prolactin release, while TRH and other releasing factors can stimulate it. Prolactin targets the mammary glands and stimulates **milk production** (lactation) after priming by estrogen and progesterone during pregnancy. Suckling reduces dopamine release and increases prolactin release through a neuroendocrine reflex. High prolactin levels inhibit GnRH, suppressing ovulation during breastfeeding (lactational amenorrhea).

Clinically, **hyperprolactinemia** is the most common pituitary hormone disorder. Causes include prolactinoma (a pituitary adenoma) and drugs that block dopamine (antipsychotics). Effects include galactorrhea (inappropriate milk production), amenorrhea in females, infertility, and decreased libido. Treatment involves dopamine agonists (cabergoline, bromocriptine), which shrink prolactinomas and reduce prolactin levels.

<image>A comprehensive diagram of the hypothalamic-pituitary axes. The hypothalamus is shown at the top with its regulatory hormones listed (GHRH, somatostatin, TRH, CRH, GnRH, dopamine). Arrows descend via the portal system to the anterior pituitary, which is divided into its cell types. Each cell type releases its hormone: somatotrophs release GH (arrow to liver producing IGF-1, and to bone/muscle/fat showing growth and metabolic effects), thyrotrophs release TSH (arrow to the thyroid gland producing T3/T4), corticotrophs release ACTH (arrow to the adrenal cortex producing cortisol), gonadotrophs release FSH and LH (arrows to the ovary producing estrogen/progesterone and to the testis producing testosterone), and lactotrophs release PRL (arrow to the mammary gland producing milk). Negative feedback loops are shown as dashed lines: IGF-1 and GH feed back to inhibit GHRH and GH release; T3/T4 inhibit TRH and TSH; cortisol inhibits CRH and ACTH; sex steroids and inhibin inhibit GnRH and FSH/LH. Dopamine's tonic inhibition of prolactin is highlighted with a minus sign. The posterior pituitary is shown separately with ADH targeting the kidney and oxytocin targeting the uterus and mammary gland.</image>

### IX. Clinical Correlations

#### Pituitary Adenomas

Pituitary adenomas are the most common pituitary pathology, representing benign tumors arising from anterior pituitary cells. **Functional adenomas** secrete excess hormones, with prolactinoma being the most common, followed by GH-secreting and ACTH-secreting adenomas. **Non-functional adenomas** do not secrete hormones but can compress surrounding structures. Mass effects include compression of the optic chiasm causing **bitemporal hemianopia** (loss of peripheral vision in both eyes) and compression of normal pituitary tissue causing **hypopituitarism**. **Hypopituitarism (panhypopituitarism)** is a deficiency of all anterior pituitary hormones and can result from pituitary adenomas, surgery, radiation, Sheehan syndrome (postpartum pituitary necrosis from hemorrhage), or traumatic brain injury.

#### Diabetes Insipidus vs. SIADH

**Diabetes insipidus** results from ADH deficiency (central) or resistance (nephrogenic) and manifests as polyuria (up to 20 L/day of dilute urine), polydipsia, hypernatremia, and high serum osmolality. **SIADH** involves excess ADH causing water retention, concentrated urine, hyponatremia, and low serum osmolality. Causes include CNS disorders, lung disease, certain drugs, and ectopic production by tumors.

#### Cushing Disease vs. Cushing Syndrome

**Cushing disease** refers specifically to an ACTH-secreting pituitary adenoma driving excess cortisol production. **Cushing syndrome** is the broader clinical syndrome of cortisol excess from any cause, including pituitary adenoma, adrenal tumor, ectopic ACTH, or exogenous corticosteroids. Clinical features include moon face, buffalo hump, central obesity, purple striae, hyperglycemia, hypertension, osteoporosis, immunosuppression, and muscle wasting.