Thyroid Gland: Anatomy, Physiology, Pathology, Imaging, and Biopsy

Thyroid Gland: Anatomy, Physiology, Pathology, Imaging, and Biopsy

Anatomy of the thyroid

  • Location: in the neck, close to the first part of the trachea.
  • Shape: butterfly-like with two lateral lobes connected by the isthmus.
  • Isthmus: narrow tissue linking the lobes; forms the midline connection.
  • Pyramidal lobe: a third lobe arising from the superior portion of the isthmus; ascends to the level of the hyoid bone.
  • Variants and relations (as seen in ultrasound curricula):
    • Parathyroid glands typically lie posterior to the thyroid lobes (not always detailed in the provided transcript but commonly discussed in anatomy for ultrasound).
    • Surrounding structures include trachea anterior to esophagus, deep cervical fascia, strap muscles, sternocleidomastoid, longus colli, platysma, etc.

Thyroid histology

  • Thyroid epithelial cells are responsible for synthesis of thyroid hormones.
  • Cells are arranged in spherical units called follicles.
  • Follicles are filled with colloid, a proteinaceous depot of thyroid hormone precursor.

Thyroid hormones and regulation

  • Hormones produced:
    • Thyroxine (T4)
    • Triiodothyronine (T3)
    • Calcitonin (thyrocalcitonin)
  • Synthesis requires iodine; follicular cells chemically process iodine to form T3 and T4.
  • Synthesis and secretion are primarily controlled by thyroid-stimulating hormone (TSH) from the pituitary.
  • Maintenance of circulating T3 and T4 is achieved via the hypothalamus–pituitary–thyroid axis (hypothalamic releasing factors stimulate pituitary to secrete TSH; TSH stimulates thyroid hormone production).
  • Euthyroid state: normal thyroid function with appropriate hormone production.

Thyroid physiology and transport

  • Thyroid hormones are poorly soluble in water.
  • >99% of circulating T3 and T4 are bound to carrier proteins; a small fraction is free and biologically active.
  • Main carrier: thyroxine-binding globulin (TBG), a liver-synthesized glycoprotein.
  • Other carriers: transthyretin and albumin.
  • Carrier proteins maintain a stable reservoir from which free (active) hormones are released to target tissues.
  • Calcitonin is produced by C cells of the thyroid; participates in Ca2+ and phosphorus metabolism.
    • Mechanism to lower blood calcium: (1) decrease intestinal calcium absorption, (2) promote calcium storage in bones, (3) increase renal calcium excretion.

Regulation of thyroid hormone synthesis & secretion

  • TSH from the anterior pituitary stimulates:
    • Synthesis of iodine transporters, thyroid peroxidase, and thyroglobulin.
  • Effects of thyroid hormones:
    • All body cells are targets; THs are essential for development, growth, and metabolism; they are not strictly required for life but have widespread physiological impact.
  • Hormonal effects span multiple systems and processes (examples below).

Physiologic effects of thyroid hormones

  • Lipid metabolism: ↑ TH leads to fat mobilization and higher plasma free fatty acids.
  • Carbohydrate metabolism: TH enhances many aspects of carbohydrate metabolism, promoting glucose uptake into cells (via insulin-dependent pathways), gluconeogenesis, and glycogenolysis to generate free glucose.
  • Growth and development: TH is necessary for normal growth in children; deficiency causes growth retardation; metamorphosis in amphibians is classic evidence of TH role in development.
  • Cardiovascular system: TH increases heart rate, cardiac contractility, and cardiac output; promotes vasodilation and increased organ perfusion.
  • Central nervous system: Both hypothyroid and hyperthyroid states alter mental state; too little TH → lethargy; too much TH → anxiety and nervousness.
  • Reproductive system: Normal reproductive behavior and physiology depend on normal TH levels; hypothyroidism is often associated with infertility.

Thyroid states: euthyroid, hypothyroidism, hyperthyroidism

  • Euthyroid: normal thyroid hormone production and function.
  • Hypothyroidism – etiology: underproduction of thyroid hormones; most common thyroid disorder.
    • Primary hypothyroidism: defective hormone synthesis; iodine deficiency or excess; inflammatory thyroiditis; congenital disorders.
    • Secondary hypothyroidism: pituitary or hypothalamic dysfunction (tumors, irradiation, medications).
  • Hypothyroidism clinical signs and symptoms (examples):
    • Myxedema (diffuse swelling of skin), weight gain, hair loss, periorbital edema, lethargy, intellectual and motor slowing, cold intolerance, constipation, deep husky voice, muscle cramps, arthritis, slow metabolic rate, decreased heart rate.
  • Hyperthyroidism (thyrotoxicosis) – etiology: overproduction of thyroid hormones; often due to diffuse gland hyperactivity or nodular disease; Graves’ disease is the underlying cause in 50–80% of cases; more common in females.
  • Hyperthyroidism clinical signs and symptoms (examples): nervousness, weight loss with increased appetite, tremors, sweating, heat intolerance, palpitations, tachycardia, exophthalmos, diarrhea.
  • Primary vs secondary disorders:
    • Primary hyperthyroidism: excess hormone produced by thyroid gland itself.
    • Secondary hyperthyroidism: TSH-secreting pituitary adenoma (rare).

Thyroid laboratory testing

  • Common thyroid labs/tests:
    • TSH
    • Free thyroxine (free T4)
    • Total thyroxine (total T4)
    • Thyroxine-binding globulin (TBG)
    • Total triiodothyronine (total T3)
    • Calcitonin
  • Normal reference ranges (examples from the material):
    • TSH: 3−30extng/mL3-30 ext{ ng/mL}
    • Free T4: 0.8−2.4extng/dL0.8-2.4 ext{ ng/dL}
    • Total T4: 4−11extng/mL4-11 ext{ ng/mL}
    • Total T3: 75−220extng/mL75-220 ext{ ng/mL}
    • Calcitonin: <100 ext{ pg/mL}
  • Iodine requirement for normal thyroxine production: 100−200extmg/week100-200 ext{ mg/week}
  • Other notes:
    • Iodine status and pituitary/thyroid axis abnormalities influence results.
    • TSH abnormalities are often the earliest sign of thyroid dysfunction.

Secondary and differential lab patterns (hypo/hyper)

  • Euthyroid: T4/T3 normal; TSH normal.
  • Hyperthyroidism: T4/T3 high; TSH low (often due to pituitary suppression).
  • Hypothyroidism: T4/T3 low; TSH high (pituitary responds to low thyroid hormone by increasing TSH).
  • Hashimoto’s thyroiditis: autoimmune hypothyroidism; ultrasound often shows diffuse coarse parenchymal texture, hypoechoic areas, nodularity with progression to inhomogeneous enlargement; color Doppler may show hypervascularity during acute onset with later decreased flow.
  • Graves’ disease: autoimmune hyperthyroidism; diffuse goiter, hypoechoic tissue, increased vascularity on color Doppler; may have exophthalmos.

Thyroid imaging: ultrasound basics and normal appearance

  • Thyroid ultrasound as a primary imaging modality:
    • Determines gland size and location of masses
    • Differentiates cystic vs solid lesions
    • Guides fine-needle aspiration (FNA) biopsy
  • Normal thyroid ultrasound appearance:
    • Granular, homogeneous, medium-level echogenicity across trabeculae of both lobes
    • Slightly more echogenic than surrounding muscles
    • Smooth fibrous capsule not normally visualized unless thickened
  • Normal dimensions (approximate ranges):
    • Length: 4.0−6.0extcm4.0-6.0 ext{ cm}
    • Anteroposterior height: 1.3−1.8extcm1.3-1.8 ext{ cm}
    • Width: 1.5−2.0extcm1.5-2.0 ext{ cm}
    • Isthmus thickness: 4.0−6.0extmm4.0-6.0 ext{ mm}
  • Surrounding structures to identify on ultrasound:
    • Trachea (medial bright reflector with reverberation and shadowing), esophagus (posterolateral to left, hypoechoic with echogenic center, moves with swallowing).
    • Surrounding musculature: longus colli, platysma, sternocleidomastoid, strap muscles.
  • Vascular anatomy on ultrasound:
    • External carotid artery and its superior thyroid artery branch; inferior thyroid artery from thyrocervical trunk.
    • Superior and middle thyroid veins drain into the internal jugular vein; inferior thyroid veins drain into the innominate vein.
    • Color Doppler helps visualize arterial branches; normal glandular vascularity is usually modest.

Variants and congenital/anatomic variants

  • Athyrosis (absence of the thyroid gland): rare, critical abnormality leading to congenital hypothyroidism; early detection and hormone replacement improve outcomes.
  • Pyramidal lobe: common anatomic variant in about 50% of people; arises from caudal portion of thyroglossal tract; extension upward toward hyoid bone; can originate from either lobe (more often left).
  • Absent isthmus: complete absence of the isthmus; gland appears as two independent lobes.
  • Ectopic thyroid tissue: thyroid tissue can be located anywhere along the descent path from the foramen cecum to its final position; most common ectopic site is lingual thyroid; other sites include sublingual, substernal, tracheal or Esophageal walls.

Nodular thyroid disease (goiter and nodules)

  • Goiter: enlargement of the thyroid; most common thyroid abnormality; occurs in about 5% of the population; can be diffuse (non-nodular) or nodular.
    • Nontoxic goiter refers to gland enlargement without nodularity and without functional disturbance.
  • Nodular hyperplasia and multinodular goiter: multiple nodules within a diffusely enlarged gland; many are benign.
  • Common etiologies: iodine deficiency; autoimmune processes; goiter can compress the esophagus or trachea.
  • Epidemiology: more common in women, typically ages 30-50.
  • Cystic degeneration and nodularity: hyperplasia can form nodules that may liquefy; cystic nodules are common in multinodular disease.
  • Ultrasound features of nodular disease:
    • Most nodules are isoechoic relative to normal thyroid tissue; gland tends to become more hyperechoic with enlargement.
    • Degenerative changes: focal scarring, ischemia, necrosis, cystic formation; fibrosis or calcifications may be present.
    • Perinodular blood vessels may create a thin peripheral hypoechoic halo around nodules.
    • Increased intranodular vascularity may be present.
    • Approximately 20% of solitary nodules are cystic.

Benign thyroid lesions

  • Cyst: True thyroid cysts are uncommon; when present, most are benign and may arise from cystic degeneration of nodules (e.g., adenomas).
    • True cysts include thyroglossal duct cysts and branchial cleft cysts (not thyroid nodules; covered elsewhere).
    • Ultrasound features: well-defined, smooth borders, anechoic (simple) or with internal echoes if degenerative; posterior acoustic enhancement; may be solitary or multiple.
  • Adenoma: benign solid neoplasm of thyroid epithelium; encapsulated; represents 5-10% of nodular thyroid disease.
    • More common in females; typically solitary and slow growing; may have areas of hemorrhage or necrosis.
    • Ultrasound appearances range from cystic to solid depending on degeneration; most are solid and well circumscribed; sometimes a peripheral halo is present.
    • Hyperfunctioning adenomas may show increased peripheral or intralesional vascularity; calcifications (eggshell) and posterior acoustic shadowing can be seen.

Malignant thyroid lesions

  • Major types (4 primary): Papillary, Follicular, Medullary, Anaplastic; plus lymphoma.
  • General etiology: thyroid cancer is relatively rare (5-6.5% of nodules malignant); solitary nodules carry a higher malignancy risk than multiple nodules; biopsy is necessary for definitive diagnosis.
  • Clinical signs suggesting malignancy: deforming neck lump, fixation, hoarseness, history of enlarging goiter, compressive symptoms (dysphagia, dyspnea).
  • Ultrasound features of thyroid cancer: highly variable by type; lesions may be solid, cystic, or complex; often hypoechoic relative to normal thyroid; calcifications common in several types; increased vascularity around or within lesion.

Papillary carcinoma

  • Most common and least aggressive (approximately 75-85% of thyroid cancers).
  • More common in females; typically 20–50 years old.
  • Lymphatic spread is common; about 20% have cervical lymphadenopathy.
  • Ultrasound features: usually hypoechoic; microcalcifications (tiny punctate echogenic foci); disorganized vascularity; microcalcifications may be seen in affected cervical nodes.

Follicular carcinoma

  • Second most common type (about 10-20%).
  • More common in females; typically 4th–5th decades.
  • Spreads hematogenously (not primarily via lymphatics) with distant metastases to bone, lungs, brain, liver.
  • Usually solitary mass.
  • Ultrasound features: irregular margins, thick irregular halo, nodular enlargement with tortuous intralesional vessels; differentiation from follicular adenoma requires histology (cannot reliably distinguish by ultrasound or fine-needle aspiration).

Medullary carcinoma

  • Etiology: rare (~5%); neuroendocrine tumor producing calcitonin; serum calcitonin is a useful tumor marker.
  • 20% familial and linked to MEN II (parathyroidism, episodic hypertension, pheochromocytoma).
  • May be multicentric or bilateral in familial cases; often aggressive with nodal metastasis.
  • Ultrasound: often hypoechoic solid mass; calcifications common; coarse calcifications may be seen in primary tumor or involved nodes.

Anaplastic carcinoma

  • Etiology: rare (<5%); highly aggressive and rapidly enlarging.
  • Predominantly in older women (mean age ~65).
  • Presents as hard, fixed mass with rapid growth; local invasion often causes airway compromise; metastases common; survival poor.
  • Ultrasound: large, solid, hypoechoic mass; color Doppler may show invasion of vessels; extensive invasion may require CT/MRI for extent.

Lymphoma of the thyroid

  • Etiology: uncommon; predominantly non-Hodgkin’s type (<5% of thyroid cancers).
  • More common in older women (7th decade).
  • Presentation: rapidly enlarging neck mass with potential airway obstruction; often associated with Hashimoto’s thyroiditis.
  • Ultrasound: large, solid, hypoechoic, lobulated mass; may compress or infiltrate thyroid parenchyma; color Doppler often shows hypovascularity; cystic necrosis may occur; adjacent thyroiditis may cause heterogeneous appearance.

Diffuse thyroid disease: thyroiditis

  • Types in the transcript: Subacute (de Quervain’s), Hashimoto’s (chronic autoimmune), Graves’ disease (diffuse toxic goiter), and chronic (Riedel’s) thyroiditis.
  • Thyroiditis etiology: swelling and tenderness of the gland; often post-viral or autoimmune; more common in females 30–50 years.
  • Clinical signs: diffusely enlarged, soft thyroid; possible hoarseness or neck swelling; discomfort or pain; sometimes decreased T3/T4.
  • Ultrasound features: diffusely enlarged gland; mildly irregular echo pattern; generally hypoechoic relative to adjacent muscles; calcifications may occur after inflammation.

Subacute (de Quervain’s) thyroiditis

  • Etiology: granulomatous diffuse inflammation following viral infection; benign and usually transient.
  • Clinical: predominantly in females 20–50; tender, enlarged thyroid with fever; may present acutely or gradually; thyroid hormone may be transiently elevated in about 50% of cases; WBC count often normal; typically resolves spontaneously.
  • Ultrasound: diffusely enlarged, hypoechoic gland; may be asymmetric; patchy areas of variable echogenicity.

Chronic (Riedel’s) thyroiditis

  • AKA invasive fibrous thyroiditis.
  • Etiology: chronic fibrotic inflammatory process; replacement of thyroid tissue with fibrotic tissue and scarring; rare and least common inflammatory thyroiditis.
  • Clinical signs: insidious onset; gland adheres to trachea and subcutaneous neck tissues causing palpable compression.
  • Ultrasound: diffuse enlargement with heterogeneous echo from fibrosis.

Hashimoto’s thyroiditis

  • Etiology: chronic autoimmune thyroiditis; most common form of thyroiditis.
  • Clinical signs: painless diffuse enlargement; more common in young to middle-aged females; mild-to-moderate tenderness.
  • Ultrasound: diffuse coarse parenchymal texture; initially homogeneous enlargement with nodularity; later becomes inhomogeneous; ill-defined hypoechoic areas separated by fibrous strands; color Doppler shows hypervascularity in acute onset and normal-to-decreased flow in late stages; cervical lymphadenopathy may be present.

Graves’ disease

  • Etiology: autoimmune hyperfunction causing thyrotoxicosis; most frequent cause of hyperthyroidism; diffuse hyperplasia.
  • Clinical signs: more common in women >30; hypermetabolism; diffuse toxic goiter; exophthalmos; pretibial and dorsum foot skin changes; ↑ T3 and T4.
  • Ultrasound: diffuse thyroid enlargement with homogeneous texture; often hypoechoic; color Doppler shows increased vascularity.
  • Associated specific feature: exophthalmos.

Diagnostic imaging and reporting: TI-RADS

  • TI-RADS stands for Thyroid Imaging Reporting and Data System; developed to standardize assessment and reduce unnecessary biopsies.
  • Key points:
    • Nodules are common; risk-based biopsy decisions are guided by scores.
    • Categories and management (as per the provided TI-RADS table):
    • TR1: Benign — No FNA.
    • TR2: Not suspicious — No FNA.
    • TR3: Mildly suspicious — FNA if the nodule is ≥ 2.5 cm; follow if ≥ 1.5 cm.
    • TR4: Moderately suspicious — FNA if ≥ 1.5 cm; follow if ≥ 1 cm.
    • TR5: Highly suspicious — FNA if ≥ 1 cm; follow if ≥ 0.5 cm (special notes apply).
  • Scoring components (summary):
    • COMPOSITION: cystic, spongiform, mixed cystic/solid, solid components.
    • ECHOGENICITY: anechoic, hyperechoic/isoechoic, hypoechoic, very hypoechoic.
    • SHAPE: taller-than-wide vs wider-than-tall.
    • MARGIN: smooth, ill-defined, lobulated, irregular, extrathyroidal extension.
    • ECHOGENIC FOCI: macrocalcifications, peripheral calcifications, punctate echogenic foci, large comet-tail artifacts.
  • Purpose: helps determine biopsy necessity and follow-up strategy.
  • Source reference: DOI for TI-RADS framework: https://doi.org/10.1016/j.jacr.2017.01.046

Biopsy techniques for thyroid nodules

  • Fine-Needle Aspiration (FNA)
    • Ultrasound-guided; use 20–27 gauge needles attached to a syringe.
    • Specimens collected via capillary action technique (up-and-down movement within the mass).
    • Cytologic evaluation by a pathologist; may be performed in the room to ensure adequate sample.
    • Benefits: minimally invasive, rapid results; helps differentiate benign from malignant lesions.
  • Core Needle Biopsy (CNB)
    • Ultrasound-guided; uses 14–19 gauge needles.
    • Tissue samples obtained using a hollow biopsy needle (gun).
    • Provides preserved histology and spatial tissue architecture; 3–5 samples commonly obtained.
    • May be louder and require patient warning; used when FNA is nondiagnostic or when histology is necessary.

Ultrasound features of specific thyroid lesions (summary references)

  • Benign cysts: simple cysts are anechoic with posterior acoustic enhancement; walls smooth; may be solitary or multiple.
  • Cystic degenerations of nodules: common cause of cystic thyroid lesions; may contain blood, serous fluid, colloid; complex cysts may have internal debris; internal septa may be present.
  • Adenomas: variable appearance from cystic to solid; well-circumscribed; peripheral halo may be present; hyperfunction lesions show increased peripheral or intralesional vascularity; eggshell calcifications possible.
  • Papillary carcinoma: microcalcifications; hypoechoic; disorganized vasculature; cervical lymphadenopathy possible with malignant nodes.
  • Follicular carcinoma: irregular margins; thick irregular halo; often solitary; may be difficult to distinguish from follicular adenoma on ultrasound or FNA; histology required.
  • Medullary carcinoma: hypoechoic solid mass with calcifications; may be multicentric/bilateral in familial cases; lymph node involvement common.
  • Anaplastic carcinoma: large, aggressive mass; invasion of surrounding muscles and vessels; CT/MRI often needed for extent.
  • Lymphoma: large, solid, hypoechoic, lobulated mass; hypovascular on color Doppler; may be associated with Hashimoto’s thyroiditis.

Practical notes on clinical correlations

  • Goiter and nodules may compress the esophagus or trachea, causing dysphagia or dyspnea.
  • Graves’ disease often presents with exophthalmos and diffuse goiter; hyperthyroid symptoms with increased T3/T4.
  • Hashimoto’s thyroiditis can present with diffuse enlargement and nodularity that progresses to inhomogeneous texture; vascularity is variable across disease stages.
  • Thyroid cancer management typically requires biopsy confirmation (FNA or CNB) due to variable ultrasound appearances across tumor types.

Quick reference: key numerical values

  • Normal ranges:
    • TSH: 3−30  ng/mL3-30\ \,\mathrm{ng/mL}
    • Free T4: 0.8−2.4  ng/dL0.8-2.4\ \,\mathrm{ng/dL}
    • Total T4: 4−11  ng/mL4-11\ \,\mathrm{ng/mL}
    • T3: 75−220  ng/mL75-220\ \,\mathrm{ng/mL}
    • Calcitonin: <100\ \,\mathrm{pg/mL}
  • Iodine requirement for normal thyroxine production: 100−200  mg/week100-200\ \,\mathrm{mg/week}
  • Ultrasound size references (gland):
    • Length: 4.0−6.0 cm4.0-6.0\ \mathrm{cm}
    • AP height: 1.3−1.8 cm1.3-1.8\ \mathrm{cm}
    • Width: 1.5−2.0 cm1.5-2.0\ \mathrm{cm}
    • Isthmus thickness: 4.0−6.0 mm4.0-6.0\ \mathrm{mm}

Quick glossary of terms

  • Euthyroid: normal thyroid function.
  • Hypothyroidism: underproduction of thyroid hormones; commonly elevated TSH with low T4/T3.
  • Hyperthyroidism: overproduction of thyroid hormones; commonly suppressed TSH with high T4/T3.
  • TI-RADS: a standardized reporting system for thyroid nodules to assess malignancy risk and guide biopsy decisions.