Benign thyroid diseases and malignant thyroid disease

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Thyroid Diseases: Benign and Malignant

ANATOMY & PHYSIOLOGY (Overview)

The thyroid gland consists of two lobes connected by an isthmus, located below and anterior to the larynx. Each lobe is divided into lobules containing 20–40 follicles, lined by cuboidal-to-columnar epithelial cells that synthesize thyroglobulin (the iodinated precursor stored as colloid). Under TSH stimulation, follicular cells convert thyroglobulin into T4 (thyroxine) and lesser T3 (triiodothyronine). In peripheral tissues, T4 is deiodinated to T3, which binds nuclear thyroid hormone receptors with 10-fold greater affinity and drives increased carbohydrate and lipid catabolism, protein synthesis, and elevated basal metabolic rate.

Robbins & Kumar Basic Pathology, p. 729

BENIGN THYROID DISEASES

1. Hyperthyroidism (Thyrotoxicosis)

Definition: Elevated circulating thyroid hormones causing a hypermetabolic state.

Causes

Cause	Frequency
Graves' disease (autoimmune, diffuse hyperplasia)	~85%
Hyperfunctioning ("toxic") multinodular goiter	Common
Hyperfunctional ("toxic") adenoma	Less common
Thyroiditis (transient)	Occasional
TSH-producing pituitary adenoma	Rare

Clinical Features

System	Manifestations
Constitutional	Soft, warm, flushed skin; heat intolerance; excessive sweating; weight loss despite increased appetite
GI	Hypermotility, diarrhea, malabsorption, steatorrhea
Cardiac	Palpitations, tachycardia, increased contractility; CHF in older patients
Neuromuscular	Anxiety, tremor, irritability; proximal muscle weakness (~50%)
Ocular	Wide staring gaze, lid lag (sympathetic overactivity); exophthalmos in Graves' disease

Special Forms

Thyroid storm: Abrupt severe hyperthyroidism (most often Graves'), precipitated by infection, surgery, stress, or cessation of antithyroid drugs. Medical emergency — significant mortality from cardiac arrhythmias.
Apathetic hyperthyroidism: In older adults; classic features blunted; often detected during workup for weight loss or worsening cardiovascular disease.

Diagnosis

Serum TSH is the single best screening test — suppressed in primary hyperthyroidism.
Elevated free T4 (or T3 in T3-toxicosis).
Radioactive iodine uptake (RAIU): Diffusely increased in Graves'; focal "hot" nodule in toxic adenoma; decreased in thyroiditis.
Robbins & Kumar Basic Pathology, p. 730

2. Hypothyroidism

Definition: Deficient thyroid hormone production; may be primary (thyroid) or secondary (pituitary/hypothalamic).

Causes

Category	Cause	Mechanism
Primary	Surgery / radiation	Loss of thyroid tissue
Primary	Hashimoto's thyroiditis	Autoimmune destruction
Primary	Iodine deficiency	Decreased hormone synthesis
Primary	Drugs (lithium, iodides)	Interference with synthesis
Primary	Dyshormonogenetic goiter	Congenital enzyme defect
Secondary	Pituitary failure	Defective TSH production
Tertiary	Hypothalamic failure	Defective TRH/TSH production

Clinical Features by Age

Neonates (Cretinism): Neurologic impairment, intellectual disability, characteristic facies (similar to Down syndrome), dwarfism. Failure to thrive; newborn screening + early T4 replacement reduces deficits.
Children: Delayed growth and development; abdominal distension, umbilical hernia.
Adults (Myxedema):
- Fatigue, weight gain, cold intolerance, constipation, menorrhagia
- Facial/periorbital puffiness (glycosaminoglycan deposition in subcutaneous tissue)
- Rough, dry, yellowish skin (↓ carotene conversion); dry, brittle hair; loss of outer ⅓ of eyebrows
- Enlarged tongue, slowed speech
- Cardiovascular: Bradycardia, cardiomegaly, pericardial effusion, reduced cardiac output
- Impaired libido and fertility

Diagnosis

Low T4 and T3; elevated TSH (primary); low TSH unresponsive to TRH (secondary).
Thyroid autoantibodies (anti-TPO, anti-TG) in Hashimoto's.
Schwartz's Principles of Surgery, p. 1666

3. Graves' Disease

Definition: Autoimmune hyperthyroidism caused by stimulatory IgG antibodies against the TSH receptor (TSI — thyroid-stimulating immunoglobulins), driving unregulated thyroid hormone synthesis.

Pathology

Gross: Diffusely enlarged, beefy-red gland (hyperemia).
Histology: Tall columnar follicular cells, papillary infoldings into colloid; scalloped colloid ("scalloping"); lymphocytic infiltration; germinal centers.

Classic Triad

Hyperthyroidism with diffuse goiter
Ophthalmopathy (exophthalmos) — caused by lymphocyte/GAG accumulation in retroorbital tissue
Pretibial myxedema (dermopathy) — thickening of skin over shins

Management

Antithyroid drugs (propylthiouracil, methimazole)
Radioactive iodine (I-131) ablation
Thyroidectomy

4. Hashimoto's Thyroiditis (Chronic Lymphocytic Thyroiditis)

Definition: Organ-specific autoimmune disease; the most common cause of hypothyroidism in iodine-sufficient regions.

Pathogenesis

Loss of self-tolerance to thyroid antigens → CD4+ T-cell activation → anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin antibodies → progressive follicular destruction.

Histology

Diffuse lymphocytic infiltrate with prominent germinal center formation
Hürthle cell (oxyphilic) change — follicular cells enlarged with abundant eosinophilic granular cytoplasm
Follicular atrophy; fibrosis in advanced disease

Clinical

Middle-aged women predominantly
Painless diffuse firm goiter (early) → atrophic gland (late)
Hypothyroidism (main outcome); rarely transient hyperthyroid phase ("Hashitoxicosis")
Increased risk of primary thyroid B-cell lymphoma

5. Subacute (De Quervain) Thyroiditis

Pathogenesis: Viral infection (mumps, coxsackievirus, adenovirus) triggers granulomatous inflammation.

Histology

Disrupted follicles surrounded by epithelioid histiocytes and multinucleated giant cells (granulomatous reaction)

Clinical

Painful, tender neck; fever; preceded by upper respiratory infection
Triphasic course: Transient hyperthyroid → euthyroid → transient hypothyroid → recovery
ESR markedly elevated; RAIU suppressed
Self-limiting; NSAIDs for pain; steroids for severe cases

6. Goiter

Simple (Non-toxic) Goiter

Diffuse, non-inflammatory, non-neoplastic enlargement; may be endemic (iodine deficiency) or sporadic.
Compensatory TSH stimulation drives hyperplasia when hormone synthesis is inadequate.
Evolves from diffuse hyperplastic goiter → multinodular goiter over time.

Multinodular Goiter (MNG)

Multiple nodules of varying size; most are non-functioning.
Toxic MNG (Plummer's disease): one or more nodules become autonomously hyperfunctioning.
Compression symptoms: dysphagia, dyspnea, stridor (especially intrathoracic/substernal extension).

7. Thyroid Adenoma

Benign solitary follicular neoplasm; most are non-functioning "cold" nodules on scan.
Follicular adenoma is the most common type — encapsulated, with intact capsule (key distinction from follicular carcinoma).
Rare toxic (hyperfunctioning) adenoma — TSH-receptor activating mutation; suppresses normal thyroid tissue on scan.

MALIGNANT THYROID DISEASES

Classification of Thyroid Cancers

Type	Cell of Origin	Frequency	Prognosis
Papillary carcinoma	Follicular cell	~85%	Excellent
Follicular carcinoma	Follicular cell	~10%	Good
Hürthle cell carcinoma	Follicular cell (oncocytic)	~3%	Intermediate
Medullary carcinoma	Parafollicular C cells	~5%	Intermediate
Anaplastic (undifferentiated)	Follicular cell	<1–2%	Very poor
Primary thyroid lymphoma	B cells	Rare	Variable

1. Papillary Thyroid Carcinoma (PTC)

Most common thyroid malignancy. Can occur at any age; peak 30–50 years; female predominance (3:1).

Risk Factors

Radiation exposure (especially childhood head/neck irradiation or nuclear fallout — Chernobyl)
RET/PTC rearrangements, BRAF V600E mutation (most common somatic mutation in PTC)

Pathology

Gross: Firm, pale, poorly delineated; may be cystic; often infiltrative; 20% are multifocal.
Histology (diagnostic features):
- Papillary architecture with fibrovascular cores
- "Orphan Annie eye" nuclei — enlarged, oval, overlapping nuclei with optically clear ("ground-glass") chromatin
- Nuclear grooves and pseudoinclusions (cytoplasmic invaginations into nucleus)
- Psammoma bodies — concentric calcified lamellations (pathognomonic when present)

Behavior

Spreads via lymphatics → regional cervical lymph nodes (common, does not worsen prognosis significantly)
Hematogenous spread to lung, bone (less common)

Prognosis

10-year survival >95% for low-risk patients
Microcarcinomas (≤1 cm) often found incidentally

Treatment

Total thyroidectomy ± central/lateral neck dissection
Radioactive iodine (I-131) ablation for high-risk patients
TSH suppression with levothyroxine

2. Follicular Thyroid Carcinoma (FTC)

Second most common; peak age 40–60 years; more common in iodine-deficient regions.

Pathology

Gross: Encapsulated, resembles adenoma — diagnosis requires histologic evidence of capsular invasion and/or vascular invasion (key distinction from adenoma; FNA cannot reliably distinguish).
Histology: Uniform follicles; lacks papillary nuclear features.
Molecular: RAS mutations; PAX8-PPARγ rearrangement (~30–40%).

Behavior

Hematogenous spread to bone, lung, liver (lymph node involvement uncommon — unlike PTC)
"Metastases that take up iodine" — responds to I-131

Prognosis

Slightly worse than PTC; depends on degree of invasion (minimally vs. widely invasive)
10-year survival ~85–90% (minimally invasive)

3. Hürthle Cell (Oncocytic) Carcinoma

Variant of follicular carcinoma composed of cells with abundant eosinophilic granular cytoplasm (mitochondria-rich).
Less iodine-avid than follicular carcinoma → I-131 less effective.
Slightly more aggressive; higher recurrence rate.

4. Medullary Thyroid Carcinoma (MTC)

Cell of origin: Parafollicular (C) cells → secretes calcitonin (tumor marker).

Forms

Form	Frequency	Notes
Sporadic	~75%	Usually unilateral; older adults
Familial/MEN 2A	~25%	RET proto-oncogene mutation; bilateral; younger; associated with pheochromocytoma + hyperparathyroidism
Familial/MEN 2B	Rare	RET mutation; bilateral; associated with pheochromocytoma + mucosal neuromas + marfanoid habitus
Familial MTC (FMTC)	Rare	RET mutation; MTC only, no other endocrinopathy

Pathology

Histology: Sheets/nests of polygonal cells; amyloid stroma (derived from calcitonin precursor — Congo red positive).
Cells may be spindle-shaped or polygonal.

Diagnosis

Elevated serum calcitonin (screening and follow-up marker)
RET mutation testing (all patients — family screening if positive)
CEA also elevated

Prognosis

10-year survival ~75%; worse than differentiated cancers
Does NOT take up radioactive iodine

Treatment

Total thyroidectomy + central neck dissection (surgery is the only curative option)
Vandetanib or cabozantinib for advanced/metastatic disease (RET/VEGFR inhibitors)

5. Anaplastic (Undifferentiated) Thyroid Carcinoma

The most aggressive thyroid malignancy. Rare (<5% of thyroid cancers); typically occurs in elderly patients (>65 years).

Pathogenesis

May arise de novo or from dedifferentiation of a pre-existing well-differentiated carcinoma.
TP53 mutations, BRAF mutations common.

Pathology

Gross: Large, rapidly enlarging neck mass with extrathyroidal extension, necrosis.
Histology: Pleomorphic undifferentiated cells — spindle cell, giant cell, and squamoid patterns; extensive necrosis and mitoses.

Clinical

Rapidly enlarging neck mass causing dysphagia, hoarseness, stridor.
Often unresectable at presentation.

Prognosis

Median survival 3–6 months; nearly 100% cancer-specific mortality at 1 year.
Rare cases downstaged by BRAF-targeted therapy (vemurafenib/dabrafenib + trametinib) if BRAF V600E mutation present.

6. Primary Thyroid Lymphoma

Almost always diffuse large B-cell lymphoma (DLBCL) or mucosa-associated lymphoid tissue (MALT) lymphoma.
Arises almost exclusively in the setting of Hashimoto's thyroiditis (background of chronic lymphocytic inflammation).
Presents as rapidly enlarging neck mass in older women with known Hashimoto's.
Treated with chemoradiation (not surgery); responds well to R-CHOP.

Approach to the Thyroid Nodule

Step	Action
History	Risk factors: radiation exposure, family history (MTC/MEN), rapid growth, hoarseness
TSH	If low → RAIU scan (hot nodule = almost always benign adenoma)
Ultrasound	Size, composition, calcifications, borders, vascularity; TIRADS scoring
FNA cytology	Bethesda classification (I–VI); guides management
Molecular testing	ThyroSeq/Afirma for indeterminate Bethesda III–IV

Red flags for malignancy: Firm/hard nodule, rapid growth, fixed to adjacent structures, vocal cord paralysis, cervical lymphadenopathy, family history of MTC/MEN2, age <20 or >70 years.

Summary Table: Benign vs. Malignant Thyroid Disease

Feature	Benign	Malignant
Presentation	Usually soft, functional change (hyper/hypothyroid)	Hard nodule, rapid growth, hoarseness
RAIU scan	Hot or normal uptake	Cold nodule (most thyroid cancers)
FNA	Benign cytology	Malignant/suspicious cells
Capsule	Intact (adenoma)	Invasion of capsule/vessels (FTC); no capsule (PTC, anaplastic)
Lymphatics	Not involved	PTC spreads via lymphatics
Vasculature	Not invaded	FTC spreads hematogenously
Calcitonin	Normal	Elevated in MTC
Prognosis	Excellent (treated)	Variable (excellent PTC → dismal anaplastic)

Sources:

Robbins & Kumar Basic Pathology, pp. 729–735
Schwartz's Principles of Surgery (11th ed.), pp. 1625–1662
Goldman-Cecil Medicine

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