The Endocrine System: Hormones and Glands

Detailed Notes

21.1 Overview of the Endocrine System

The endocrine system is one of the body's two major control systems, alongside the nervous system. While both systems coordinate and regulate the activities of cells, tissues, and organs, they do so through fundamentally different mechanisms.

Table 21.1 — Comparison of the Nervous and Endocrine Systems

The endocrine system includes endocrine glands—ductless organs that secrete hormones directly into the interstitial fluid, from which they diffuse into blood capillaries. These glands are widely distributed throughout the body and include the pituitary, thyroid, parathyroid, adrenal, pineal, and the endocrine pancreas (islets of Langerhans). Additionally, several organs that have primary non-endocrine functions also contain endocrine tissue: the hypothalamus (part of the brain), heart (atrial natriuretic peptide), kidneys (erythropoietin, renin), adipose tissue (leptin), and the gastrointestinal tract (gastrin, secretin, CCK, and others).

Hormones exert their effects at very low concentrations and influence target cells with remarkable specificity. A given hormone circulates throughout the entire body, yet only cells that possess receptors for that specific hormone—referred to as target cells—will respond to it. This is often compared to a radio broadcast: the signal is everywhere, but only radios tuned to the correct frequency can receive it.

21.2 What Are Hormones?

A hormone is a chemical messenger released by an endocrine gland or specialized cell into the bloodstream, which travels to distant target cells and elicits a specific physiological response by binding to receptors. Hormones regulate nearly every major physiological process, including growth and development, metabolism, reproduction, electrolyte and water balance, and the body's response to stress.

Hormones can be classified by the distance they travel from their site of release to their site of action:

1. Endocrine signaling (classical): The hormone is secreted into the bloodstream and travels throughout the body to reach distant target cells. This is the defining mode of the endocrine system. Example: thyroid-stimulating hormone (TSH) released from the anterior pituitary travels via the blood to the thyroid gland.

1. Paracrine signaling: The signaling molecule diffuses through the interstitial fluid to act on neighboring cells of a different type. The molecule does not enter the general circulation. Example: somatostatin released by pancreatic delta cells acts locally on nearby alpha and beta cells to inhibit glucagon and insulin secretion.

1. Autocrine signaling: The signaling molecule acts on the same cell that released it, or on cells of the same type. Example: certain prostaglandins released by smooth muscle cells can bind to receptors on the same cell that produced them, modulating its own contraction.

The target cell specificity of a hormone is determined entirely by the presence or absence of receptor proteins on or in the target cell. Receptors are large proteins or glycoproteins with a binding site that is complementary in shape and chemical properties to the hormone molecule—analogous to a lock and key. The strength of binding between a hormone and its receptor is termed affinity. Because hormones circulate at extremely low concentrations (typically nanomolar to picomolar), receptors must have high affinity to capture the hormone from the blood effectively.

When a hormone binds to its receptor, it triggers a cascade of intracellular events—the signal transduction pathway—that ultimately changes the target cell's activity. This change may involve activation or inhibition of enzymes, opening or closing of ion channels, alterations in gene transcription and protein synthesis, or changes in the rate of cellular secretion.

21.3 Hormone Chemical Classes

Hormones can be divided into four major chemical classes based on their molecular structure, which in turn determines their solubility, how they are transported in the blood, and their mechanism of action at the target cell.

Table 21.2 — The Four Chemical Classes of Hormones

21.3.1 Amino Acid Derivatives

Amino acid-derived hormones are synthesized from the amino acid tyrosine (with the exception of melatonin, which is derived from tryptophan). This class includes:

• Catecholamines: Epinephrine (adrenaline), norepinephrine (noradrenaline), and dopamine. These are water-soluble molecules synthesized in the adrenal medulla and certain neurons. They travel free in the plasma and bind to cell-surface receptors.

• Thyroid hormones: Triiodothyronine (T₃) and thyroxine (T₄). Despite being amino acid derivatives, thyroid hormones are uniquely lipid-soluble. They are synthesized in the thyroid gland by iodination of tyrosine residues within the large protein thyroglobulin. In the bloodstream, T₃ and T₄ are mostly bound to thyroxine-binding globulin (TBG) and other carrier proteins. Because they are lipid-soluble, their receptors are located inside target cells (in the nucleus), where they regulate gene transcription.

• Melatonin: Derived from tryptophan (not tyrosine), melatonin is produced by the pineal gland. It is water-soluble and plays a central role in regulating circadian rhythms and the sleep-wake cycle.

21.3.2 Peptide and Protein Hormones

This is the largest and most structurally diverse class of hormones. They are chains of amino acids linked by peptide bonds:

• Peptide hormones: Short chains, typically fewer than 50 amino acids. Examples include antidiuretic hormone (ADH) and oxytocin (each 9 amino acids).

• Protein hormones: Longer polypeptide chains, often with complex tertiary and quaternary structure. Examples include insulin (51 amino acids, two chains linked by disulfide bridges), growth hormone (GH) (191 amino acids), and thyroid-stimulating hormone (TSH) (a glycoprotein with α and β subunits).

• Glycoprotein hormones: A subgroup in which carbohydrate groups are attached to the protein. Examples include TSH, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

Peptide and protein hormones are water-soluble and therefore cannot cross the hydrophobic lipid bilayer of the plasma membrane. They are synthesized as larger, inactive preprohormones on the rough endoplasmic reticulum, cleaved to prohormones in the ER lumen, and then packaged into secretory vesicles in the Golgi apparatus, where they are further cleaved to the active hormone. They are released by exocytosis and travel free (unbound) in the plasma. Because they are water-soluble, their receptors are located on the cell surface, and they exert their effects through second-messenger systems (see Section 21.4.2).

21.3.3 Steroid Hormones

Steroid hormones are derived from cholesterol. They share a characteristic four-ring hydrocarbon core structure. This class includes:

• Adrenal cortex hormones: Cortisol (a glucocorticoid), aldosterone (a mineralocorticoid), and adrenal androgens (e.g., DHEA).
• Gonadal hormones: Testosterone (primary male androgen), estradiol (primary estrogen), and progesterone.
• Calcitriol: The active form of vitamin D, synthesized in the kidneys.

Steroid hormones are lipid-soluble. They are not stored in significant amounts within the producing cells; instead, they are synthesized on demand from cholesterol stores and diffuse freely across the plasma membrane as they are produced. In the bloodstream, steroid hormones are mostly bound to carrier proteins (e.g., corticosteroid-binding globulin, sex hormone-binding globulin, albumin). Only the small free (unbound) fraction is biologically active and capable of diffusing into target cells.

Inside the target cell, steroid hormones bind to intracellular receptors located in the cytoplasm or nucleus. The hormone-receptor complex then acts as a transcription factor, binding to specific DNA sequences called hormone response elements (HREs) and regulating the transcription of target genes (see Section 21.4.1).

21.3.4 Eicosanoids

Eicosanoids are a family of locally acting, lipid-soluble signaling molecules derived from the 20-carbon fatty acid arachidonic acid, which is released from membrane phospholipids by the enzyme phospholipase A₂. The two major synthetic pathways are:

1. Cyclooxygenase (COX) pathway: Produces prostaglandins and thromboxanes.
2. Lipoxygenase pathway: Produces leukotrienes.

Eicosanoids are unique among hormones in that they are not synthesized in advance and stored; rather, they are produced on demand and act primarily as paracrine or autocrine agents in the immediate vicinity of their release. They have an extremely short half-life (seconds to minutes) and are rapidly degraded by local enzymes. Clinically, nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and aspirin work by inhibiting the cyclooxygenase (COX) enzymes, thereby reducing prostaglandin synthesis and alleviating pain, fever, and inflammation.

21.4 Hormone Mechanisms of Action

The mechanism by which a hormone alters target cell function depends critically on whether the hormone is lipid-soluble or water-soluble. This distinction determines where the hormone's receptor is located and the nature of the intracellular signaling cascade.

21.4.1 Lipid-Soluble Hormones: The Intracellular Receptor Pathway

Lipid-soluble hormones—steroid hormones and thyroid hormones—can diffuse directly through the plasma membrane. Their receptors are located inside the cell, either in the cytoplasm or, more commonly, in the nucleus.

Mechanism (Direct Gene Activation):

1. The hormone dissociates from its carrier protein in the bloodstream and diffuses across the plasma membrane of the target cell because it is lipid-soluble.

1. Inside the cell, the hormone binds to its specific intracellular receptor. For steroid hormones, the receptor is often located in the cytoplasm, complexed with chaperone proteins (such as heat-shock proteins) that keep the receptor in an inactive conformation. Hormone binding causes the chaperone proteins to dissociate, exposing a nuclear localization signal.

1. The hormone-receptor complex translocates to the nucleus (if not already there). Thyroid hormone receptors are already bound to DNA in the nucleus even in the absence of hormone.

1. In the nucleus, the hormone-receptor complex binds to specific DNA sequences called hormone response elements (HREs) located in the regulatory (promoter) regions of target genes.

1. Binding to the HRE either activates or represses the transcription of specific genes, leading to increased or decreased synthesis of particular mRNA molecules.

1. The newly transcribed mRNA is translated on ribosomes, resulting in altered production of specific proteins (enzymes, structural proteins, transporters, or other hormones).

1. These newly synthesized proteins mediate the physiological response.

Key features: The intracellular receptor pathway is relatively slow—taking hours to days for the full effect to manifest—because it requires transcription, mRNA processing, translation, and protein accumulation. However, the effects are typically long-lasting, because the newly synthesized proteins continue to function well after the hormone has been cleared from the circulation.

Example: Cortisol diffuses into target cells, binds to the glucocorticoid receptor in the cytoplasm, and the complex enters the nucleus to stimulate transcription of genes involved in gluconeogenesis (glucose synthesis) and to suppress transcription of genes involved in immune and inflammatory responses.

21.4.2 Water-Soluble Hormones: Second Messenger Systems

Water-soluble hormones—peptide/protein hormones, catecholamines, and melatonin—cannot cross the hydrophobic plasma membrane. Their receptors are integral membrane proteins located on the cell surface. When the hormone (the first messenger) binds to its receptor, the signal must be relayed across the membrane into the cell interior by a cascade of intracellular molecules called second messengers.

21.4.2.1 The cAMP Second Messenger System

The cyclic adenosine monophosphate (cAMP) pathway is one of the most widespread second messenger systems in the body. Hormones that use this pathway include epinephrine (acting through β-adrenergic receptors), glucagon, ADH (acting through V2 receptors in the kidney), TSH, ACTH, FSH, LH, PTH, and calcitonin.

Mechanism (cAMP Pathway):

1. The water-soluble hormone (first messenger) binds to its specific receptor on the cell surface.

1. The hormone-bound receptor activates a G protein (a guanine nucleotide-binding protein) located on the inner face of the plasma membrane. G proteins are trimeric (α, β, γ subunits). In the resting state, the α subunit binds GDP. Receptor activation causes the G protein to exchange GDP for GTP, and the activated α subunit (with its bound GTP) dissociates from the βγ dimer.

1. The activated Gα subunit diffuses laterally in the membrane and stimulates (Gαₛ, stimulatory) or inhibits (Gαᵢ, inhibitory) the enzyme adenylyl cyclase (also called adenylate cyclase).

1. Activated adenylyl cyclase converts ATP into cAMP (cyclic AMP) — the second messenger.

1. cAMP binds to and activates protein kinase A (PKA). In its inactive state, PKA consists of two regulatory subunits and two catalytic subunits. cAMP binding to the regulatory subunits causes them to dissociate, freeing the catalytic subunits, which are now active.

1. Activated PKA phosphorylates (adds a phosphate group to) specific intracellular proteins—enzymes, ion channels, or transcription factors—using ATP as the phosphate donor. This phosphorylation alters the activity of these target proteins, producing the cellular response.

1. The response is terminated when the hormone dissociates from the receptor. The Gα subunit possesses intrinsic GTPase activity and hydrolyzes GTP to GDP, returning the G protein to its inactive state. Additionally, the enzyme phosphodiesterase (PDE) rapidly degrades cAMP to AMP, shutting off the signal.

Amplification: A key feature of this cascade is enormous signal amplification. A single hormone-receptor complex can activate multiple G proteins, each of which activates an adenylyl cyclase enzyme that produces many cAMP molecules. Each cAMP activates one PKA, and each PKA phosphorylates many target proteins. Thus, one hormone molecule can ultimately trigger the production of millions of product molecules inside the cell—explaining why hormones are effective at extremely low (picomolar) concentrations.

21.4.2.2 The PIP₂ (PIP2)-Calcium Second Messenger System

A second major G-protein-coupled pathway uses the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP₂) to generate two distinct second messengers, and leads to an increase in intracellular calcium ions (Ca²⁺). Hormones that use this pathway include epinephrine (acting through α₁-adrenergic receptors), oxytocin, ADH (acting through V1 receptors on vascular smooth muscle), and gonadotropin-releasing hormone (GnRH) from the hypothalamus.

Mechanism (PIP₂-Ca²⁺ Pathway):

1. The hormone binds to its cell-surface receptor, which is coupled to a Gq protein (a specific G protein subtype).

1. Activated Gαq stimulates the membrane-bound enzyme phospholipase C (PLC).

1. PLC cleaves PIP₂ (a minor membrane phospholipid) into two products: diacylglycerol (DAG) and inositol trisphosphate (IP₃). Both act as second messengers.

1. IP₃ is water-soluble and diffuses through the cytoplasm to the endoplasmic reticulum (ER). It binds to IP₃-gated calcium channels on the ER membrane, triggering the release of stored Ca²⁺ into the cytoplasm. This raises the intracellular Ca²⁺ concentration from a resting level of approximately 0.1 µM to 1–10 µM.

1. DAG remains embedded in the plasma membrane (it is lipid-soluble) and, together with the elevated Ca²⁺, activates protein kinase C (PKC). PKC then phosphorylates a distinct set of target proteins, producing cellular responses.

1. The elevated calcium also binds to the protein calmodulin. The Ca²⁺–calmodulin complex activates various enzymes, including Ca²⁺/calmodulin-dependent protein kinases (CaMK), which phosphorylate additional target proteins and amplify the response.

1. The signal is terminated when IP₃ is rapidly dephosphorylated, Ca²⁺ is pumped back into the ER by SERCA pumps and out of the cell, and DAG is metabolized.

Note on calcium's dual role: Calcium (Ca²⁺) is sometimes considered a third messenger in this pathway. It also acts as a second messenger in other systems—for example, in smooth muscle contraction, where it binds to calmodulin and activates MLCK.

21.5 The Hypothalamic-Pituitary Axis

The hypothalamus and pituitary gland together form the hypothalamic-pituitary axis, the master command center of the endocrine system. The hypothalamus is a region of the diencephalon in the brain that receives input from virtually all areas of the central nervous system and integrates neural, hormonal, and humoral signals to maintain homeostasis. It directly controls the pituitary gland, which in turn regulates most of the body's other endocrine glands.

The pituitary gland (also called the hypophysis) is a pea-sized structure located in the sella turcica of the sphenoid bone, suspended from the hypothalamus by a stalk called the infundibulum. It consists of two anatomically and functionally distinct lobes: the posterior pituitary (neurohypophysis) and the anterior pituitary (adenohypophysis).

Table 21.3 — Posterior vs. Anterior Pituitary: Key Differences

21.5.1 The Posterior Pituitary

The posterior pituitary does not synthesize hormones; it stores and releases two hormones that are synthesized by neurons in the supraoptic and paraventricular nuclei of the hypothalamus. These hormones are transported down the axons of the hypothalamic-hypophyseal tract and stored in axon terminals within the posterior pituitary until a nerve signal triggers their release by exocytosis.

1. Antidiuretic Hormone (ADH) — also called vasopressin:
   • Produced by: Primarily the supraoptic nucleus of the hypothalamus.
   • Targets: Kidneys (collecting ducts and distal convoluted tubules), and vascular smooth muscle (at higher concentrations).
   • Actions: In the kidneys, ADH promotes insertion of aquaporin-2 water channels into the apical membrane of collecting duct principal cells, increasing water reabsorption, reducing urine volume, and concentrating the urine — hence the name antidiuretic hormone. At higher concentrations, it causes vasoconstriction (hence the alternate name vasopressin), which can raise blood pressure.
   • Regulation: ADH release is stimulated by increased plasma osmolality (detected by osmoreceptors in the hypothalamus), decreased blood volume or blood pressure, and by factors such as pain, stress, and certain drugs. It is inhibited by adequate hydration (decreased osmolality) and alcohol.

1. Oxytocin:
   • Produced by: Primarily the paraventricular nucleus of the hypothalamus.
   • Targets: Uterine smooth muscle and mammary glands.
   • Actions: During childbirth, oxytocin stimulates powerful rhythmic contractions of uterine smooth muscle (labor). After delivery, it triggers the milk let-down reflex — contraction of myoepithelial cells surrounding the mammary alveoli, ejecting milk toward the nipple. Oxytocin also plays a role in social bonding, maternal behavior, and sexual arousal in both sexes.
   • Regulation: Oxytocin release is triggered by stretching of the uterine cervix (during labor) and by suckling of an infant at the breast. Both stimuli activate a positive feedback loop (see Section 21.7.2).

21.5.2 The Anterior Pituitary

The anterior pituitary synthesizes and secretes six major hormones under the control of releasing hormones and inhibiting hormones from the hypothalamus. These hypothalamic regulatory hormones reach the anterior pituitary via the hypothalamic-hypophyseal portal system — a specialized capillary network in which blood flows from capillaries in the median eminence of the hypothalamus, down the infundibulum through portal veins, to a second capillary bed in the anterior pituitary. This portal system ensures that the hypothalamic hormones reach the anterior pituitary cells at high concentrations without being diluted in the general circulation.

Table 21.4 — Hypothalamic Regulatory Hormones and Their Anterior Pituitary Targets

Anterior Pituitary Hormones and Their Functions:

1. Growth Hormone (GH) — also called somatotropin:
   • Targets: Liver (primary), bone, muscle, adipose tissue, and most body cells.
   • Actions: GH stimulates the liver to produce insulin-like growth factors (IGFs), primarily IGF-1, which mediate many of GH's growth-promoting effects. GH directly stimulates protein synthesis, lipolysis (fat breakdown), and increases blood glucose (a hyperglycemic effect — it is a glucose-sparing and diabetogenic hormone). It promotes linear growth of long bones at the epiphyseal plates in children and adolescents.
   • Disorders: Gigantism (GH excess before epiphyseal plate closure), acromegaly (GH excess after plate closure), pituitary dwarfism (GH deficiency in childhood).

1. Thyroid-Stimulating Hormone (TSH) — also called thyrotropin:
   • Target: Thyroid gland follicular cells.
   • Actions: Stimulates the thyroid to release T₃ and T₄, and promotes growth and maintenance of the thyroid gland. TSH exerts its effects via the cAMP second messenger pathway.
   • Regulation: TSH release is stimulated by TRH from the hypothalamus, and inhibited by rising levels of T₃ and T₄ (negative feedback) and by somatostatin.

1. Adrenocorticotropic Hormone (ACTH) — also called corticotropin:
   • Target: Adrenal cortex (zona fasciculata and zona reticularis).
   • Actions: Stimulates the release of cortisol (a glucocorticoid) and, to a lesser extent, adrenal androgens. ACTH also promotes the growth and maintenance of the adrenal cortex. Its effects are mediated by cAMP.
   • Regulation: ACTH release is stimulated by CRH from the hypothalamus and inhibited by rising cortisol levels (negative feedback). ACTH also follows a circadian rhythm, peaking in the early morning hours and declining throughout the day.

1. Follicle-Stimulating Hormone (FSH):
   • Targets: Gonads (ovaries in females, testes in males).
   • Actions: In females, FSH stimulates development of ovarian follicles and, together with LH, promotes estrogen secretion. In males, FSH stimulates Sertoli cells to support spermatogenesis (sperm production).
   • Regulation: FSH release is stimulated by GnRH from the hypothalamus and inhibited by gonadal hormones — inhibin (a peptide hormone from the gonads that selectively inhibits FSH) and, in males, testosterone.

1. Luteinizing Hormone (LH):
   • Targets: Gonads.
   • Actions: In females, LH triggers ovulation (release of the mature oocyte) and promotes formation of the corpus luteum, which secretes progesterone and estrogen. In males, LH stimulates Leydig cells (interstitial cells of the testes) to produce testosterone.
   • Regulation: LH release is stimulated by GnRH and regulated by gonadal hormone feedback.

1. Prolactin (PRL):
   • Target: Mammary glands.
   • Actions: Stimulates milk production (lactation) in the mammary glands after childbirth. PRL may also play a role in immune function and osmoregulation.
   • Regulation: PRL is unique in that its primary hypothalamic regulation is inhibitory — dopamine (PIH) from the hypothalamus tonically suppresses PRL secretion. TRH can stimulate PRL release (hence the occasional galactorrhea in hypothyroidism). Suckling by an infant reduces hypothalamic dopamine secretion, disinhibiting PRL release and promoting milk production.

21.6 Major Endocrine Glands

21.6.1 The Thyroid Gland

The thyroid gland is a butterfly-shaped organ located in the anterior neck, just inferior to the larynx, consisting of two lateral lobes connected by a narrow isthmus. Histologically, it is composed of spherical thyroid follicles — each a single layer of follicular cells (simple cuboidal epithelium) surrounding a lumen filled with a protein-rich, gel-like material called colloid.

Hormones of the Thyroid:

1. Triiodothyronine (T₃) and Thyroxine (T₄):
   • Source: Follicular cells.
   • Chemical class: Amino acid derivatives (from tyrosine); uniquely lipid-soluble.
   • Synthesis: Follicular cells synthesize the large glycoprotein thyroglobulin and secrete it into the follicular lumen as colloid. Iodide (I⁻) is actively transported into follicular cells and oxidized to iodine (I₂) by the enzyme thyroid peroxidase. The iodine is then attached to tyrosine residues within thyroglobulin. Coupling of iodinated tyrosines yields T₃ (3 iodine atoms) and T₄ (4 iodine atoms), which remain attached to thyroglobulin in the colloid as a storage reservoir. When TSH stimulates the gland, follicular cells endocytose colloid droplets, lysosomal enzymes cleave T₃ and T₄ from thyroglobulin, and the hormones are released into the bloodstream.
   • Actions: T₃ and T₄ increase the basal metabolic rate (BMR) of nearly every cell in the body by increasing oxygen consumption and heat production (calorigenic effect). They are essential for normal growth and development, particularly of the nervous and skeletal systems in children. They also enhance sympathetic nervous system activity by upregulating adrenergic receptors (making target cells more sensitive to epinephrine and norepinephrine). T₃ is about 4–5 times more potent than T₄, but T₄ is produced in much greater quantity. In peripheral tissues, T₄ is converted to the more active T₃ by the enzyme deiodinase.
   • Regulation: TSH (from the anterior pituitary) is the primary regulator. Rising T₃ and T₄ levels feed back to inhibit both TRH release from the hypothalamus and TSH release from the anterior pituitary — a classic negative feedback loop.

1. Calcitonin:
   • Source: Parafollicular cells (also called C cells) scattered between the thyroid follicles.
   • Chemical class: Peptide hormone (32 amino acids).
   • Actions: Calcitonin lowers blood calcium (Ca²⁺) levels. It does so mainly by inhibiting osteoclast activity (reducing bone resorption and calcium release from bone) and by stimulating renal calcium excretion. In humans, calcitonin plays a relatively minor role in day-to-day calcium homeostasis compared to PTH; its effects are most prominent during childhood (bone growth) and pregnancy.
   • Regulation: Calcitonin secretion is directly stimulated by elevated blood Ca²⁺ levels. When Ca²⁺ falls, secretion is inhibited.

21.6.2 The Parathyroid Glands

The parathyroid glands are four (typically) small, oval structures embedded in the posterior surface of the thyroid gland. Despite their proximity, they are anatomically and functionally distinct from the thyroid.

Hormone of the Parathyroid:

• Parathyroid Hormone (PTH):
  • Source: Chief cells of the parathyroid glands.
  • Chemical class: Peptide hormone (84 amino acids).
  • Actions: PTH is the single most important hormone regulating blood calcium ion (Ca²⁺) concentration. PTH elevates blood Ca²⁺ through three synergistic mechanisms:
    1. Bone: PTH stimulates osteoclast activity and inhibits osteoblast activity, increasing bone resorption and releasing Ca²⁺ and phosphate (PO₄³⁻) into the blood.
    2. Kidneys: PTH enhances Ca²⁺ reabsorption in the distal convoluted tubule (reducing urinary calcium loss) and inhibits phosphate reabsorption in the proximal tubule (increasing urinary phosphate excretion — the phosphaturic effect). This is critical because calcium and phosphate tend to form insoluble complexes; by eliminating excess phosphate, PTH ensures that plasma Ca²⁺ can rise without precipitating.
    3. Intestine (indirectly): PTH stimulates the kidney to activate vitamin D by converting 25-hydroxyvitamin D₃ to calcitriol (1,25-dihydroxyvitamin D₃), the active form of vitamin D. Calcitriol then increases intestinal absorption of dietary calcium and phosphate.
  • Regulation: PTH secretion is directly stimulated by low blood Ca²⁺ and inhibited by high blood Ca²⁺, detected by calcium-sensing receptors (CaSR) on the chief cells. This is a simple negative feedback loop independent of the pituitary.

21.6.3 The Adrenal Glands

The adrenal glands (suprarenal glands) are paired, pyramid-shaped organs perched atop each kidney. Each gland is structurally and functionally divided into two distinct regions: an outer adrenal cortex and an inner adrenal medulla.

Table 21.5 — Adrenal Cortex Zones and Their Hormones

Adrenal Medulla Hormones:

The adrenal medulla is essentially a modified sympathetic ganglion. Its chromaffin cells are directly innervated by preganglionic sympathetic neurons and release catecholamines into the blood upon stimulation:

• Epinephrine (adrenaline): Accounts for approximately 80% of the medullary secretion. Increases heart rate and contractility, dilates airways (bronchodilation), stimulates glycogenolysis and lipolysis, and redirects blood flow to skeletal muscles, heart, and brain while constricting vessels in skin and viscera — the classic "fight-or-flight" response.
• Norepinephrine (noradrenaline): Accounts for approximately 20%. Contributes to vasoconstriction and blood pressure elevation.
• Both are water-soluble amino acid derivatives that act through cell-surface receptors (α and β adrenergic receptors) coupled to G proteins and second messenger systems.

21.6.4 The Endocrine Pancreas

The pancreas is both an exocrine gland (secreting digestive enzymes through ducts into the duodenum) and an endocrine gland. The endocrine tissue is organized into roughly one million microscopic clusters called pancreatic islets (islets of Langerhans), which are scattered throughout the exocrine tissue like islands in a sea. Each islet contains four main cell types:

Table 21.6 — Pancreatic Islet Cell Types and Their Hormones

Insulin is the primary anabolic hormone of the body. Its release is stimulated by elevated blood glucose, amino acids, and gastrointestinal hormones (incretins). Insulin binds to a receptor tyrosine kinase on target cells, triggering a cascade that translocates GLUT4 glucose transporters to the cell membrane, enabling glucose uptake. Insulin also promotes storage of excess nutrients: glycogen in liver and muscle (glycogenesis), triglycerides in adipose (lipogenesis), and protein synthesis.

Glucagon is the primary catabolic hormone opposing insulin. Released when blood glucose falls (e.g., fasting, exercise), glucagon stimulates the liver to break down glycogen (glycogenolysis) and synthesize new glucose from non-carbohydrate precursors (gluconeogenesis), releasing glucose into the blood. It also promotes lipolysis and ketone body production.

The balance between insulin and glucagon maintains blood glucose within a normal range of approximately 70–110 mg/dL. In type 1 diabetes mellitus, autoimmune destruction of beta cells results in absolute insulin deficiency. In type 2 diabetes mellitus, target cells become resistant to insulin's effects (insulin resistance).

21.6.5 The Pineal Gland

The pineal gland is a small, pinecone-shaped structure located in the epithalamus of the brain, near the posterior end of the third ventricle.

• Hormone: Melatonin (an amino acid derivative synthesized from tryptophan).
• Actions: Melatonin is the key hormone of circadian rhythm regulation. It signals darkness to the body, promoting sleepiness and lowering core body temperature. Melatonin secretion increases in the evening, peaks around midnight to early morning, and declines to near-undetectable levels during daylight hours.
• Regulation: Melatonin synthesis and release are controlled by the suprachiasmatic nucleus (SCN) of the hypothalamus — the body's master circadian clock. The SCN receives input from specialized retinal ganglion cells that detect light via the photopigment melanopsin. Light exposure inhibits melatonin secretion (which is why light exposure at night can disrupt sleep), while darkness stimulates it.

21.7 Feedback Loops

Hormone secretion is primarily regulated by feedback mechanisms that maintain homeostasis. These are classified as negative feedback (most common) or positive feedback (rarer, found in specific circumstances).

21.7.1 Negative Feedback

In a negative feedback loop, a change in a regulated variable triggers a response that opposes the initial change, driving the variable back toward its set point. Negative feedback is the predominant mechanism by which the endocrine system maintains stable internal conditions.

Generic sequence of a negative feedback loop:

1. A stimulus causes a deviation from the set point (e.g., blood glucose rises after a meal).
2. This deviation is detected by a sensor (e.g., beta cells of the pancreatic islets detect elevated glucose).
3. The sensor triggers a response that counteracts the deviation (e.g., beta cells release insulin, which drives glucose into cells and lowers blood glucose).
4. As the variable returns toward the set point, the stimulus for hormone release diminishes, and secretion declines.

Classic endocrine examples of negative feedback:

• Blood glucose regulation: Elevated glucose → insulin release (lowers glucose); low glucose → glucagon release (raises glucose).
• Blood calcium regulation: Low Ca²⁺ → PTH release → raises Ca²⁺; high Ca²⁺ → calcitonin release → lowers Ca²⁺.
• Thyroid axis: Low T₃/T₄ → TRH release → TSH release → T₃/T₄ release → rising T₃/T₄ inhibits TRH and TSH (long-loop negative feedback). TSH can also feed back to inhibit TRH (short-loop negative feedback).
• Cortisol axis: Low cortisol → CRH release → ACTH release → cortisol release → rising cortisol inhibits CRH and ACTH.
• Blood osmolality: High osmolality → ADH release → water reabsorption → osmolality falls; low osmolality inhibits ADH → diuresis → osmolality rises.

21.7.2 Positive Feedback

In a positive feedback loop, the response amplifies the initial stimulus rather than opposing it, driving the variable further away from the set point. Positive feedback loops in physiology are typically self-limiting — they run to completion and then terminate, often because the stimulus is removed.

Classic endocrine examples of positive feedback:

• Oxytocin during childbirth: The baby's head stretches the cervix → stretch-sensitive neurons send signals to the hypothalamus → oxytocin is released from the posterior pituitary → oxytocin stimulates uterine contractions → contractions push the baby further down, stretching the cervix even more → more oxytocin is released. This cycle continues, with increasing intensity, until the baby is delivered and the cervical stretching ceases — terminating the loop.

• Milk ejection (let-down reflex): Suckling by the infant stimulates nerve endings in the nipple → signals to the hypothalamus → oxytocin release → contraction of myoepithelial cells in the mammary gland → milk ejection → the infant continues suckling, maintaining the stimulus → ongoing oxytocin release. The loop ends when feeding stops.

• LH surge and ovulation: In the menstrual cycle, rising estrogen from the developing follicle initially exerts negative feedback on GnRH and LH. However, when estrogen levels exceed a critical threshold for a sustained period, the feedback switches to positive — high estrogen stimulates a massive surge of GnRH and LH, which triggers ovulation. Once the oocyte is released and the follicle transforms into the corpus luteum (which secretes progesterone), the positive feedback loop terminates.

21.8 Hormone Clearance and Half-Life

Once a hormone has been released into the bloodstream and exerted its effects on target tissues, it must be removed to prevent overstimulation. Hormone clearance refers to the processes that remove hormones from the circulation.

Routes of hormone clearance:

1. Enzymatic degradation: Hormones are broken down by enzymes, primarily in the liver and kidneys. Peptide hormones are degraded by proteases; catecholamines are metabolized by enzymes such as monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT); steroid hormones are inactivated by hepatic cytochrome P450 enzymes and conjugated for elimination.

1. Renal excretion: Many hormone metabolites are excreted in the urine. Small peptide hormones and catecholamines may also be filtered at the glomerulus and appear in the urine unmetabolized.

1. Receptor-mediated endocytosis: Hormone-receptor complexes on the cell surface may be internalized and degraded in lysosomes, removing both the hormone and downregulating the receptor.

Hormone half-life is the time required for the plasma concentration of a hormone to decrease by 50%. Half-life varies enormously among hormones and is largely determined by their chemical class:

• Water-soluble hormones (peptides, catecholamines): Typically have short half-lives (seconds to minutes). Because they travel free in the plasma, they are rapidly accessible to degrading enzymes and renal filtration. For example, epinephrine has a half-life of about 2–3 minutes; insulin, about 4–6 minutes.

• Lipid-soluble hormones (steroids, thyroid hormones): Typically have long half-lives (hours to days). Because they are largely bound to carrier proteins, they are protected from enzymatic degradation and renal clearance. Only the small free fraction is available for degradation. The bound reservoir acts as a buffer, releasing hormone slowly. For example, cortisol has a half-life of about 60–90 minutes; T₄ has a half-life of about 6–7 days.

The rate of hormone secretion and the rate of clearance together determine the hormone's plasma concentration at any given moment. A gland must continuously synthesize and release hormones with short half-lives to maintain a steady concentration, whereas hormones with long half-lives can maintain stable levels with less frequent secretion.