Endocrine Control: Hypothalamic-Pituitary Axis and Thyroid Hormone Synthesis

Comprehensive practice flashcards detailing the neuroendocrine regulation of the hypothalamic-pituitary axis and the cellular mechanisms of thyroid hormone synthesis, storage, and release.

Environmental stimuli

External environmental cues and inputs received by the brain that influence hypothalamic neuroendocrine signaling.

Internal stimuli

Endogenous physiological signals and homeostatic conditions within the body that act on the brain to modulate hypothalamic activity.

Hypothalamus

The central neuroendocrine brain structure that integrates internal and environmental stimuli to control the anterior pituitary via releasing and inhibiting hormones, and acts as a primary target for feedback regulation.

Hypothalamic hormones

Neurohormones produced by the hypothalamus that exert positive (stimulatory) or negative (inhibitory) control over hormone synthesis and secretion in the anterior lobe of the pituitary gland.

Anterior lobe (Pituitary gland)

The anterior glandular portion of the pituitary gland that synthesizes and releases tropic hormones into the circulation in response to hypothalamic hormones.

Adrenal cortex, gonads, thyroid gland, liver

The primary peripheral target endocrine glands stimulated by anterior pituitary tropic hormones (ACTH, FSH, LH, TSH, GH) to produce gland hormones or metabolic mediators.

Target organs

The peripheral effector tissues and organs that respond directly to circulating gland hormones to produce specific physiological and metabolic effects.

Short-loop feedback

A negative feedback mechanism in which hormones secreted by the anterior pituitary gland directly inhibit hypothalamic hormone secretion.

Long-loop feedback

A negative feedback mechanism in which hormones secreted by peripheral target glands inhibit hormone secretion at both the anterior pituitary lobe and the hypothalamus.

ACTH, FSH, LH, TSH, GH

The principal hormones produced by the anterior lobe of the pituitary gland: adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone, and growth hormone.

Gland hormones

Chemical messengers secreted by peripheral endocrine glands (adrenal cortex, gonads, thyroid gland, liver) that act on target organs and mediate long-loop negative feedback.

Active (TSH-stimulated) thyroid follicle

A thyroid follicle undergoing active hormone synthesis and resorption under TSH stimulation, characterized by taller epithelial cells with apical pseudopods and depleted, scalloped colloid.

Inactive thyroid follicle

A resting thyroid follicle lined by flattened, low cuboidal epithelial cells enclosing a large, distended reservoir of luminal colloid.

Colloid (Follicular lumen)

The central protein-rich luminal reservoir within a thyroid follicle primarily composed of thyroglobulin and stored thyroid hormone precursors.

Colloid (Apical interface)

The extracellular luminal fluid adjacent to the apical microvilli of thyroid follicular cells where iodide oxidation, organification of thyroglobulin, and apical endocytosis take place.

Thyroglobulin

A large precursor glycoprotein synthesized in thyroid follicular cells and secreted across the apical membrane into the colloid to serve as the protein backbone for thyroid hormone synthesis.

Endocytotic vesicle

An intracellular membrane vesicle formed at the apical border of the thyroid follicular cell to internalize droplets of iodinated thyroglobulin from the colloid.

$T_3$ and $T_4$

Triiodothyronine and thyroxine, the active iodinated hormones cleaved from thyroglobulin inside the thyroid follicular cell and released basally into circulation.

Capillary

The microvascular blood vessel adjacent to the basal lamina of the thyroid follicle that delivers $I^-$ and $Na^+$ and absorbs released $T_3$ and $T_4$.

Thyroid hormone binding globulin

A plasma glycoprotein in the capillary blood that binds circulating $T_3$ and $T_4$ to facilitate their transport through the vascular system.

Thyroid peroxidase (TPO)

An enzyme located at the apical plasma membrane of thyroid cells that catalyzes the oxidation of iodide ($I^-$) to iodine ($I_2$) using $H_2O_2$ and couples iodotyrosine residues on thyroglobulin.

Iodide transporter

A basal membrane transport protein that cotransports iodide ($I^-$) into thyroid follicular cells alongside sodium ($Na^+$) against the iodide concentration gradient.

Lysosome

An intracellular digestive organelle containing acid hydrolases that fuses with the endocytotic vesicle to proteolytically cleave iodinated thyroglobulin into free $T_3$ and $T_4$.

Iodinated thyroglobulin

Thyroglobulin in the colloid that has undergone iodination at tyrosine residues by thyroid peroxidase (TPO) to form hormone precursors prior to endocytosis.

Basal lamina

The extracellular basement membrane supporting the basal aspect of thyroid follicular cells and separating them from the adjacent capillary endothelium.

Bound $T_3$ and $T_4$

Circulating triiodothyronine and thyroxine reversibly complexed with thyroid hormone binding globulin in capillary blood.