Thyroid Adenomas

Slide 1 · Thyroid Adenomas

Benign follicular neoplasms of the thyroid

Pathogenesis • morphology • diagnostic boundaries

A pathology lecture grounded in Robbins

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Classify a solitary thyroid nodule without equating it with malignancy

Explain TSH-independent hormone secretion in toxic adenomas

Recognize characteristic gross and microscopic morphology

Apply the invasion-based distinction between adenoma and carcinoma

Explain why FNA cannot confirm a follicular adenoma

Slide 3 · What is a follicular adenoma?

A benign, discrete neoplasm of thyroid follicular epithelium

Usually solitary, spherical or ovoid, and sharply circumscribed

Most are nonfunctioning; A small minority are autonomous

Diagnosis is established by tissue architecture and absence of invasion

Discrete nodule

Schematic

Adenoma is a histologic diagnosis—not a synonym for any thyroid nodule.

Slide 4 · Where does adenoma sit in thyroid pathology?

Thyroid nodule

Neoplastic or nonneoplastic

Follicular-patterned lesion

Assess complete lesion–host interface

A clinical nodule is a presentation; Follicular adenoma is a specific diagnosis.

Slide 5 · Solitary nodule ≠ solitary neoplasm

Clinical presentation

Palpable or incidental focal thyroid swelling

May be a cyst, nodular hyperplasia, thyroiditis, or neoplasm

Physical examination cannot establish invasion

Histologic entity

Follicular adenoma is a benign follicular-cell tumor

Discrete growth with noninvasive interface

Resection establishes a definitive diagnosis

Slide 6 · Major questions for a thyroid nodule

Is the lesion functional or nonfunctional?

Is it a neoplasm or a nonneoplastic nodule?

If follicular-patterned, is there capsular or vascular invasion?

Which investigations inform—but do not replace—histology?

Palpable thyroid nodule

Clinical and tissue assessment

Follicular-patterned lesion

Boundary + invasion status

Answer these questions separately.

Slide 7 · Two independent descriptive axes

Functional axis

Nonfunctioning adenoma: Usual presentation

Toxic adenoma: Autonomous hormone secretion

Scintigraphic uptake reflects function

Biologic axis

Adenoma: No invasion

Follicular carcinoma: Invasion present

Hot/cold status is not histologic classification

Slide 8 · Why the boundary of a lesion matters

A noninvasive follicular proliferation is assessed at its periphery.

A bland central field alone does not tell whether tumor penetrates its capsule or vessels.

Tumor

Capsule

Adjacent thyroid

Schematic

Slide 9 · Follicular nodular disease: Key mimic

Often contains multiple nodules across the gland

A dominant nodule may feel clinically solitary

Nodules may show degeneration and variable follicles

A distinct complete tumor capsule is often absent

Single nodule

Multiple nodules

Schematic

Clinical multinodular goiter and true solitary adenoma can overlap in appearance.

Slide 10 · Adenoma versus follicular nodular disease

Follicular adenoma

Usually solitary

Well circumscribed, often encapsulated

Growth pattern typically distinct from background thyroid

Follicular nodular disease

Multiple nodules may coexist

Dominant nodule may simulate neoplasm

Heterogeneous follicles and secondary changes

Slide 11 · Functional classification of adenomas

Most follicular adenomas do not cause thyrotoxicosis

A small minority secrete thyroid hormone independently of TSH

An autonomously functioning adenoma is called a toxic adenoma

Hormone output and morphologic invasion are different properties

Function

Biologic behavior

Nonfunctioning

Toxic / autonomous

No invasion: Adenoma

Invasion: Carcinoma

Classify on separate axes

“Toxic” indicates hormone excess, not malignant behavior.

Slide 12 · Normal TSH signaling: Conceptual overview

TSH binds TSH receptor

Gs-linked intracellular signaling

Follicular-cell hormone production

Feedback restrains TSH

Constitutive signaling bypasses dependence on pituitary TSH.

Slide 13 · Toxic adenomas arise through autonomy

Somatic gain-of-function changes activate the TSH receptor pathway

Activation occurs even when circulating TSH stimulation is reduced

Follicular cells maintain thyroid hormone secretion

Persistent autonomy can produce hyperthyroidism

Activating TSHR or GNAS mutation

Constitutive Gs signaling

Thyroid hormone secretion

Without normal TSH dependence

Schematic

This is acquired follicular-cell signaling autonomy.

Slide 14 · TSHR mutations

Activating TSHR variants can make the receptor signal without ordinary TSH stimulation.

TSHR encodes the thyroid-stimulating hormone receptor; Mutated receptors help drive autonomous hormone production.

Activating TSHR or GNAS mutation

Constitutive Gs signaling

Thyroid hormone secretion

Without normal TSH dependence

Schematic

Slide 15 · GNAS mutations

GNAS alterations can activate the Gsα signaling component downstream of TSHR.

The source describes GNAS as a less common activating mechanism than TSHR mutations.

TSH / receptor

Gsα pathway

Thyroid hormone secretion

Schematic

Slide 16 · From molecular change to a hot nodule

TSHR / GNAS activation

Constitutive follicular signaling

TSH-independent hormone secretion

Hyperthyroidism + hot uptake

The scan is functional evidence, not proof of a specific genetic variant.

Slide 17 · Normal regulation versus autonomous secretion

Normal follicle

TSH-dependent receptor activation

Hormone output tracks pituitary stimulation

Negative feedback acts through TSH

Toxic adenoma

Signal persists with little TSH stimulation

Hormone output becomes autonomous

Nodule often appears “hot” on radionuclide study

Slide 18 · What the molecular frequency means

TSHR or GNAS mutations in 50%–80% of toxic adenomas

This is a reported range, not a universal diagnostic sensitivity

A mutation is not required to diagnose every toxic adenoma

Molecular status does not replace the invasion assessment

Function

Biologic behavior

Nonfunctioning

Toxic / autonomous

No invasion: Adenoma

Invasion: Carcinoma

Classify on separate axes

Source-specific proportion: Robbins, 11th ed., p. 987.

Slide 19 · EZH1: A proposed second hit

A study proposed that EZH1 mutation may cooperate with an activating TSH pathway mutation.

This is a suggested two-hit model, not an established universal requirement for follicular adenoma.

Activating TSHR / GNAS

EZH1: Proposed cooperation

A proposed two-hit model

Hypothesis—not an obligatory criterion

Schematic

Slide 20 · Molecular association is not diagnostic proof

What Robbins supports

Activating TSHR / GNAS changes in many toxic adenomas

Suggested cooperating EZH1 alteration

Those mutations are uncommon in follicular carcinomas

What it does not establish

A molecular test is sufficient to diagnose adenoma

Every adenoma has an EZH1 mutation

Toxic adenomas commonly transform into carcinomas

Slide 21 · Toxic adenoma is not regarded as a precursor of carcinoma

TSHR, GNAS, and EZH1 alterations described in toxic adenomas are rare in follicular carcinomas.

This distinction to argue against toxic adenomas as precancerous lesions.

Function

Biologic behavior

Nonfunctioning

Toxic / autonomous

No invasion: Adenoma

Invasion: Carcinoma

Classify on separate axes

Slide 22 · Nonfunctioning adenomas: An important contrast

Do not secrete enough thyroid hormone to produce overt thyrotoxicosis

Often present as an isolated painless thyroid nodule

Usually exhibit relatively low iodine uptake on scintigraphy

“Cold” describes function and does not establish malignancy

Functional schematic—not a radionuclide scan

HOT

COLD

Relative uptake ↑

Relative uptake ↓

Schematic

Do not infer benign or malignant histology from uptake alone.

Slide 23 · Hot and cold: Interpret function correctly

Hot nodule

Higher uptake on radionuclide imaging

Suggests functional autonomy

Much more likely to be benign, not guaranteed

Cold nodule

Reduced tracer uptake

Typical of many nonfunctioning adenomas

Also occurs in other benign or malignant nodules

Slide 24 · Typical gross appearance

Solitary, spherical or ovoid mass

Clear separation from adjacent thyroid

Often encircled by a fibrous capsule

Can compress the surrounding thyroid parenchyma

Discrete nodule

Schematic

Schematic illustrates relationships; It is not a specimen photograph.

Slide 25 · The adenoma often bulges on cut section

An expansile, well-demarcated neoplasm displaces neighboring thyroid tissue.

The lesion as bulging from a freshly resected cut surface and compressing adjacent thyroid.

Expansile border

Color depends on cellularity and colloid

Schematic

Slide 26 · The capsule is a structural clue

A fibrous capsule often sharply separates tumor from normal thyroid

The capsule encircles many—but not all—adenomas

Tumor–capsule–thyroid relationships must be examined carefully

An intact capsule supports, but does not alone prove, benign behavior

Tumor

Capsule

Adjacent thyroid

Schematic

Capsular integrity requires histologic evaluation.

Slide 27 · Encapsulated versus unencapsulated adenoma

Encapsulated pattern

Well-formed continuous peripheral fibrous tissue

Clear lesion–host boundary

Capsular invasion must be excluded

Circumscribed without full capsule

Capsule is not necessarily present in every adenoma

Absence of infiltrative growth remains essential

Peripheral interface still determines diagnosis

Slide 28 · Why tumor color varies

Cut surfaces may appear gray-white, tan, or red-brown

Cellularity and stored colloid influence the appearance

Gross color is not a reliable discriminator of malignancy

Interpret color alongside encapsulation and growth pattern

Expansile border

Color depends on cellularity and colloid

Schematic

Macroscopic variation alone does not determine biologic behavior.

Slide 29 · Secondary changes may dominate grossly

Hemorrhage

Fibrosis

Calcification

Cystic change

Large adenomas particularly may display degenerative areas

Secondary changes may coexist

Schematic

These changes are also seen in follicular nodular disease.

Slide 30 · Secondary change is not invasion

Common degenerative findings

Hemorrhage within an adenoma

Fibrotic or calcified foci

Cystic breakdown in a larger nodule

Findings requiring tumor-border review

Definite extension through capsule

True vascular invasion

Infiltration of adjacent thyroid

Slide 31 · Microscopic architecture: Follicles

Well-differentiated follicular epithelial cells form follicles

Follicles may contain eosinophilic colloid

Architecture often differs from that of adjacent nonneoplastic thyroid

Peripheral relationship is needed to interpret the central pattern

Follicles containing colloid

Schematic — not histology

Original editable schematic; Not an H&E photomicrograph.

Slide 32 · Follicular uniformity is characteristic

Neoplastic follicles often show a relatively monotonous architecture.

The source describes follicles that may resemble normal thyroid but form a growth pattern distinct from the surrounding gland.

Follicles containing colloid

Schematic — not histology

Slide 33 · Cytology is usually bland

Cells typically vary little in size and shape

Nuclear morphology is comparatively uniform

Marked atypia is not a defining feature of ordinary adenoma

Bland appearance cannot by itself exclude follicular carcinoma

Follicles containing colloid

Schematic — not histology

Review the tumor margin even when nuclei appear innocuous.

Slide 34 · Mitotic figures are typically rare

Low mitotic activity supports the conventional benign cytologic impression.

Marked activity prompts greater suspicion and more careful evaluation; This is not itself a substitute for demonstrating invasion.

Follicles containing colloid

Schematic — not histology

Slide 35 · Colloid is a variable constituent

Follicles may contain pink, proteinaceous colloid

Cellularity and colloid contribute to gross color and consistency

Follicular size and colloid amount can vary among lesions

Colloid-containing follicles alone do not establish a neoplasm

Follicles containing colloid

Schematic — not histology

Evaluate pattern, border, and surrounding parenchyma.

Slide 36 · Oncocytic change in an adenoma

Some tumor cells have abundant granular eosinophilic cytoplasm

This cytoplasmic phenotype is termed oncocytic change

An oncocytic adenoma

Oncocytic cytoplasm alone does not demonstrate malignancy

Oncocytic cytoplasm (conceptual)

Schematic — not histology

Diagnosis still depends on complete structural assessment.

Slide 37 · Architecture and cytology answer different questions

Central tumor fields

Follicular growth pattern

Colloid formation

Uniform cells and rare mitoses

Possible oncocytic cytoplasm

Tumor periphery

Presence / character of capsule

Complete versus penetrative interface

Vascular and intrathyroidal invasion

Required for final biologic classification

Slide 38 · The decisive distinction: Invasion

Follicular carcinoma exhibits capsular, vascular, or intrathyroidal invasion.

An adenoma must be circumscribed and show no invasion; The absence of invasion cannot be established by routine cytology.

No invasion

Invasion

Capsule traversed

Conceptual boundary schematic

Slide 39 · Adenoma and carcinoma may look alike centrally

Follicular adenoma

Follicular cells can look bland

Encapsulated or sharply circumscribed

No capsular, vascular, or intrathyroidal invasion

Follicular carcinoma

May also exhibit differentiated follicles

Invasion establishes malignant behavior

Requires deliberate histologic study of tumor edge

Slide 40 · Capsular invasion: What is the question?

Does neoplastic tissue actually penetrate the capsule?

Tangential sections and irregular contours require cautious interpretation. The schematic distinguishes intact from penetrated capsule.

No invasion

Invasion

Capsule traversed

Conceptual boundary schematic

Slide 41 · Vascular invasion: What is the question?

Are neoplastic cells invading vessels at or beyond the tumor periphery?

Vascular invasion as a diagnostic feature of follicular carcinoma; Central bland histology does not negate it.

No invasion

Invasion

Vessel at edge

Conceptual boundary schematic

Slide 42 · Intrathyroidal invasion: What is the question?

Does the tumor infiltrate surrounding nonneoplastic thyroid?

Circumscription without invasion is required for adenoma, even when no complete fibrous capsule is visible.

No invasion

Invasion

Infiltration outside lesion

Conceptual boundary schematic

Slide 43 · A systematic peripheral sampling logic

Identify the lesion boundary

Examine tumor–capsule interface

Assess peripheral vessels

Exclude infiltration beyond lesion

This is a conceptual checklist, not a prescribed specimen-block count.

Slide 44 · Why focal invasion can be missed

A lesion is three-dimensional; Each histologic section is only a plane

The center cannot reveal every peripheral tumor–host relationship

The concerning focus may be small relative to the tumor circumference

Adequate sampling of the interface is therefore critical

Inspect the full interface, not only the center

Schematic

A reassuring single field does not constitute full exclusion of invasion.

Slide 45 · Concerning histology versus proven carcinoma

Raises suspicion

Increased cellularity

Notably elevated mitotic activity

Necrosis

Atypical architecture

Diagnostic distinction in the source

Capsular invasion

Vascular invasion

Intrathyroidal infiltration

These findings require tissue architecture

Slide 46 · What FNA can show

Follicular epithelial cells and colloid in an aspirated sample

Cytologic architecture and nuclear characteristics

Suspicious features prompting further diagnostic workup

Useful information for clinical nodule triage

Aspirated cells

No complete capsule in aspirate

Schematic

FNA samples cells, not the complete tumor boundary.

Slide 47 · What FNA cannot establish

Follicular adenoma versus follicular carcinoma cannot be definitively distinguished by FNA alone.

Capsular integrity and vascular invasion require evaluation of a resected lesion.

Aspirated cells

No complete capsule in aspirate

Schematic

Slide 48 · Ultrasound, FNA and resection

Preoperative evaluation

Ultrasound characterizes the nodule

Suspicious features guide FNA selection

FNA may classify many nodules but leaves some indeterminate

Definitive tissue diagnosis

Histology demonstrates architecture

Peripheral interface can be inspected

Adenoma requires exclusion of invasion

Slide 49 · Ultrasound provides risk information

Nodule composition and contour can be documented

Hypoechogenicity and irregular margins may be suspicious

Imaging can guide decisions about FNA

Ultrasound cannot directly prove absent microscopic capsular invasion

Ultrasound

FNA ± molecular

Resection histology

Boundary + invasion

Conceptual diagnostic sequence

No ultrasound pattern alone confirms follicular adenoma.

Slide 50 · Molecular assessment: A supporting role

Indeterminate thyroid nodules can be evaluated with molecular diagnostic tests

These studies may help select nodules needing further surgery

Molecular results are adjunctive to morphology

The textbook does not supply a single universal marker of follicular adenoma

Ultrasound

FNA ± molecular

Resection histology

Boundary + invasion

Conceptual diagnostic sequence

Do not confuse triage performance with proof of absent invasion.

Slide 51 · Clinical-to-pathology diagnostic pathway

Painless thyroid nodule

Functional + sonographic evaluation

FNA / molecular triage as indicated

Resection + periphery review when required

The pathway illustrates principle, not a substitute for current local clinical guidelines.

Slide 52 · Most adenomas present as a mass

Often painless

Frequently found on routine physical examination

May be incidentally detected during radiologic investigation

Clinical features overlap with many benign and malignant nodules

Discrete nodule

Schematic

Symptoms do not establish an invasion status.

Slide 53 · Larger lesions may cause dysphagia

An expanding thyroid mass can exert pressure on adjacent cervical structures.

Difficulty swallowing as one possible local symptom of a larger adenoma.

Discrete nodule

Schematic

Expansile mass → local pressure

Slide 54 · Clinical phenotype: Nonfunctioning versus toxic

Nonfunctioning follicular adenoma

Typically euthyroid clinical presentation

Painless nodule or incidental discovery

Often reduced radionuclide uptake

Toxic follicular adenoma

Autonomous thyroid hormone secretion

Can cause clinical thyrotoxicosis

Typically functionally “hot”

Slide 55 · Functional imaging: What to conclude

Hot nodule: Autonomous uptake is favored

Cold nodule: Reduced iodine uptake is identified

Hot nodules are much more likely to be benign

A cold nodule does not by itself prove carcinoma

Functional schematic—not a radionuclide scan

HOT

COLD

Relative uptake ↑

Relative uptake ↓

Schematic

Scintigraphy assesses function rather than capsular integrity.

Slide 56 · Resection allows definitive classification

The examined specimen permits assessment of the tumor boundary and any vascular invasion.

Definitive adenoma diagnosis requires careful histologic examination of the excised lesion.

Inspect the full interface, not only the center

Schematic

Slide 57 · Prognosis after a confirmed adenoma

Follicular adenoma is a benign neoplasm

It does not metastasize

Recurrence is not expected for a definitively diagnosed, adequately treated lesion

The expected prognosis is excellent

Discrete nodule

Schematic

These statements apply to a securely established adenoma, not an indeterminate nodule.

Slide 58 · Why adenoma and carcinoma must not be conflated

Shared appearances

Follicular epithelial differentiation

Sometimes a solitary circumscribed thyroid mass

A similar cytologic appearance on FNA

Essential difference

Adenoma lacks invasion

Carcinoma demonstrates invasion

Resection histology establishes the distinction

Slide 59 · Case 1: A bland follicular aspirate

An asymptomatic adult has a well-defined thyroid nodule. FNA yields uniform follicular cells and colloid.

Interpretation question

Can the aspirate certify follicular adenoma?

No. The diagnosis requires assessment of the resected lesion–host interface for invasion.

Aspirated cells

No complete capsule in aspirate

Schematic

Slide 60 · Case 2: A functionally hot nodule

A patient with thyrotoxicosis has one focally hyperfunctioning thyroid nodule on radionuclide imaging.

Interpretation question

Which mechanism is biologically plausible?

Activating TSHR or GNAS pathway alteration causing hormone secretion independent of TSH.

Activating TSHR or GNAS mutation

Constitutive Gs signaling

Thyroid hormone secretion

Without normal TSH dependence

Schematic

Slide 61 · Case 3: A focal capsular breach

A resected follicular-patterned tumor contains bland follicles centrally, but one peripheral field shows unequivocal capsular invasion.

Interpretation question

Does bland cytology retain an adenoma diagnosis?

No. Demonstrated invasion overrides bland cytologic appearance and raises carcinoma classification.

No invasion

Invasion

Capsule traversed

Conceptual boundary schematic

Slide 62 · Case 4: Dominant nodule in a large gland

A palpable dominant nodule is present, but examination reveals several other nodules with heterogeneous architecture.

Interpretation question

What is a key differential for a “solitary” adenoma?

Follicular nodular disease (multinodular goiter) with a dominant nodule.

Single nodule

Multiple nodules

Schematic

Slide 63 · Case 5: Oncocytic cells

A well-circumscribed follicular tumor shows abundant granular eosinophilic cytoplasm. No invasion is identified after peripheral assessment.

Interpretation question

What does the oncocytic phenotype indicate?

Oncocytic change; It does not alone establish malignancy.

Oncocytic cytoplasm (conceptual)

Schematic — not histology

Slide 64 · Decision framework: What each modality contributes

Method

Can support

Cannot by itself prove

Clinical assessment

Nodule / pressure / thyrotoxicosis

Adenoma versus carcinoma

Radionuclide scan

Functional activity (hot / cold)

Absence of invasion

Ultrasound

Nodule architecture and suspicious features

Microscopic capsular status

FNA

Cytology and triage

Capsular / vascular invasion

Resection histology

Tumor boundary and invasion status

—

Slide 65 · Adenoma versus principal mimics

Feature

Follicular adenoma

Follicular nodular disease

Follicular carcinoma

Distribution

Usually solitary

Typically multinodular

May be solitary

Capsule

Often present

Usually no distinct complete tumor capsule

May be present

Follicles

Relatively uniform

Variable / heterogeneous

May look differentiated

Invasion

Absent

Not a criterion of a benign nodule

Present by definition

Slide 66 · Five defining checkpoints

Follicular-cell neoplasm, commonly solitary

Well-circumscribed, often encapsulated mass

Follicles often uniform, with bland cytology

Most are nonfunctional; A minority are toxic

No capsular, vascular, or intrathyroidal invasion

Tumor

Capsule

Adjacent thyroid

Schematic

Adenoma is confirmed by the complete morphologic interpretation.

Slide 67 · Final diagnostic principle

A beautiful central micrograph does not substitute for examining the tumor boundary.

The diagnosis of follicular adenoma ultimately rests on circumscription without invasion.

No invasion

Invasion

Capsule traversed

Conceptual boundary schematic

Slide 68 · Source and visual provenance

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026). Chapter 24, “Thyroid Adenomas,” pp. 987–988.

Parent context: “Neoplasms of the Thyroid” and “Follicular Nodular Disease,” Chapter 24.

All diagrams are original, editable teaching schematics; no diagnostic specimen images are reproduced.

TSHR/GNAS frequencies and EZH1 two-hit discussion are reported as stated in the supplied edition.