Thyroid Pathology — Goiter, Thyroiditis, Carcinoma

GOITER (Diffuse Thyroid Hyperplasia)

Simple Goiter (Nontoxic Diffuse Goiter)

• Iodine deficiency → impaired thyroid hormone synthesis → decreased negative feedback on TSH → compensatory TSH-stimulated thyroid follicular hyperplasia and hypertrophy
• Follicular cells enlarge and proliferate; thyroid mass increases without autonomous hormone production
• Results in diffuse thyroid enlargement without hyperthyroidism
• Histology shows hyperplastic follicles with columnar epithelium, increased vascularity, and relative decrease in colloid

Toxic Multinodular Goiter (Plummer's Disease)

• Develops from longstanding simple goiter with somatic mutations in TSH receptor (TSHR) and GNAS genes creating autonomous thyroid nodules
• Mutated follicular cells produce thyroid hormone independent of TSH → suppressed TSH, elevated free T3/T4
• Nodules become increasingly independent, causing chronic thyrotoxicosis
• Histology shows nodules of varying sizes with focal areas of autonomy; some nodules may show hemorrhage, fibrosis, or calcification

THYROIDITIS

Hashimoto's Thyroiditis (Chronic Lymphocytic Thyroiditis)

• Autoimmune destruction via Th1-mediated cellular immunity and B cell-derived antibodies (anti-TPO, anti-thyroglobulin)
• Cytotoxic CD8+ T cells infiltrate and destroy thyroid follicles; B cells produce thyroid peroxidase (TPO) and thyroglobulin autoantibodies
• Progressive follicular destruction → hypothyroidism; TgAb and TPOAb are pathognomonic
• Histology: dense lymphoplasmacytic infiltration, Hürthle cells (oncocytic metaplasia), and progressive fibrosis leading to atrophy
• HLA association (HLA-DR3, HLA-DR5) indicates genetic predisposition

Graves' Disease (Diffuse Toxic Goiter)

• TSH receptor-stimulating antibodies (TRAb/TSI) bind TSH receptor and activate Gs protein → increase cAMP → follicular cell hyperplasia and hormone overproduction
• Orbital fibroblasts also express TSH receptor; antibody binding causes orbital inflammation, adipose tissue expansion, and extraocular muscle enlargement → exophthalmos
• Thyroid shows diffuse hyperplasia with tall columnar epithelium, increased vascularity, and scalloped colloid margins
• Results in persistent thyroid stimulation independent of physiologic TSH suppression

Acute Thyroiditis (De Quervain's Thyroiditis)

• Viral infection (coxsackievirus, rubella, mumps, EBV) triggers viral replication in follicular cells → direct cytotoxicity and secondary immune activation
• Destruction of follicles with release of preformed thyroid hormone → transient thyrotoxicosis
• Followed by hormone depletion phase with transient hypothyroidism and eventual recovery
• Histology: granulomatous inflammation with giant cells (initially viral-induced), follicular destruction, and preserved follicular architecture

Riedel's Thyroiditis (Fibrous/Invasive Thyroiditis)

• Chronic fibroinflammatory disease of unknown etiology, possibly related to IgG4-mediated autoimmunity
• Progressive replacement of thyroid parenchyma with dense fibrosis extending beyond thyroid capsule into surrounding structures (mediastinum, neck muscles)
• Associated with other fibrotic conditions (retroperitoneal fibrosis, orbital pseudotumor)
• Histology: dense fibrous tissue with minimal inflammation, hyalinization, and destruction of normal thyroid architecture

Suppurative (Bacterial) Thyroiditis

• Bacterial infection (S. aureus, S. pyogenes, E. coli) via hematogenous spread or direct extension from pharyngitis
• Often occurs in immunocompromised patients or those with preexisting thyroid disease
• Acute suppurative inflammation with abscess formation
• Histology: acute inflammatory infiltrate with neutrophils, necrosis, and potential abscess formation

THYROID CARCINOMA

Papillary Thyroid Carcinoma (PTC)

• Most common thyroid malignancy (80% of cases); arises from follicular epithelium
• Pathognomonic driver mutations: BRAF V600E (most common, ~45%), RET/PTC rearrangement (especially in radiation-induced), NTRK fusions
• Slow growth with early lymph node metastasis (50% at presentation) but late hematogenous spread
• Mutations activate MAPK/ERK signaling → increased cell proliferation, decreased differentiation, reduced apoptosis
• Excellent prognosis with 10-year survival >90%; mortality risk stratified by age, size, extrathyroidal extension, and distant metastasis

Follicular Thyroid Carcinoma (FTC)

• Second most common thyroid malignancy; arises from follicular epithelium
• Characterized by vascular invasion and distant hematogenous metastasis (lungs, bone, brain); minimal lymph node involvement distinguishes from PTC
• Key mutations: PI3K/AKT/mTOR pathway alterations, PAX8-PPAR-gamma translocation (30%)
• Activating mutations promote cell survival and suppress apoptosis
• Prognosis intermediate between papillary and anaplastic; 10-year survival ~85%

Medullary Thyroid Carcinoma (MTC)

• Arises from parafollicular C cells (neuroendocrine origin); secretes calcitonin (pathognomonic marker)
• RET proto-oncogene gain-of-function mutations in ~50% sporadic and 100% familial cases (MEN 2A, MEN 2B, familial MTC)
• Germline RET mutations activate tyrosine kinase → increased proliferation and decreased apoptosis
• Amyloid stroma (composed of calcitonin) is histopathologic hallmark
• Earlier diagnosis in familial cases via genetic screening improves outcomes

Anaplastic Thyroid Carcinoma (ATC)

• Most aggressive thyroid malignancy with rapid progression and poor prognosis (median survival <1 year)
• Arises from dedifferentiation of preexisting differentiated thyroid cancer or de novo
• TP53 mutations (70%), BRAF V600E, RAS mutations, and complex genomic alterations drive undifferentiation
• Loss of PAX8 and thyroglobulin expression; activation of EMT pathways
• Rapid invasion beyond thyroid capsule with airway compromise and distant metastases at presentation

Goiter

• Iodine deficiency (most common worldwide cause of goiter; rare in iodized salt regions)
• TSH elevation from primary hypothyroidism (Hashimoto's thyroiditis, iodine deficiency, dyshormonogenesis)
• Medications: lithium (inhibits hormone synthesis and release), amiodarone, interferon-alpha
• Hormonal factors: estrogen, pregnancy (increased iodine demands, increased TBG)
• Environmental goitrogens: cruciferous vegetables (cabbage, Brussels sprouts), cassava, thiocyanates in tobacco smoke
• Somatic mutations in TSH receptor and GNAS → toxic multinodular goiter

Thyroiditis

• Hashimoto's thyroiditis: female predominance (10:1), HLA-DR3/DR5 association, family history of autoimmune disease, concurrent autoimmune conditions (type 1 diabetes, celiac disease, Addison's disease)
• Graves' disease: female predominance (5-10:1), HLA-B8/DR3 association, stress, infection, pregnancy (postpartum flare), family history, iodine intake
• De Quervain's thyroiditis: viral prodrome, HLA-B35 association, male:female ratio 1:5
• Riedel's thyroiditis: rare, female predominance, IgG4 dysregulation
• Suppurative thyroiditis: immunocompromise, preexisting thyroid disease, pharyngitis, dental infection

Thyroid Carcinoma

• Radiation exposure (ionizing radiation in childhood → RET/PTC rearrangement; most significant risk for PTC)
• Gender: females affected more frequently for differentiated cancers (papillary, follicular); males have worse prognosis
• Age: pediatric and older adult ages associated with more aggressive variants
• Preexisting thyroid disease: benign nodules, goiter
• Familial syndromes: MEN 2A/2B (RET mutations → medullary carcinoma), familial adenomatous polyposis (papillary carcinoma), Cowden syndrome (PTEN mutations)
• Hormone therapy: prolonged estrogen use may increase papillary carcinoma risk
• Obesity, metabolic syndrome
• Genetic susceptibility: BRAF, RAS, TP53, PAX8-PPAR mutations

GOITER

Cardinal Symptoms

• Neck enlargement (visible swelling, neck tightness, difficulty with clothing collar fit)
• Dysphagia (difficulty swallowing, sensation of neck fullness)
• Dyspnea (shortness of breath, stridor, difficulty breathing when lying flat—orthopnea)
• Voice hoarseness (recurrent laryngeal nerve compression)
• Neck pain or tenderness (less common in simple goiter)

Physical Examination Findings

• Diffuse, symmetric thyroid enlargement without nodules (simple goiter) vs. palpable nodules of varying size and firmness (multinodular goiter)
• Thyroid bruit (systemic flow murmur audible with stethoscope, suggests increased vascularity in Graves' disease)
• Thyroid tenderness (minimal in chronic goiter)
• Lymphadenopathy (absent in simple goiter; present in thyroiditis and malignancy)
• Signs of hyperthyroidism (in toxic multinodular goiter): tachycardia, tremor, hyperreflexia, warm moist skin

Laboratory & Imaging Correlates

• TSH normal or suppressed in simple goiter; suppressed in toxic multinodular goiter
• Free T4/T3 normal in simple goiter; elevated in toxic multinodular goiter
• Thyroid ultrasound: diffuse enlargement, heterogeneous echotexture, or multiple nodules of varying echogenicity
• CT/MRI: assess tracheal compression, retrosternal extension, invasion of adjacent structures
• Radioiodine scan (if toxic multinodular): patchy uptake in autonomous nodules with suppressed background

THYROIDITIS

Hashimoto's Thyroiditis

• Insidious onset of fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss (hypothyroid symptoms)
• Goiter (firm, diffuse, nontender)
• Myxedema (puffy face, periorbital edema, macroglossia) in advanced cases
• Delayed hyporeflexia ("hung up" reflexes—slow both contraction and relaxation phases)
• Elevated TSH, low free T4 (primary hypothyroidism); positive TPO and thyroglobulin antibodies
• Ultrasound: hypoechoic, heterogeneous pattern ("hailstorm" appearance from lymphocytic infiltration)

Graves' Disease

• Insidious onset of palpitations, anxiety, weight loss, heat intolerance, tremor, hyperreflexia (hyperthyroid symptoms)
• Diffuse goiter (soft, tender with bruit)
• Exophthalmos (bilateral eye protrusion, proptosis, stare—from orbital tissue expansion); lid lag and lid retraction
• Pretibial myxedema (localized subcutaneous edema on anterior shins, orange-peel appearance)
• Thyroid acropachy (digital swelling with periosteal new bone formation)
• Low TSH, elevated free T4/T3; positive TSI/TRAb (pathognomonic)
• Ultrasound: diffuse enlargement, increased vascularity ("thyroid inferno" on Doppler)

De Quervain's Thyroiditis

• Abrupt onset of severe neck pain and tenderness (often preceded by viral prodrome)
• Constitutional symptoms: fever, malaise, myalgias
• Triphasic course: (1) thyrotoxic phase (days to weeks)—elevated free T4/T3, suppressed TSH; (2) hypothyroid phase—low free T4, elevated TSH; (3) recovery phase—normalization
• Elevated inflammatory markers: ESR markedly elevated (>50), CRP elevated; elevated thyroid peroxidase may be present but antibody titers lower than Hashimoto's
• Ultrasound: heterogeneous, hypoechoic infiltration with absent Doppler flow

Riedel's Thyroiditis

• Insidious onset of thyroid hardness and neck stiffness; often presents with hypothyroidism
• Dense, rock-hard thyroid (may be difficult to distinguish from carcinoma)
• Associated fibrotic conditions: retroperitoneal fibrosis, orbital pseudotumor, mediastinal fibrosis, pulmonary fibrosis
• TSH elevated, free T4 low (from fibrotic destruction of thyroid tissue)
• CT/MRI: thyroid appears dense, hypodense, with extension beyond capsule; may mimic carcinoma

Suppurative Thyroiditis

• Acute onset of severe neck pain, fever, malaise (often with preceding pharyngitis or upper respiratory infection)
• Unilateral thyroid swelling and tenderness
• Leukocytosis with left shift; elevated inflammatory markers
• Ultrasound/CT: focal hypoechoic or hypodense area suggesting abscess

THYROID CARCINOMA

Papillary Thyroid Carcinoma

• Often asymptomatic at diagnosis; frequently found incidentally
• Solitary or multiple thyroid nodules (firm, irregular borders)
• Cervical lymphadenopathy (50% at presentation); neck mass may be initial symptom
• If advanced: dysphagia, dyspnea, voice hoarseness (recurrent laryngeal nerve invasion)
• Distant metastases (lungs, bone, brain) less common at presentation than nodal disease
• TSH typically normal; thyroglobulin mildly elevated (less specific than in follicular carcinoma)
• Ultrasound: hypoechoic, irregular borders, microcalcifications (intranuclear pseudoinclusions), taller-than-wide shape, increased vascularity
• FNA cytology: "Orphan Annie eye" nuclei (pale, optically clear chromatin)

Follicular Thyroid Carcinoma

• Usually presents as solitary thyroid nodule without lymphadenopathy (unlike papillary)
• Often asymptomatic until distant metastases develop (lungs, bone)
• Thyroglobulin markedly elevated (more specific

Step 1 — biochemical triage (always first)

• Serum TSH: the single most sensitive initial test because pituitary TSH responds logarithmically to small changes in free T4. Suppressed TSH → thyrotoxicosis; elevated TSH → primary hypothyroidism; normal TSH with a nodule → proceed straight to imaging.
• Free T4 ± total T3: distinguishes overt from subclinical disease, and identifies T3 toxicosis (early Graves, toxic nodule).

Step 2 — if TSH is suppressed, determine the mechanism with radioiodine uptake (RAIU) and scan

• Diffuse increased uptake = Graves disease (confirm with TRAb/TSI, which is also the test of choice in pregnancy where RAIU is contraindicated).
• Patchy/multifocal "hot" areas with suppressed background = toxic multinodular goiter; a single hot nodule = toxic adenoma.
• Near-absent uptake = destructive thyroiditis (subacute granulomatous, painless/postpartum, amiodarone type 2) or exogenous hormone. Low thyroglobulin separates factitious thyrotoxicosis from thyroiditis.

Step 4 — the nodule pathway (ATA 2015 thyroid nodule/DTC guideline)

• Neck ultrasound with sonographic risk stratification (ATA patterns or ACR TI-RADS) is mandatory for every nodule with a non-suppressed TSH. Features driving risk: marked hypoechogenicity, microcalcifications, irregular/infiltrative margins, taller-than-wide shape, extrathyroidal extension, and abnormal nodes.
• FNA biopsy is the diagnostic gold standard, with the size threshold falling as sonographic suspicion rises (roughly ≥1 cm for high- and intermediate-suspicion patterns, larger for low/very-low-suspicion, and any size for suspicious lymph nodes).
• Bethesda System for Reporting Thyroid Cytopathology (categories I–VI) grades cytology and dictates action: nondiagnostic → repeat FNA; benign → surveillance; indeterminate (III/IV) → molecular testing or diagnostic lobectomy; suspicious/malignant → surgery.
• Key limitation: FNA cannot diagnose follicular carcinoma — capsular or vascular invasion is only visible on the resected specimen.
• Medullary carcinoma: serum calcitonin and CEA, germline RET testing, and plasma/urine metanephrines to exclude pheochromocytoma before any operation.

Immediate stabilization (emergencies first)

• Thyroid storm (Burch-Wartofsky score ≥45 is highly suggestive): beta blocker (propranolol, which also blunts peripheral T4→T3 conversion), a thionamide — propylthiouracil preferred acutely for the same conversion-blocking effect — then iodine (SSKI/Lugol) given at least one hour after the thionamide so the iodine load is not used as substrate (Wolff-Chaikoff block), plus glucocorticoids, cooling, and treatment of the precipitant. Order matters and is heavily tested.
• Myxedema coma: IV levothyroxine (± liothyronine) with stress-dose glucocorticoids given before hormone to avoid precipitating adrenal crisis; passive rewarming and ventilatory support.

Definitive therapy by entity (ATA 2016 hyperthyroidism guideline)

• Graves disease: three equivalent options — thionamide (methimazole first-line), radioactive iodine ablation, or total thyroidectomy. Beta blockade controls adrenergic symptoms while awaiting effect.
• Toxic multinodular goiter/adenoma: rarely remits on drugs → RAI or surgery is preferred definitive therapy.
• Hashimoto/hypothyroidism: levothyroxine monotherapy, titrated to TSH; ATA recommends treating overt hypothyroidism and considering treatment when TSH exceeds roughly 10 mIU/L.
• Subacute granulomatous (de Quervain) thyroiditis: NSAIDs, glucocorticoids if severe, beta blocker for symptoms — thionamides are useless because hormone is preformed and released, not synthesized.
• Suppurative thyroiditis: antistaphylococcal/antistreptococcal antibiotics plus drainage.
• Riedel thyroiditis: glucocorticoids ± tamoxifen; surgery reserved for compressive/airway disease (wedge isthmusectomy) because the fibrosis makes resection hazardous.

Disease-related

• Thyroid storm (emergency): decompensated thyrotoxicosis precipitated by surgery, infection, iodinated contrast, or amiodarone. Fever, delirium, tachyarrhythmia, and high-output heart failure; hepatic dysfunction signals severity.
• Myxedema coma (emergency): profound hypothyroidism → hypothermia, hypoventilation with CO₂ retention, hyponatremia, bradycardia, and obtundation.
• Atrial fibrillation and osteoporosis: excess T3 shortens atrial refractoriness and increases osteoclastic bone resorption — the reason both untreated thyrotoxicosis and levothyroxine over-replacement matter.
• Compressive goiter (emergency if acute): retrosternal extension → stridor, positive Pemberton sign (facial plethora on arm elevation); sudden nodular hemorrhage can obstruct the airway.
• Graves orbitopathy and optic neuropathy (sight-threatening): retro-orbital fibroblast expansion; falling color vision or an afferent pupillary defect signals compression.
• Primary thyroid lymphoma: rapidly enlarging mass in longstanding Hashimoto thyroiditis — chronic lymphocytic stimulation drives MALT-type/diffuse large B-cell transformation.
• Anaplastic carcinoma: rock-hard fixed mass with airway invasion; tracheostomy may be needed emergently.

Treatment-related

• Thionamide agranulocytosis (emergency): idiosyncratic marrow suppression — fever or sore throat mandates stopping the drug and checking an absolute neutrophil count, not empiric reassurance.
• Propylthiouracil hepatotoxicity: fulminant hepatic necrosis; also ANCA-associated vasculitis. Methimazole causes cholestasis instead.
• Post-thyroidectomy hypocalcemia (emergency if symptomatic): inadvertent parathyroid devascularization → perioral paresthesias, Chvostek and Trousseau signs, QT prolongation, laryngospasm.
• Recurrent laryngeal nerve injury: unilateral → hoarseness; bilateral injury causes vocal cord adduction and acute airway obstruction (emergency).
• Radioactive iodine: transient radiation thyroiditis, sialadenitis/xerostomia, and worsening of Graves orbitopathy — prophylactic glucocorticoids are advised in smokers with eye disease. Permanent hypothyroidism is expected, not a failure.

• TSH is always the first move: for any thyroid stem — nodule, goiter, or vague symptoms — the single best next step is serum TSH, then ultrasound if TSH is normal/high and a nodule exists, or RAIU scan if TSH is suppressed.
• Papillary carcinoma buzzwords: Orphan Annie eye nuclei, nuclear grooves, intranuclear pseudoinclusions, and psammoma bodies. Nuclear features — not architecture — make the diagnosis, which is why FNA is diagnostic for PTC. Psammoma bodies are also seen in serous papillary ovarian carcinoma, meningioma, and mesothelioma.
• The classic FNA trap: a "follicular neoplasm" on cytology cannot be called carcinoma. Capsular or vascular invasion on the resected specimen is required — so the answer is diagnostic lobectomy, never "repeat FNA."
• Medullary carcinoma: amyloid stroma (calcitonin-derived, Congo red with apple-green birefringence), elevated calcitonin, germline RET. Before thyroidectomy in suspected MEN2, exclude pheochromocytoma — operating first can precipitate hypertensive crisis. MTC does not take up radioiodine.
• Painful thyroid + markedly elevated ESR + low radioiodine uptake = subacute granulomatous (de Quervain) thyroiditis after a viral prodrome. Treat with NSAIDs/steroids; thionamides are the distractor.
• Low-uptake thyrotoxicosis differential: thyroiditis, exogenous levothyroxine (low thyroglobulin), and struma ovarii. High uptake means the gland is making hormone — Graves or toxic nodular disease.
• Hashimoto's one tested association: a rapidly enlarging, firm goiter in a patient with longstanding Hashimoto thyroiditis should raise concern for primary thyroid lymphoma, not carcinoma.
• Rock-hard, fixed, painless gland: in an elderly patient think anaplastic carcinoma; in a middle-aged woman with retroperitoneal fibrosis and IgG4 disease think Riedel thyroiditis. Biopsy distinguishes them.
• Pregnancy rule: PTU in the first trimester (methimazole → aplasia cutis), methimazole thereafter; radioiodine is absolutely contraindicated.