[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Endocrine system](https://durgical.com/en/general-pathology/areas/endocrine/) › Thyroid nodule and follicular adenoma

# Thyroid nodule and follicular adenoma

- Lesion

Thyroid lumps are common and most are harmless. A needle sample sorts out the few that may be cancer.

Updated Nov 12, 2026

## Key points

- Risk factors for malignancy: male sex, extremes of age, radiation history, solitary solid nodule, cold nodule, rapid growth, hoarseness.
- Hot (functioning) nodules are almost always benign; cold nodules carry most cancers.
- Follicular adenoma: solitary, encapsulated, compresses the surrounding gland; RAS or PAX8-PPARG in some.
- Toxic adenoma: autonomous adenoma with TSHR or GNAS activating mutations; causes hyperthyroidism.
- Oncocytic (Hürthle cell) adenoma: cells full of mitochondria; distinct mitochondrial and chromosomal changes.
- FNA cytology cannot separate adenoma from follicular carcinoma: the whole capsule must be examined.

## What it is

Thyroid nodules are found in a large share of adults, especially on ultrasound, and most are benign: nodular goitre or follicular adenoma. Nodules are more suspicious when solitary, in young or older patients, in men, after neck irradiation, and when solid and cold on scan. Follicular adenoma is a benign encapsulated tumour of follicular cells; it cannot be told from follicular carcinoma on cytology because the difference is invasion of the capsule or vessels.

## Structure

- Gross: solitary round nodule with a thin intact capsule, grey-white to brown; may show haemorrhage or cystic change.
- Micro: uniform follicles with colloid, cells without papillary nuclear features.
- The capsule and its vessels are sampled thoroughly to exclude invasion.
- Endocrine atypia (scattered big nuclei) does not mean malignancy.

## Benign or suspicious nodule

| Feature | Favours benign | Favours malignant |
| --- | --- | --- |
| [Number](https://durgical.com/en/conditions/goitre/) | Many nodules (multinodular) | Solitary nodule |
| [Patient](https://durgical.com/en/conditions/thyroid-cancer/) | Middle-aged woman | Man, child or older adult; radiation history |
| Scan | Hot (functioning) | Cold |
| Ultrasound | Spongiform, purely cystic | Solid, hypoechoic, microcalcifications, taller than wide |
| [Cytology](https://durgical.com/en/conditions/thyroid-nodule/) | Colloid, benign follicular cells | Papillary nuclear features or follicular neoplasm |

## Clinical notes

- Reporting of thyroid FNA uses a tiered system, from non-diagnostic and benign to suspicious and malignant; molecular tests can refine indeterminate results.
- A follicular neoplasm on cytology leads to lobectomy so the capsule can be examined.

## Memory aid

For follicular tumours, the cytologist can only see the bricks; whether the wall (capsule) was breached needs the whole house.

## Test yourself

Think of the answer, then tap the question to see it.

## Why can fine-needle aspiration not tell follicular adenoma from follicular carcinoma?

Both have similar follicular cells; carcinoma is defined by capsular or vascular invasion, which only histology of the whole capsule shows.

## Which mutations make a follicular adenoma toxic?

Activating mutations of the TSH receptor or GNAS, which keep cAMP signalling on.

## Are hot nodules more or less likely to be malignant?

Less likely; hot nodules are almost always benign.

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## Elsewhere in DurgiPedia

- [Cytology: the Pap test, fine-needle aspiration and body fluids](https://durgical.com/en/general-pathology/cytology-the-pap-test-fine-needle-aspiration-and-body-fluids/)
- [Benign vs malignant tumours: the comparison table](https://durgical.com/en/general-pathology/benign-vs-malignant-tumours-the-comparison-table/)
- [Thyroid follicles, C cells and parathyroid](https://durgical.com/en/histology/thyroid-follicles-c-cells-and-parathyroid/)

## Connections

## Related 4

- [Goitre: diffuse and multinodular](https://durgical.com/en/general-pathology/goitre-diffuse-and-multinodular/)
- [Follicular thyroid carcinoma and NIFTP](https://durgical.com/en/general-pathology/follicular-thyroid-carcinoma-and-niftp/)
- [Papillary thyroid carcinoma](https://durgical.com/en/general-pathology/papillary-thyroid-carcinoma/)
- [Cytology: the Pap test, fine-needle aspiration and body fluids](https://durgical.com/en/general-pathology/cytology-the-pap-test-fine-needle-aspiration-and-body-fluids/)

## Related conditions 4

- [Thyroid nodule](https://durgical.com/en/conditions/thyroid-nodule/)
- [Goitre](https://durgical.com/en/conditions/goitre/)
- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)
- [Thyroid cancer (papillary)](https://durgical.com/en/conditions/thyroid-cancer/)

[← All of general pathology](https://durgical.com/en/general-pathology/)

Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Thyroid Nodule](https://www.ncbi.nlm.nih.gov/books/NBK535422/) (StatPearls (NCBI Bookshelf), 2025); [Thyroid Cancer Treatment (PDQ®)–Health Professional Version](https://www.cancer.gov/types/thyroid/hp/thyroid-treatment-pdq) (National Cancer Institute, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/thyroid-nodule-and-follicular-adenoma/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/thyroid-nodule-and-follicular-adenoma/