Thyroid Gland Physiology and Pathophysiology

Comprehensive vocabulary flashcards covering the biosynthesis, regulation, physiologic effects, pharmacological inhibitors, and pathophysiologic conditions of the thyroid gland.

Thyroid Gland Anatomy

A butterfly-shaped endocrine gland situated anterior to the trachea and inferior to the larynx, consisting of two lateral lobes joined across the midline by an isthmus, vascularized by the superior and inferior thyroid arteries, and bordered laterally by the common carotid arteries.

Iodide Trap Mechanism

The active transport of iodide ($\text{I}^-$) from the bloodstream into thyroid follicular epithelial cells via an active pump, representing the initial step in thyroid hormone biosynthesis that is competitively blocked by thiocyanate and perchlorate anions.

Wolff-Chaikoff Effect

An autoregulatory phenomenon in which high circulating levels of iodide ($\text{I}^-$) directly inhibit the iodide transport pump, thereby transiently suppressing the biosynthesis of thyroid hormones.

Thyroid Peroxidase (TPO)

An enzyme located in the follicular cell membrane that catalyzes the oxidation of iodide ($\text{I}^-$) to active iodine ($\text{I}_2$), the iodination of tyrosine residues on thyroglobulin (organification), and the coupling of iodotyrosines.

Propylthiouracil (PTU)

A thioamide antithyroid drug that inhibits thyroid peroxidase to block hormone synthesis and prevents peripheral conversion of $\text{T}_4$ to $\text{T}_3$; used in thyroid storm and during the first trimester of pregnancy despite an FDA warning for hepatotoxicity.

Organification

The biochemical incorporation of oxidized iodine ($\text{I}_2$) into tyrosine residues of thyroglobulin at the junction of the follicular cell and lumen, producing monoiodotyrosine (MIT) and diiodotyrosine (DIT).

Coupling Reaction

The peroxidase-catalyzed combination of iodinated tyrosines within thyroglobulin: DIT + DIT forms thyroxine ($\text{T}_4$), whereas DIT + MIT forms triiodothyronine ($\text{T}_3$).

Thyroid Deiodinase

An enzyme responsible for removing iodine from residual MIT and DIT within follicular cells for iodine recycling, as well as converting circulating $\text{T}_4$ into biologically active $\text{T}_3$ or inactive reverse $\text{T}_3$ ($\text{rT}_3$) in peripheral tissues.

Potassium Iodide (KI)

A pharmacologic agent administered before or immediately following nuclear emissions that saturates the thyroid gland to block the uptake of radioactive iodine isotopes, preventing radiation-induced thyroid malignancies.

Thyroxine-Binding Globulin (TBG)

A hepatic glycoprotein that transports circulating $\text{T}_3$ and $\text{T}_4$; its levels drop in hepatic failure and rise during pregnancy, with the latter maintaining a euthyroid state due to normal levels of free active hormone.

Triiodothyronine ($\text{T}_3$)

The biologically active thyroid hormone that possesses greater affinity for the intracellular nuclear thyroid hormone receptor than $\text{T}_4$, despite circulating at lower concentrations and exhibiting a shorter half-life.

Radioactive Iodine Uptake (RAIU)

A diagnostic evaluation measuring the percentage of an oral tracer isotope ($^{\text{123}}\text{I}$) absorbed by the thyroid over time (normal: $10\text{--}35\%$ at $24\,\text{hours}$); elevated in glandular hyperthyroidism and suppressed in exogenous hormone consumption or thyroiditis.

Thyrotropin-Releasing Hormone (TRH)

A hypothalamic tripeptide that stimulates the synthesis and release of TSH from the anterior pituitary gland; excessive secretion due to primary hypothyroidism also stimulates lactotrophs to elevate serum prolactin.

Thyroid-Stimulating Hormone (TSH)

An anterior pituitary glycoprotein hormone that acts through cyclic AMP ($\text{cAMP}$) to stimulate all steps of thyroid hormone production and secretion, while also increasing thyroid vascularity, cellular hypertrophy, and hyperplasia.

Primary Hypothyroidism

End-organ failure of the thyroid gland resulting in decreased $\text{T}_3$ and $\text{T}_4$, paired with elevated serum TSH and elevated TRH due to the loss of systemic negative feedback.

Secondary Hypothyroidism

Hypothyroidism stemming from anterior pituitary disease, characterized by decreased levels of TSH alongside decreased $\text{T}_3$ and $\text{T}_4$, and elevated TRH; identified clinically when administered exogenous TSH successfully raises $\text{T}_3$ and $\text{T}_4$.

Tertiary Hypothyroidism

Hypothyroidism secondary to hypothalamic insufficiency, characterized biochemically by concurrent decreases in serum TRH, TSH, and thyroid hormones ($\text{T}_3$ and $\text{T}_4$).

Primary Hyperthyroidism

Autonomous thyroid hyperfunction (such as toxic adenoma or Graves' disease) resulting in elevated circulating $\text{T}_3$ and $\text{T}_4$, which exerts strong negative feedback on the pituitary and hypothalamus to suppress TSH and TRH.

Secondary Hyperthyroidism

Pituitary hypersecretion of TSH (e.g., from a TSH-secreting pituitary adenoma) leading to increased circulating TSH, which subsequently drives excessive glandular release of $\text{T}_3$ and $\text{T}_4$ while suppressing hypothalamic TRH.

Calorigenic Action

The physiological effect of thyroid hormones that elevates basal metabolic rate (BMR) and body temperature by upregulating $\text{Na}^+\text{-}\text{K}^+\,\text{ATPase}$ synthesis and cellular oxygen consumption across all tissues except the brain, gonads, and spleen.

Thyroid-Stimulating Immunoglobulins (TSI)

Pathogenic Immunoglobulin G ($\text{IgG}$) autoantibodies characteristic of Graves' disease that bind directly to and activate TSH receptors, driving autonomous, unregulated thyroid hormone hypersecretion.

Exophthalmos

Bilateral forward protrusion of the globes accompanied by a staring look, caused by cytokine-mediated peri-orbital and retro-orbital edema along with sympathetic-induced upper eyelid retraction in Graves' disease.

Hashimoto's Thyroiditis

An autoimmune cause of primary hypothyroidism involving autoantibody production directed against thyroglobulin and thyroid peroxidase (TPO), culminating in progressive inflammatory follicular destruction.

Myxedema

Advanced hypothyroidism presenting in adults, characterized clinically by reduced basal metabolism, cold intolerance, weight gain, mental slowness, ptosis, and characteristic non-pitting cutaneous edema.

Non-Pitting Edema

A form of diffuse subcutaneous swelling seen in myxedema that resists persistent finger indentation upon pressure, caused by diminished protein catabolism leading to interstitial accumulation of hyaluronic acid and mucopolysaccharides.

Pitting Edema

Subcutaneous fluid accumulation in which external compressive pressure displaces interstitial water, leaving a sustained indentation that persists for a period of time before resolving.

Cretinism

A congenital or infantile hypothyroid syndrome marked by irreversible mental retardation, deaf-mutism, and motor rigidity caused by defective axonal myelination and cortical, basal ganglionic, and cochlear dysfunction.

Infantile Body Proportions

A manifestation of congenital hypothyroidism in which skeletal growth and epiphyseal ossification fail to progress normally, resulting in a delayed bone age relative to chronological age due to loss of thyroid synergy with growth hormone.

Beta-Carotenemia

Yellow-orange cutaneous pigmentation observed in hypothyroidism due to impaired enzymatic conversion of $\beta$-carotene to vitamin A, distinguished clinically from jaundice by the absence of scleral and mucous membrane discoloration.

Goiter

Macroscopic enlargement of the thyroid gland resulting from sustained elevation of TSH (as seen in endemic iodine deficiency or secondary hyperthyroidism), stimulating autoantibodies, or neoplasia; absent in secondary hypothyroidism due to deficient TSH.