Endocrine Physiology & Hypothalamic-Pituitary Regulation

Comprehensive vocabulary flashcards covering hormone structural classifications, plasma transport, receptor mechanisms, feedback regulation, hypothalamic-pituitary axes, and diagnostic principles of primary, secondary, and tertiary endocrine disorders.

Peptides & Proteins

A hormone class composed of amino acid chains synthesized as preprohormones that convert to prohormones; they are water-soluble, circulate unbound with a short half-life of minutes, bind cell surface receptors, and activate second-messenger cascades ($cAMP$, $IP_3$/DAG, tyrosine kinase).

Steroids

A hormone class synthesized from cholesterol in the adrenal cortex, gonads, and placenta; they are lipid-soluble, circulate bound to carrier proteins with a long half-life of hours to days, and bind intracellular receptors to alter gene transcription and protein synthesis.

Amines

A hormone class derived from the amino acid tyrosine, encompassing both water-soluble catecholamines and lipid-soluble thyroid hormones.

Catecholamines

Tyrosine-derived amine hormones (epinephrine, norepinephrine, dopamine) that are water-soluble, circulate free in plasma, and bind cell membrane receptors to activate second messengers.

Thyroid Hormones ($T_3$ and $T_4$)

Tyrosine-derived amine hormones that are lipid-soluble, circulate bound to thyroxine-binding globulin (TBG), and bind nuclear receptors to regulate nuclear gene transcription.

Free Hormone Fraction

The unbound, biologically active fraction of circulating hormone that is capable of binding target-cell receptors and triggering negative feedback.

Down-Regulation

A reduction in the number or affinity of target-cell receptors resulting from chronic hormone excess, which decreases target cell sensitivity.

Up-Regulation

An increase in target-cell receptor density provoked by chronic hormone deficiency to maximize cellular sensitivity to scarce signals.

Permissiveness

A hormone interaction where the presence of one hormone is required for a second hormone to exert its full biological effect (e.g., thyroid hormone increasing $\beta$-adrenergic receptors for epinephrine).

Negative Feedback

The most common endocrine control mechanism, in which rising end-organ hormone levels inhibit upstream hypothalamic and anterior pituitary secretion to maintain homeostasis.

Positive Feedback

A rare endocrine feedback mechanism in which rising hormone levels amplify further hormone secretion until a discrete physiological event is completed.

Posterior Pituitary (Neurohypophysis)

The pituitary lobe connected directly to the hypothalamus via the hypothalamic-hypophyseal tract; it stores and releases oxytocin and ADH synthesized in the supraoptic and paraventricular nuclei.

Antidiuretic Hormone (ADH / Vasopressin)

A posterior pituitary hormone stimulated by increased plasma osmolality or severe volume depletion that inserts aquaporin-2 water channels in renal collecting ducts and induces arteriolar vasoconstriction at high concentrations.

Oxytocin

A posterior pituitary hormone that stimulates uterine smooth muscle contraction during labor and myoepithelial cell contraction for milk ejection during lactation.

Hypothalamic-Hypophyseal Portal System

A specialized vascular capillary network through which hypothalamic releasing and inhibiting hormones travel directly to act upon anterior pituitary endocrine cells.

Anterior Pituitary (Adenohypophysis)

The glandular portion of the pituitary that synthesizes and releases trophic hormones under the control of hypothalamic factors delivered by the portal circulation.

Thyrotropin-Releasing Hormone (TRH)

A hypothalamic hormone that stimulates anterior pituitary secretion of TSH and also exerts stimulatory effects on prolactin secretion.

Thyroid-Stimulating Hormone (TSH)

An anterior pituitary hormone that stimulates thyroid follicular cells to synthesize and release $T_3$ and $T_4$, thereby elevating basal metabolic rate.

Corticotropin-Releasing Hormone (CRH)

A hypothalamic hormone that stimulates the anterior pituitary to synthesize and secrete adrenocorticotropic hormone (ACTH).

Adrenocorticotropic Hormone (ACTH)

An anterior pituitary hormone that stimulates the zona fasciculata of the adrenal cortex to secrete cortisol.

Gonadotropin-Releasing Hormone (GnRH)

A hypothalamic hormone that stimulates anterior pituitary secretion of the gonadotropins FSH and LH.

FSH & LH (Gonadotropins)

Anterior pituitary hormones that stimulate gametogenesis (sperm and follicle maturation) and sex steroid synthesis (estrogen, progesterone, testosterone) in the gonads.

Growth Hormone-Releasing Hormone (GHRH)

A hypothalamic hormone that stimulates anterior pituitary somatotrophs to synthesize and release growth hormone (GH).

Somatostatin (GHIH)

A hypothalamic hormone that exerts an inhibitory effect on growth hormone secretion by the anterior pituitary.

Growth Hormone (GH / Somatotropin)

An anterior pituitary hormone that causes lipolysis and insulin antagonism directly, and stimulates the liver to produce IGF-1 for bone and soft-tissue growth.

Insulin-Like Growth Factor-1 (IGF-1)

A peptide produced by the liver in response to GH that stimulates bone and soft-tissue growth.

Dopamine / Prolactin-Inhibiting Hormone (PIH)

A hypothalamic agent that maintains continuous tonic inhibitory control over anterior pituitary prolactin secretion.

Prolactin (PRL)

An anterior pituitary hormone uniquely under tonic inhibitory dopamine control that promotes mammary gland development and milk production.

Primary Endocrine Disorder

An endocrine dysfunction originating from an intrinsic defect in the peripheral target gland itself, resulting in divergent levels of target hormone and pituitary trophic hormone.

Secondary Endocrine Disorder

An endocrine pathology localized to the anterior pituitary gland, resulting in concordant changes in trophic and target gland hormones.

Tertiary Endocrine Disorder

An endocrine disorder originating in the hypothalamus due to altered secretion of hypothalamic releasing or inhibiting hormones.