Comprehensive Thyroid Disease: Physiology, Hyperthyroidism, Hypothyroidism, Nodules, and Cancer

Vocabulary practice flashcards covering thyroid physiology, hypothyroidism, hyperthyroidism, thyroiditis, thyroid nodules, ultrasound risk stratification, and thyroid malignancies based on lecture notes.

Thyroid Follicle

The fundamental structural and functional unit of the thyroid gland, composed of follicular cells arranged around a central colloid cavity that stores thyroglobulin for thyroid hormone synthesis.

Thyroglobulin (Tg)

A large precursor glycoprotein synthesized and secreted by thyroid follicular cells that binds with oxidized iodine to form the thyroid hormones thyroxine ($T_4$) and triiodothyronine ($T_3$).

Triiodothyronine ($T_3$)

The biologically active and more potent form of thyroid hormone possessing greatest metabolic activity, with the vast majority produced in peripheral tissues via the enzymatic deiodination of $T_4$.

Thyroxine ($T_4$)

The primary prohormone form of thyroid hormone synthesized and secreted exclusively by the thyroid gland, which is converted peripherally into active $T_3$.

Hypothalamic-Pituitary-Thyroid (HPT) Negative Feedback Loop

The endocrine control pathway where low circulating levels of $T_3$ and $T_4$ stimulate the hypothalamus to release TRH, prompting the anterior pituitary to secrete TSH, which signals the thyroid gland to synthesize and release thyroid hormones until rising hormone levels inhibit further TRH and TSH secretion.

Primary Thyroid Dysfunction

A pathological state arising directly from intrinsic disease or failure of the thyroid gland itself (the "pilots"), in contrast to secondary disease resulting from pituitary or hypothalamic regulatory failure.

Secondary Thyroid Dysfunction

A disorder caused by dysfunction of the anterior pituitary gland or hypothalamus (the "air traffic control"), resulting in aberrant TSH or TRH secretion that inappropriately drives or understimulates an otherwise intact thyroid gland.

Thyroid Hormone-Binding Proteins

Circulating carrier proteins—predominantly thyroxine-binding globulin (TBG), transthyretin (TTR), albumin, and lipoproteins—that bind $>99\%$ of circulating thyroid hormone to maintain an immediately available reservoir while only the free fraction remains bioavailable.

Hashimoto Thyroiditis

A chronic autoimmune destructive thyroiditis and the leading cause of primary hypothyroidism in iodine-sufficient areas, characterized by autoantibodies against thyroid peroxidase (anti-TPO) and thyroglobulin (anti-Tg) and a diffusely enlarged gland with a pebbly texture.

Overt Hypothyroidism

A clinical syndrome characterized by classic hypometabolic symptoms accompanied by laboratory findings of elevated serum TSH and decreased free $T_4$.

Subclinical Hypothyroidism

An asymptomatic or mildly symptomatic state defined biochemically by an elevated serum TSH level in the setting of normal circulating free $T_4$ concentrations.

Pretibial Myxedema

Non-pitting, indurated dermal edema and plaque formation over the anterior tibia caused by excessive glycosaminoglycan accumulation in interstitial tissues, seen classically in severe hypothyroidism and Graves disease.

Myxedema Crisis

A medical emergency representing the most severe, life-threatening progression of decompensated hypothyroidism, characterized by altered mental status, hypothermia, multiorgan dysfunction, and high mortality.

Levothyroxine

Synthetic $T_4$ replacement therapy prescribed as the first-line lifelong treatment for primary hypothyroidism, typically initiated at approximately $1.6\,\mu\text{g/kg/day}$ in healthy adults ($25\text{--}50\,\mu\text{g/day}$ in older adults) taken on an empty stomach $1\text{ hour}$ before food or other medications.

Hyperthyroidism

A hypermetabolic clinical condition caused by excessive production and secretion of thyroid hormones ($T_3$, $T_4$) by the thyroid gland, typically manifested on laboratory testing as a suppressed TSH with elevated free $T_4$ and $T_3$.

Graves Disease

The most common etiology of hyperthyroidism, caused by stimulating autoantibodies (TSI/TRAb) targeting and constitutively activating the TSH receptor on thyroid follicular cells, resulting in unregulated overproduction of thyroid hormones.

Exophthalmos

Proptosis or abnormal forward protrusion of the eyes accompanied by scleral injection and eyelid retraction resulting from autoimmune retro-orbital inflammation and edema, specifically pathognomonic for Graves disease.

Lid Lag

A physical exam finding of hyperthyroidism where the descent of the upper eyelid is delayed relative to downward movement of the globe, exposing the white sclera above the limbus.

Euthyroid Hyperthyroxinemia

A non-thyroidal differential diagnosis where abnormalities in serum thyroid hormone-binding proteins cause elevated total serum $T_4$ levels while free $T_4$, $T_3$, and TSH concentrations remain within normal physiological limits.

Biotin Therapy Assay Interference

A laboratory artifact where high-dose biotin ingestion interferes with competitive and immunometric assay platforms, causing falsely suppressed TSH alongside falsely elevated $T_4$, $T_3$, and TBII levels.

Diffusely High Radioactive Iodine Uptake

The characteristic nuclear medicine scintigraphy pattern pathognomonic for Graves disease, demonstrating homogeneous, intensely elevated radiotracer uptake throughout both lobes of the thyroid gland.

Thyroid Inferno Pattern

The dramatic increase in intrathyroidal vascular blood flow visualized on color-flow Doppler ultrasound, characteristic of a hyperactive, hyperthyroid gland in Graves disease.

Methimazole

The primary first-line thionamide antithyroid drug used to inhibit thyroid peroxidase-mediated hormone synthesis in Graves disease, requiring regular CBC and LFT monitoring for agranulocytosis and hepatotoxicity.

Propylthiouracil (PTU)

A thionamide antithyroid medication indicated as the preferred agent during early pregnancy to treat maternal hyperthyroidism due to a lower risk of specific teratogenic birth defects relative to methimazole.

Subacute Thyroiditis

A post-viral, painful granulomatous inflammation of the thyroid gland presenting with severe neck tenderness, markedly elevated ESR, transient hyperthyroidism ($3\text{--}6\text{ weeks}$), subsequent transient hypothyroidism ($\le 6\text{ months}$), and eventual euthyroid resolution.

Postpartum Thyroiditis

A painless autoimmune thyroiditis developing within months of childbirth characterized by positive anti-TPO/anti-Tg antibodies, transient hyperthyroidism followed by hypothyroidism resolving within $12\text{--}18\text{ months}$, with $50\%$ of patients developing permanent hypothyroidism within $9\text{ years}$.

Superior Vena Cava (SVC) Syndrome

Vascular compromise caused by extrinsic compression of the superior vena cava from a large substernal goiter or invasive thyroid mass, leading to facial edema, jugular venous distention, dyspnea, and cyanosis upon elevating the arms above the head.

Hot Thyroid Nodule

An autonomous, hyperfunctioning thyroid nodule demonstrating increased radiotracer uptake and suppression of uptake in the remainder of the gland on scintigraphy; hot nodules carry an exceedingly low risk of malignancy.

Cold Thyroid Nodule

A non-functioning thyroid nodule showing decreased or absent radionuclide uptake relative to surrounding tissue on radioactive iodine scan, carrying a significantly higher suspicion of malignancy requiring diagnostic biopsy.

High-Risk Sonographic Features

Ultrasound characteristics of a thyroid nodule strongly predictive of malignancy: solid composition, hypoechogenicity (darker appearance), microcalcifications, irregular or infiltrative margins, and a taller-than-wide dimension.

Fine-Needle Aspiration (FNA) Biopsy

The definitive pre-surgical diagnostic procedure indicated for thyroid nodules $>1\,\text{cm}$ with normal/high TSH, nodules $>1\,\text{cm}$ that are cold on radioiodine scan, or any nodule exhibiting suspicious sonographic features.

Papillary Thyroid Carcinoma

The most common form of thyroid cancer, exhibiting an indolent course and favorable prognosis, found in up to $10\%$ of first-degree relatives of affected patients.

Anaplastic Thyroid Carcinoma

An uncommon, highly aggressive, undifferentiated thyroid malignancy characterized by rapid locally invasive growth over weeks to months and an extremely poor clinical prognosis.

Recurrent Laryngeal Nerve Injury

A surgical injury to a branch of the vagus nerve during thyroidectomy resulting in vocal cord paralysis or paresis, clinically manifesting as persistent hoarseness, dysphonia, and airway obstruction.

Serum Thyroglobulin Surveillance

The laboratory measurement of serum thyroglobulin used following total thyroidectomy and radioiodine ablation as a tumor marker to monitor for recurrent or residual thyroid carcinoma.