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Thyroid adenoma.

Although the vast majority of adenomas are nonfunctional, a small proportion produces thyroid hormones ("toxic adenomas")

Activating ("gain of function") somatic mutations in the TSH receptor itself
well-defined, intact capsule

cells are arranged in uniform follicles that contain colloid

Variant: brightly eosinophilic granular cytoplasm (oxyphil or Hürthle cell change)
exhibit focal nuclear pleomorphism, atypia, and prominent nucleoli (endocrine atypia)

Careful evaluation of the integrity of the capsule is critical in the distinction of follicular adenomas from follicular carcinomas, that’s why FNAC is useless in differentiating between the two.

On radionuclide scanning, adenomas appear as "cold" nodules relative to the adjacent normal thyroid gland. Toxic adenomas, however, will appear as "warm" or "hot" nodules

malignancy is virtually nonexistent in "hot" nodules.